Читать книгу Farm drainage - Henry F. French - Страница 16
ОглавлениеFertilizing Substances in Rain Water.—Amount of Rain Fall in United States—in England.—Tables of Rain Fall.—Number of Rainy Days, and Quantity of Rain each Month.—Snow, how Computed as Water.—Proportion of Rain Evaporated.—What Quantity of Water Dry Soil will Hold.—Dew Point.—How Evaporation Cools Bodies.—Artificial Heat Underground.—Tables of Filtration and Evaporation.
Although we usually regard drainage as a means of rendering land sufficiently dry for cultivation, that is by no means a comprehensive view of the objects of the operation.
Rain is the principal source of moisture, and a surplus of moisture is the evil against which we contend in draining. But rain is also a principal source of fertility, not only because it affords the necessary moisture to dissolve the elements of fertility already in the soil, but also because it contains in itself, or brings with it from the atmosphere, valuable fertilizing substances. In a learned article by Mr. Caird, in the Cyclopedia of Agriculture, on the Rotation of Crops, he says:
"The surprising effects of a fallow, even when unaided by any manure, has received some explanation by the recent discovery of Mr. Barral, that rain-water contains within itself, and conveys into the soil, fertilizing substances of the utmost importance, equivalent, in a fall of rain of 24 inches per annum, to the quantity of ammonia contained in 2 cwt. of Peruvian guano, with 150 lbs. of nitrogeneous matter besides, all suited to the nutrition of our crops."
About 42 inches of rain may be taken as a fair general average of the rain-fall in the United States. If this supplies as much ammonia to the soil as 3 cwt. of Peruvian guano to the acre, which is considered a liberal manuring, and which is valuable principally for its ammonia, we at once see the importance of retaining the rain-water long enough upon our fields, at least, to rob it of its treasures. But rain-water has a farther value than has yet been suggested:
"Rain-water always contains in solution, air, carbonic acid, and ammonia. The two first ingredients are among the most powerful disintegrators of a soil. The oxygen of the air, and the carbonic acid being both in a highly condensed form, by being dissolved, possess very powerful affinities for the ingredients of the soil. The oxygen attacks and oxydizes the iron; the carbonic acid seizing the lime and potash and other alkaline ingredients of the soil, produces a further disintegration, and renders available the locked-up ingredients of this magazine of nutriment. Before these can be used by plants, they must be rendered soluble; and this is only affected by the free and renewed access of rain and air. The ready passage of both of these, therefore, enables the soil to yield up its concealed nutriment."
We see, then, that the rains of heaven bring us not only water, but food for our plants, and that, while we would remove by proper drainage the surplus moisture, we should take care to first conduct it through the soil far enough to fulfill its mission of fertility. We cannot suppose that all rain-water brings to our fields precisely the same proportion of the elements of fertility, because the foreign properties with which it is charged, must continually vary with the condition of the atmosphere through which it falls, whether it be the thick and murky cloud which overhangs the coal-burning city, or the transparent ether of the mountain tops. We may see, too, by the tables, that the quantity of rain that falls, varies much, not only with the varying seasons of the year, and with the different seasons of different years, but with the distance from the equator, the diversity of mountain and river, and lake and wood, and especially with locality as to the ocean. Yet the average results of nature's operations through a series of years, are startlingly constant and uniform, and we may deduce from tables of rain-falls, as from bills of mortality and tables of longevity, conclusions almost as reliable as from mathematical premises.
The quantity of rain is generally increased by the locality of mountain ranges. "Thus, at the Edinburgh Water Company's works, on the Pentland Hills, there fell in 1849, nearly twice as much rain as at Edinburgh, although the distance between the two places is only seven miles."
Although a much greater quantity of rain falls in mountainous districts (within certain limits of elevation) than in the plains, yet a greater quantity of rain falls at the surface of the ground than at an elevation of a few hundred feet. Thus, from experiments which were carefully made at York, it was ascertained that there fell eight and a half inches more rain at the surface of the ground, in the course of twelve months, than at the top of the Minster, which is 212 feet high. Similar results have been obtained in many other places.
Some observations upon this point may also be found in the Report of the Smithsonian Institution for 1855, at p. 210, given by Professor C. W. Morris, of New York.
Again, the evaporation from the surface of water being much greater than from the land, clouds that are wafted by the winds from the sea to the land, condense their vapor upon the colder hills and mountain sides, and yield rain, so that high lands near the sea or other large bodies of water, from which the winds generally blow, have a greater proportion of rainy days and a greater fall of rain than lands more remote from water. The annual rain-fall in the lake districts in Cumberland County, in England, sometimes amounts to more than 150 inches.
With a desire to contribute as much as possible to the stock of accurate knowledge on this subject, we availed ourselves of the kindly offered services of our friends, Shedd and Edson, in preparing a carefully considered article upon a part of our general subject, which has much engaged their attention. Neither the article itself, nor the observations of Dr. Hobbs, which form a part of its basis, has ever before been published, and we believe our pages cannot be better occupied than by giving them in the language of our friends:
"All vegetables, in the various stages of growth, require warmth, air, and moisture, to support life and health.
Below the surface of the ground there is a body of stagnant water, sometimes at a great depth, but in retentive soils usually within a foot or two of the surface. This stagnant water not only excludes the air, but renders the soil much colder, and, being in itself of no benefit, without warmth and air, its removal to a greater depth is very desirable.
A knowledge of the depth to which this water-table should be removed, and of the means of removing it, constitutes the science of draining, and in its discussion, a knowledge of the rain-fall, humidity of the atmosphere, and amount of evaporation, is very important.
The amount of rain-fall, as shown by the hyetal, or rain-chart, of North America, by Lorin Blodget, is thirty inches vertical depth in the basin of the great lakes; thirty-two inches on Lake Erie and Lake Champlain; thirty-six inches in the valley of the Hudson, on the head waters of the Ohio, through the middle portions of Pennsylvania and Virginia, and western portion of North Carolina; forty inches in the extreme eastern and the northern portion of Maine, northern portions of New Hampshire and Vermont, south-eastern counties of Massachusetts, Central New York, north-east portion of Pennsylvania, south-east portion of New Jersey and Delaware; also, on a narrow belt running down from the western portion of Maryland, through Virginia and North Carolina, to the north-western portion of South Carolina; thence, up through the western portion of Virginia, north-east portion of Ohio, Northern Indiana and Illinois, to Prairie du Chien; forty-two inches on the east coast of Maine, Eastern Massachusetts, Rhode Island, and Connecticut, and middle portion of Maryland; thence, on a narrow belt to South Carolina; thence, up through Eastern Tennessee, through Central Ohio, Indiana, and Illinois, to Iowa; thence, down through Western Missouri and Texas to the Gulf of Mexico; forty-five inches from Concord, New Hampshire, through Worcester, Mass., Western Connecticut, and the City of New York, to the Susquehanna River, just north of Maryland; also, at Richmond, Va., Raleigh, N. C., Augusta, Geo., Knoxville, Tenn., Indianopolis, Ind., Springfield, Ill., St. Louis, Mo.; thence, through Western Arkansas, across Red River to the Gulf of Mexico. From the belt just described, the rain-fall increases inland and southward, until at Mobile, Ala., the rain-fall is sixty-three inches. The same amount also falls in the extreme southern portion of Florida.
In England, the average rain-fall in the eastern portion is represented at twenty inches; in the middle portion, twenty-two inches; in the southern and western, thirty inches; in the extreme south-western, forty-five inches; and in Wales, fifty inches. In the eastern portion of Ireland, it is twenty-five inches; and in the western, forty inches.
Observations at London for forty years, by Dalton, gave average fall of 20.69 inches. Observations at New Bedford, Mass., for forty-three years, by S. Rodman, gave average fall of 41.03 inches—about double the amount in London. The mean quantity for each month, at both places, is as follows:
| New Bedford. | London. | |
|---|---|---|
| January | 3.36 | 1.46 |
| February | 3.32 | 1.25 |
| March | 3.44 | 1.17 |
| April | 3.60 | 1.28 |
| May | 3.63 | 1.64 |
| June | 2.71 | 1.74 |
| July | 2.86 | 2.45 |
| August | 3.61 | 1.81 |
| September | 3.33 | 1.84 |
| October | 3.46 | 2.09 |
| November | 3.97 | 2.22 |
| December | 3.74 | 1.74 |
| Spring | 10.67 | 4.09 |
| Summer | 9.18 | 6.00 |
| Autumn | 10.76 | 6.15 |
| Winter | 10.42 | 4.45 |
| Year | 41.03 | 20.69 |
Another very striking difference between the two countries is shown by a comparison of the quantity of water falling in single days. The following table, given in the Radcliffe Observatory Reports, Oxford, England, 15th volume, shows the proportion of very light rains there. The observation was in the year 1854. Rain fell on 156 days:
| 73 | days gave | less than | .05 | inch. | ||
| 30 | " | between | that | and | .10 | " |
| 27 | " | between | .10 | " | .20 | " |
| 9 | " | " | .20 | " | .30 | " |
| 9 | " | " | .30 | " | .40 | " |
| 4 | " | " | .40 | " | .50 | " |
| 1 | gave | .60 | " | |||
| 2 | " | .80 | " | |||
| 1 | " | 1.00 | " |
Nearly half the number gave less fall than five-hundredths of an inch, and more than four-fifths the number gave less than one-fifth of an inch, and none gave over an inch.
There is more rain in the United States, by a large measure, than there; but the amount falls in less time, and the average of saturation is certainly much less here. From manuscript records, furnished us by Dr. Hobbs, of Waltham, Mass., we find, that the quantity falling in the year 1854, was equal to the average quantity for thirty years, and that rain fell on fifty-four days, in the proportion as follows:
Number of rainy days, 54; total rain-fall, 41.29.
| 0 | days gave | less than | .05 | inch. | ||
| 2 | " | between | that | and | .10 | " |
| 8 | " | between | .10 | " | .20 | " |
| 7 | " | " | .20 | " | .30 | " |
| 5 | " | " | .30 | " | .40 | " |
| 4 | " | " | .40 | " | .50 | " |
| 2 | " | " | .50 | " | .60 | " |
| 4 | " | " | .60 | " | .70 | " |
| 4 | " | " | .70 | " | .80 | " |
| 3 | " | " | .80 | " | .90 | " |
| 0 | " | " | .90 | " | 1.00 | " |
| 0 | " | " | 1.00 | " | 1.10 | " |
| 2 | " | " | 1.10 | " | 1.20 | " |
| 1 | " | " | 1.20 | " | 1.30 | " |
| 1 | " | " | 1.30 | " | 1.40 | " |
| 3 | " | " | 1.40 | " | 1.50 | " |
| 2 | " | " | 1.50 | " | 1.60 | " |
| 1 | " | " | 1.60 | " | 1.70 | " |
| 2 | " | " | 1.80 | " | 1.90 | " |
| 1 | " | " | 2.30 | " | 2.40 | " |
| 1 | " | " | 2.50 | " | 2.60 | " |
| 1 | " | " | 3.20 | " | 3.30 | " |
No rain-fall gave less than five-hundredths of an inch; and more than one-fourth the number of days gave more than one inch. In 1850, four years earlier, the rain-fall for the year, in Waltham, was 62.13 inches, the greatest recorded by observations kept since 1824. It fell as shown in the table:
Number of rainy days, 58; total rain-fall, 62.13.
| 3 | days gave between | .05 | and | .10 | inches. |
| 4 | " | .10 | " | .20 | " |
| 6 | " | .20 | " | .30 | " |
| 3 | " | .30 | " | .40 | " |
| 5 | " | .40 | " | .50 | " |
| 3 | " | .50 | " | .60 | " |
| 3 | " | .60 | " | .70 | " |
| 3 | " | .70 | " | .80 | " |
| 2 | " | .80 | " | .90 | " |
| 1 | " | .90 | " | 1.00 | " |
| 3 | " | 1.00 | " | 1.10 | " |
| 7 | " | 1.20 | " | 1.30 | " |
| 2 | " | 1.80 | " | 1.90 | " |
| 2 | " | 1.90 | " | 2.00 | " |
| 3 | " | 2.00 | " | 2.10 | " |
| 2 | " | 2.10 | " | 2.20 | " |
| 1 | " | 2.30 | " | 2.40 | " |
| 1 | " | 2.50 | " | 2.60 | " |
| 1 | " | 2.60 | " | 2.70 | " |
| 1 | " | 2.80 | " | 2.90 | " |
| 1 | " | 3.60 | " | 3.70 | " |
| 1 | " | 4.50 | " | 4.60 | " |
Sept. 7th and 8th, in 24 hours, 6.88 inches of rain fell, the greatest quantity recorded in one day.
In 1846—still earlier by four years—the rain-fall in Waltham was 26.90 inches—the least recorded by the same observations. It fell, as shown in the table: Number of rainy days, 49; total rain-fall, 26.90.
| 3 | days gave between | .05 | and | .10 | inches. |
| 7 | " | .10 | " | .20 | " |
| 10 | " | .20 | " | .30 | " |
| 6 | " | .30 | " | .40 | " |
| 4 | " | .40 | " | .50 | " |
| 3 | " | .50 | " | .60 | " |
| 2 | " | .70 | " | .80 | " |
| 3 | " | .80 | " | .90 | " |
| 1 | " | .90 | " | 1.00 | " |
| 3 | " | 1.00 | " | 1.10 | " |
| 2 | " | 1.10 | " | 1.20 | " |
| 1 | " | 1.20 | " | 1.30 | " |
| 2 | " | 1.40 | " | 1.50 | " |
| 1 | " | 1.50 | " | 1.60 | " |
| 1 | " | 2.40 | " | 2.50 | " |
The rain-fall in 1852 was very near the average for thirty years; and the quantity falling in single storms, on sixty-three different occasions, as registered by Dr. Hobbs, was as follows: Number of storms, 63; total rain-fall, 42.24.
| 7 | storms gave | less than | .10 | inches. | ||||
| 11 | " | between | .10 | and | .20 | " | ||
| 9 | " | " | .20 | " | .30 | " | ||
| 5 | " | " | .30 | " | .40 | " | ||
| 6 | " | " | .40 | " | .50 | " | ||
| 5 | " | " | .50 | " | .60 | " | ||
| 1 | " | " | .60 | " | .70 | " | ||
| 1 | " | " | .70 | " | .80 | " | ||
| 3 | " | " | .80 | " | .90 | " | ||
| 1 | " | " | .90 | " | 1.00 | " | ||
| 5 | " | " | 1.00 | " | 1.10 | " | ||
| 1 | " | " | 1.10 | " | 1.20 | " | ||
| 1 | " | " | 1.20 | " | 1.30 | " | ||
| 1 | " | " | 1.40 | " | 1.50 | " | ||
| 3 | " | " | 1.60 | " | 1.70 | " | ||
| 1 | " | in | 5 | days | 3.16 | " | ||
| 1 | " | " | 4 | " | 4.38 | " | ||
| 1 | " | " | 6 | " | 5.35 | " |
These tables are sufficient to show that provision must be made to carry off much greater quantities of water from lands in this country than in England. We add a table of the greatest fall of rain in any one day, for each month, and for the year, from April, 1824, to 1st January, 1859. It also was abstracted from the manuscript of observations by Dr. Hobbs, and will be, we think, quite useful:
| Years | January | February | March | April | May | June | July | August | September | October | November | December | Greatest Fall in the Year |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1824 | 0.76 | 0.67 | 0.53 | 0.44 | 1.90 | 2.54 | 0.81 | 0.76 | 1.80 | 2.54 | |||
| 1825 | 2.16 | 2.61 | 0.27 | 1.23 | 1.37 | 0.91 | 2.51 | 0.89 | 1.32 | 0.71 | 2.40 | 2.61 | |
| 1826 | 1.80 | 0.56 | 1.67 | 0.89 | 0.39 | 1.78 | 0.87 | 1.80 | 1.57 | 1.37 | 1.22 | 1.41 | 1.87 |
| 1827 | 3.81 | 1.55 | 2.42 | 0.66 | 1.36 | 3.16 | 4.93 | 2.22 | 3.85 | 1.39 | 4.93 | ||
| 1828 | 0.60 | 1.48 | 1.82 | 2.06 | 2.01 | 1.44 | 1.52 | 0.14 | 1.82 | 1.52 | 1.90 | 0.29 | 2.06 |
| 1829 | 3.86 | 1.98 | 4.12 | 2.35 | 1.15 | 0.97 | 1.92 | 0.97 | 1.39 | 1.00 | 1.25 | 1.58 | 4.12 |
| 1830 | 1.31 | 1.17 | 2.68 | 2.28 | 0.78 | 1.84 | 2.45 | 2.40 | 1.20 | 2.64 | 2.44 | 2.68 | |
| 1831 | 0.64 | 1.48 | 2.32 | 2.12 | 1.79 | 1.87 | 2.27 | 1.00 | 1.00 | 2.82 | 1.24 | 0.15 | 2.82 |
| 1832 | 2.68 | 1.59 | 2.00 | 4.48 | 2.52 | 1.24 | 2.13 | 0.80 | 1.50 | 2.60 | 1.34 | 4.48 | |
| 1833 | 0.83 | 2.57 | 0.98 | 2.03 | 1.42 | 0.64 | 2.75 | 2.32 | 3.12 | 1.27 | 3.12 | ||
| 1834 | 0.64 | 1.31 | 0.94 | 2.35 | 1.87 | 2.12 | 0.73 | 1.25 | 1.89 | 2.42 | 0.92 | 2.42 | |
| 1835 | 1.44 | 0.88 | 2.48 | 2.48 | 1.18 | 1.52 | 4.72 | 1.32 | 1.57 | 3.28 | 0.74 | 2.32 | 4.72 |
| 1836 | 2.72 | 3.04 | 2.26 | 1.86 | 1.29 | 2.24 | 1.04 | 0.72 | 0.36 | 2.04 | 1.50 | 1.68 | 3.04 |
| 1837 | 3.62 | 1.50 | 1.14 | 1.68 | 1.46 | 1.30 | 0.72 | 0.78 | 0.66 | 1.46 | 0.81 | 1.68 | 3.62 |
| 1838 | 1.64 | 0.75 | 0.76 | 1.32 | 1.40 | 1.67 | 0.82 | 1.40 | 3.84 | 1.10 | 2.46 | 1.00 | 3.84 |
| 1839 | 0.70 | 0.80 | 0.58 | 4.06 | 2.98 | 0.94 | 1.08 | 3.54 | 0.70 | 1.60 | 0.80 | 1.92 | 4.06 |
| 1840 | 1.68 | 2.20 | 1.54 | 2.12 | 1.16 | 1.08 | 1.40 | 2.72 | 1.28 | 1.04 | 3.72 | 1.12 | 3.72 |
| 1841 | 1.44 | 1.12 | 1.32 | 1.64 | 0.90 | 0.75 | 0.64 | 2.82 | 2.78 | 2.66 | 1.05 | 1.70 | 2.82 |
| 1842 | 0.54 | 1.22 | 1.16 | 0.64 | 0.47 | 2.10 | 0.68 | 1.44 | 0.96 | 0.34 | 1.10 | 2.02 | 2.10 |
| 1843 | 1.60 | 1.64 | 2.50 | 1.34 | 0.34 | 1.04 | 1.98 | 2.58 | 0.52 | 1.94 | 1.28 | 2.58 | |
| 1844 | 4.14 | 2.06 | 0.24 | 0.58 | 0.78 | 0.86 | 1.34 | 1.76 | 2.30 | 1.86 | 1.28 | 4.14 | |
| 1845 | 2.42 | 1.70 | 1.14 | 0.70 | 1.02 | 1.03 | 1.20 | 1.66 | 0.88 | 1.16 | 3.32 | 1.46 | 3.32 |
| 1846 | 1.54 | 2.46 | 1.16 | 1.18 | 0.82 | 1.46 | 0.49 | 0.56 | 0.55 | 0.54 | 1.02 | 2.46 | |
| 1847 | 1.18 | 2.74 | 1.66 | 1.12 | 0.84 | 1.28 | 0.56 | 1.86 | 2.16 | 0.64 | 2.74 | 3.02 | 3.02 |
| 1848 | 1.44 | 1.56 | 2.68 | 0.68 | 2.28 | 1.00 | 0.72 | 1.24 | 1.48 | 2.96 | 0.88 | 1.00 | 2.96 |
| 1849 | 1.36 | 0.40 | 2.30 | 0.92 | 1.28 | 0.72 | 1.52 | 2.08 | 1.12 | 2.60 | 2.48 | 1.76 | 2.60 |
| 1850 | 2.56 | 1.92 | 1.84 | 2.68 | 2.80 | 1.20 | 1.20 | 3.68 | 6.88 | 1.04 | 2.16 | 1.92 | 6.88 |
| 1851 | 0.80 | 1.84 | 0.56 | 3.60 | 1.92 | 1.12 | 0.96 | 0.32 | 1.15 | 1.47 | 2.25 | 0.89 | 3.60 |
| 1852 | 1.06 | 0.88 | 1.15 | 4.38 | 1.47 | 1.69 | 0.66 | 4.16 | 1.19 | 1.61 | 1.59 | 0.89 | 4.38 |
| 1853 | 0.92 | 1.33 | 1.03 | 1.12 | 2.39 | 0.42 | 1.03 | 2.36 | 2.14 | 1.95 | 1.67 | 1.35 | 2.39 |
| 1854 | 0.83 | 1.60 | 1.25 | 1.88 | 2.57 | 1.50 | 1.58 | 0.48 | 2.33 | 1.82 | 3.25 | 1.43 | 3.25 |
| 1855 | 3.37 | 3.08 | 0.80 | 1.33 | 0.39 | 1.23 | 1.93 | 0.75 | 0.70 | 1.77 | 2.22 | 1.24 | 3.37 |
| 1856 | 1.30 | 0.63 | 1.97 | 2.93 | 0.66 | 1.30 | 4.23 | 2.42 | 0.87 | 0.88 | 1.20 | 4.23 | |
| 1857 | 1.50 | 0.54 | 1.55 | 3.68 | 1.28 | 0.96 | 2.43 | 2.00 | 0.87 | 3.54 | 0.67 | 1.28 | 3.68 |
| 1858 | 1.12 | 1.18 | 0.35 | 1.28 | 1.00 | 3.86 | 1.35 | 2.21 | 1.64 | 1.22 | 1.36 | 1.40 | 3.86 |
The following table shows the record of rain-fall, as kept for one year; it was selected as a representative year, the total quantity falling being equal to the average. For the year 1840: Number of rainy days, 50; total rain-fall, 42.00.
| Days | January 1840 | February | March | April | May | June | July | August | September | October | November | December |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 0.55 | 0.14 | 2.72 | 0.64 | ||||||||
| 2 | 0.08 | 0.05 | ||||||||||
| 3 | 0.32 | |||||||||||
| 4 | 1.08 | 0.10 | ||||||||||
| 5 | 1.16 | 0.63 | ||||||||||
| 6 | 0.50 | |||||||||||
| 7 | ||||||||||||
| 8 | 0.20 | |||||||||||
| 9 | 0.25 | 3.72 | ||||||||||
| 10 | 2.20 | 1.28 | ||||||||||
| 11 | 0.10 | |||||||||||
| 12 | 2.12 | 0.54 | ||||||||||
| 13 | 0.14 | 1.12 | ||||||||||
| 14 | 0.58 | 0.70 | ||||||||||
| 15 | 0.36 | |||||||||||
| 16 | ||||||||||||
| 17 | ||||||||||||
| 18 | ||||||||||||
| 19 | 0.82 | 0.24 | 0.68 | 1.04 | ||||||||
| 20 | 1.54 | 0.44 | ||||||||||
| 21 | 0.98 | 1.04 | ||||||||||
| 22 | 0.52 | 2.20 | ||||||||||
| 23 | 1.68 | 0.96 | 0.18 | |||||||||
| 24 | 1.40 | |||||||||||
| 25 | 0.16 | 0.35 | ||||||||||
| 26 | 0.18 | |||||||||||
| 27 | 0.17 | 0.30 | ||||||||||
| 28 | ||||||||||||
| 29 | 1.80 | 0.10 | 1.40 | |||||||||
| 30 | 1.42 | 0.08 | 1.04 | |||||||||
| 31 | ||||||||||||
| Total | 1.68 | 2.78 | 3.28 | 5.17 | 2.28 | 2.41 | 2.09 | 5.22 | 2.89 | 3.65 | 7.35 | 3.20 |
The average quantity of rain which has fallen in Waltham, during the important months of vegetation, from 1824 to 1858 inclusive—a period of thirty-five years—is for—
| April. | May. | June. | July. | Aug. | Sept. |
|---|---|---|---|---|---|
| 3.96 | 3.71 | 3.18 | 3.38 | 4.50 | 3.52 |
| Average for the six months, 22.25. |
It will be noticed, that the average for the month of August is about 33 per cent. larger than for June and July. The quantity of rain falling in each month, as registered at the Cambridge Observatory, is as follows:
| MEAN OF OBSERVATIONS FOR TWELVE YEARS. | |||||||||||
| Jan. | Feb. | Mar. | Apr. | May. | June. | July. | Aug. | Sept. | Oct. | Nov. | Dec. |
| 2.39 | 3.19 | 3.47 | 3.64 | 3.74 | 3.13 | 2.57 | 5.47 | 4.27 | 3.73 | 4.57 | 4.31 |
| Spring. | Summer. | Autumn. | Winter. | ||||||||
| 10.85 | 11.17 | 12.57 | 9.89 | ||||||||
| Average quantity per year, 44.48. |
The quantity falling from January to July, is much less than falls from July to January.
The great quantity of snow which falls in New England during the Winter months, and is carried off mainly in the Spring, usually floods the low lands, and should be taken into account in establishing the size of pipe to be used in a system of drainage. The following observations of the average depth of snow, have been made at the places cited, and are copied, by Blodget, from various published notices:
| Oxford Co., Me. | 12 | years | 90 | inches | per year. |
| Dover, N. H. | 10 | " | 68.6 | " | " |
| Montreal | 10 | " | 67 | " | " |
| Burlington, Vt. | 10 | " | 85 | " | " |
| Worcester, Mass. | 12 | " | 55 | " | " |
| Amherst, " | 7 | " | 54 | " | " |
| Hartford, Conn. | 24 | " | 43 | " | " |
| Lambertville, N. J. | 8 | " | 25.5 | " | " |
| Cincinnati | 16 | " | 19 | " | " |
| Burlington, Iowa | 4 | " | 15.5 | " | " |
| Beloit, Wisconsin | 3 | " | 25 | " | " |
One-tenth the depth of snow is taken as its equivalent in water, for general purposes, though it gives too small a quantity of water in southern latitudes, and in extreme latitudes too great a quantity. The rule of reduction of snow to water, in cold climates, is one inch of water to twelve of snow.
The proportion of the annual downfall of rain which is collectable into reservoirs—or, in other words, the per-centage of the rain-fall which drains off—is well shown in a table used by Ellwood Morris, Esq., C. E., in an article on "The Proposed Improvement of the Ohio River" (Jour. Frank. Inst., Jan., 1858), in which we find, that, in eighteen series of observations in Great Britain, the ratio, or per cent. of the rain-fall which drains off is 65½, or nearly two-thirds the rain-fall.
Seven series of observations in America are cited as follows:
| No. | Name of Drainage Area. | Annual rain-fall, in inches. | Drainage flowing away, in inches. | Ratio, or per ct. of the rain which drains off. | Authorities. |
|---|---|---|---|---|---|
| 1 | Schuylkill Navigation Reservoirs | 36 | 18 | 50 | Morris and Smith. |
| 2 | Eaton Brook | 34 | 23 | 66 | McAlpine. |
| 3 | Madison Brook | 35 | 18 | 50 | McAlpine. |
| 4 | Patroon's Brook | 46 | 25 | 55 | McAlpine. |
| 5 | Patroon's Brook | 42 | 18 | 42 | McAlpine. |
| 6 | Long Pond | 40 | 18 | 44 | Boston Water Com'rs. |
| 7 | West Fork Reservoir | 36 | 14 | 40 | W. Milnor Roberts. |
| Totals | 269 | 134 | 347 | ||
| Averages | 38 | 19 | 50 |
These examples show an average rain-fall of thirty-eight vertical inches, and an annual amount, collectable in reservoirs, of nineteen inches, or fifty per cent.
The per-centage of water of drainage from land under-drained with tile, would be greater than that which is collectable in reservoirs from ordinary gathering-grounds.
If a soil were perfectly saturated with water, that is, held as much water in suspension as possible to hold without draining off, and drains were laid at a proper depth from the surface, and in sufficient number to take off all surplus water, then the entire rain-fall upon the surface would be water of drainage—presuming, of course, the land to be level, and the air at saturation, so as to prevent evaporation. The water coming upon the surface, would force out an equal quantity of water at the bottom, through the drains—the time occupied by the process, varying according to the porous or retentive nature of the soil; but in ordinary circumstances, it would be, perhaps, about forty-eight hours. Drains usually run much longer than this after a heavy rain, and, in fact, many run constantly through the year, but they are supplied from lands at a higher level, either near by or at a distance.
If, on the other hand, the soil were perfectly dry, holding no water in suspension, then there would be no water of drainage until the soil had become saturated.
Evaporation is constantly carrying off great quantities of water during the warm months, so that under-drained soil is seldom in the condition of saturation, and, on account of the supply by capillary attraction and by dew, is never thoroughly dry; but the same soil will, at different times, be at various points between saturation and dryness, and the water of drainage will be consequently a greater or less per centage of the rain-fall.
An experiment made by the writer, to ascertain what quantity of water a dry soil would hold in suspension, resulted as follows: A soil was selected of about average porosity, so that the result might be, as nearly as possible, a mean for the various kinds of soil, and dried by several days' baking. The quantity of soil then being carefully measured, a measured quantity of water was supplied slowly, until it began to escape at the bottom. The quantity draining away was measured and deducted from the total quantity supplied. It was thus ascertained that one cubic foot of earth held 0.4826+ cubic feet of water, which is a little more than three and one-half gallons. A dry soil, four feet deep, would hold a body of water equal to a rain-fall of 23.17 inches, vertical depth, which is more than would fall in six months.
The quantity which is not drained away is used for vegetation or evaporated; and the fact, that the water of drainage is so much greater in proportion to the rain-fall in England than in this country, is owing to the humidity of that climate, in which the evaporation is only about half what it is in this country.
The evaporation from a reservoir surface at Baltimore, during the Summer months, was assumed by Colonel Abert to be to the quantity of rain as two to one.
Dr. Holyoke assigns the annual quantity evaporated at Salem, Mass., at fifty-six inches; and Colonel Abert quotes several authorities at Cambridge, Mass., stating the quantity at fifty-six inches. These facts are given by Mr. Blodget, and also the table below.
| QUANTITY OF WATER EVAPORATED, IN INCHES, VERTICAL DEPTH. | |||||||||||||
| Jan. | Feb. | Mar. | Apr. | May. | June. | July. | Aug. | Sept. | Oct. | Nov. | Dec. | Year. | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Whitehaven, England, mean of 6 years | 0.88 | 1.04 | 1.77 | 2.54 | 4.15 | 4.54 | 4.20 | 3.40 | 3.12 | 1.93 | 1.32 | 1.09 | 30.03 |
| Ogdensburg, N. Y., 1 yr. | 1.65 | 0.82 | 2.07 | 1.63 | 7.10 | 6.74 | 7.79 | 5.41 | 7.40 | 3.95 | 3.66 | 1.15 | 49.37 |
| Syracuse, N. Y., 1 year | 0.67 | 1.48 | 2.24 | 3.42 | 7.31 | 7.60 | 9.08 | 6.85 | 5.33 | 3.02 | 1.33 | 1.86 | 50.20 |
The quantity for Whitehaven, England, is reported by J. F. Miller. It was very carefully observed, from 1843 to 1848—the evaporation being from a copper vessel, protected from rain. The district is one of the wettest of England—the mean quantity of rain, for the same time, having been 45.25 inches.
This shows a great difference in the capacity of the air to absorb moisture in England and the United States; and as evaporation is a cooling process, there is greater necessity for under-draining in this country than in England, supposing circumstances in other respects to be similar.
Evaporation takes place at any point of temperature from 32°, or lower, to 212°—at which water boils. It is increased by heat, but is not caused solely by it—for a north-west wind in New-England evaporates water, and dries the earth more rapidly than the heat alone of a Summer's day; and when, under ordinary circumstances, evaporation from a water-surface is slow, it becomes quite active when brought in close proximity to sulphuric acid, or other vapor-absorbing bodies.
The cold which follows evaporation is caused by a loss of the heat which is required for evaporation, and which passes off with the vapor, as a solution, in the atmosphere; and as heat leaves the body to aid evaporation, it is evident that that body cannot be cooled by the process, below the dew-point at which evaporation ceases. The popular notion that a body may be cooled almost to the freezing-point, in a hot Summer day, by the action of heat alone, is, then, erroneous. But still, the amount of heat which is used up in evaporating stagnant water from undrained land, that might otherwise go towards warming the land and the roots of crops, is a very serious loss.
The difference in the temperature of a body, resulting from evaporation, may reach 25° in the desert interior of the American continent; but, in the Eastern States, it is not often more than 15°.
The temperature of evaporation is the reading of a wet-bulb-thermometer (the bulb being covered with moistened gauze) exposed to the natural evaporation; and the difference between that reading and the reading of a dry-thermometer, is the expression of the cold resulting from evaporation.
When the air is nearly saturated, the temperature of the air rarely goes above 74°; but, if so, the moisture in the air prevents the passing away of insensible perspiration, and the joint action of heat and humidity exhausts the vital powers, causing sun-stroke, as it is called. At New York city, August 12th to 14th, 1853, the wet-thermometer stood at 80° to 84°; the air, at 90° to 94°. The mortality, from this joint effect, was very great—over two hundred persons losing their lives in the two days, in that city.
From very careful observations, made by Lorin Blodget, in 1853, at Washington, it was found that the difference between the wet and dry thermometer was 18½° at 4 P. M., June 30th, and 16° at 2 P. M. on July 1st—the temperature of the air being 98° on the first day, and 95° on the second; but such excesses are unusual.
The following table has been compiled from Mr. Blodget's notice of the peculiarities of the Summer of 1853:
The dates are such as were selected to illustrate the extreme temperatures of the month, and the degrees represent the differences between the wet and dry thermometer. The observations were made at 3 P. M.:
| Locality. | Dates. | Differences. | |||||
|---|---|---|---|---|---|---|---|
| June, 1853. | |||||||
| Burlington, Vt. | 14th | to | 30th | ranged from | 8° | to | 17° |
| Montreal | 14th | to | 30th | " | 6 | to | 17 |
| Poultney, Iowa | 10th | to | 30th | " | 9 | to | 16 |
| Washington | 20th | to | 30th | " | 8.5 | to | 16 |
| Baltimore | 13th | to | 30th | " | 7.4 | to | 20.2 |
| Savannah | 13th | to | 30th | " | 5.2 | to | 17.3 |
| Austin, Texas | 10th | to | 30th | " | 4 | to | 24 |
| Clarkesville, Tenn. | 4th | to | 30th | " | 10.3 | to | 20.5 |
| August. | |||||||
| Bloomfield, N. J. | 9th | to | 14th | " | 5 | to | 15 |
| Austin, Texas | 6th | to | 12th | " | 0 | to | 19 |
| Philadelphia | 10th | to | 15th | " | 8 | to | 14 |
| Jacksonville, Fla. | 10th | to | 15th | " | 6 | to | 8 |
Observations by Lieut. Gillis, at Washington, give mean differences between wet and dry thermometers, from March, 1841, to June, 1842, as follows:
Observations at 3 P. M.:
| Jan. | Feb. | Mar. | Apr. | May. | June. | July. | Aug. | Sept. | Oct. | Nov. | Dec. |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 3°.08 | 4°.40 | 6°.47 | 5°.37 | 7°.05 | 8°.03 | 8°.89 | 5°.29 | 5°.63 | 4°.61 | 4°.77 | 2°.03 |
