Читать книгу The Works of Benjamin Franklin, Volume 5 - Бенджамин Франклин - Страница 5

London, 2 July, 1768.

Dear Sir:—

You must needs think the time long that your instruments have been in hand. Sundry circumstances have occasioned the delay. Mr. Short, who undertook to make the telescope, was long in a bad state of health, and much in the country for the benefit of the air. He however at length finished the material parts that required his own hand, and waited only for something about the mounting that was to have been done by another workman, when he was removed by death. I have put in my claim to the instrument, and shall obtain it from the executors as soon as his affairs can be settled. It is now become much more valuable than it would have been if he had lived, as he excelled all others in that branch. The price agreed for was one hundred pounds.

The equal altitudes and transit instrument was undertaken by Mr. Bird, who doing all his work with his own hands for the sake of greater truth and exactness, one must have patience that expects any thing from him. He is so singularly eminent in his way, that the commissioners of longitude have lately given him five hundred pounds merely to discover and make public his method of dividing instruments. I send it you herewith. But what has made him longer in producing your instrument is the great and hasty demand on him from France and Russia, and our Society here, for instruments to go to different parts of the world for observing the next transit of Venus; some to be used in Siberia, some for the observers that go to the South Seas, some for those that go to Hudson’s Bay. These are now all completed and mostly gone, it being necessary, on account of the distance, that they should go this year to be ready on the spot in time. And now he tells me he can finish yours, and that I shall have it next week. Possibly he may keep his word. But we are not to wonder if he does not.

Mr. Martin, when I called to see his panopticon, had not one ready; but was to let me know when he should have one to show me. I have not since heard from him, but will call again.

Mr. Maskelyne wishes much that some of the governments in North America would send an astronomer to Lake Superior to observe this transit. I know no one of them likely to have a spirit for such an undertaking, unless it be the Massachusetts, or that have a person and instruments suitable. He presents you one of his pamphlets, which I now send you, together with two letters from him to me relating to that observation. If your health and strength were sufficient for such an expedition, I should be glad to hear you had taken it. Possibly you may have an élève that is capable. The fitting you out to observe the former transit, was a public act for the benefit of science, that did your province great honor.

We expect soon a new volume of the Transactions, in which your piece will be printed. I have not yet got the separate ones which I ordered.

It is perhaps not so extraordinary that unlearned men, such as commonly compose our church vestries, should not yet be acquainted with, and sensible of the benefits of metal conductors in averting the stroke of lightning, and preserving our houses from its violent effects, or that they should be still prejudiced against the use of such conductors, when we see how long even philosophers, men of extensive science and great ingenuity, can hold out against the evidence of new knowledge that does not square with their preconceptions ^{Ref. 003}; and how long men can retain a practice that is conformable to their prejudices, and expect a benefit from such practice though constant experience shows its inutility. A late piece of the Abbé Nollet, printed last year in the Memoirs of the French Academy of Sciences, affords strong instances of this; for, though the very relations he gives of the effects of lightning in several churches and other buildings show clearly that it was conducted from one part to another by wires, gildings, and other pieces of metal that were within or connected with the building, yet in the same paper he objects to the providing metalline conductors without the building, as useless or dangerous. ^{Ref. 004} He cautions people not to ring the church bells during a thunder-storm, lest the lightning, in its way to the earth, should be conducted down to them by the bell-ropes, ^{Ref. 005} which are but bad conductors, and yet is against fixing metal rods on the outside of the steeple, which are known to be much better conductors, and which it would certainly choose to pass in, rather than in dry hemp. And, though for a thousand years past bells have been solemnly consecrated by the Romish Church, ^{Ref. 006} in expectation that the sound of such blessed bells would drive away the storms, and secure our buildings from the stroke of lightning; and during so long a period, it has not been found by experience that places within the reach of such blessed sound are safer than others where it is never heard; but that, on the contrary, the lightning seems to strike steeples of choice, and that at the very time the bells are ringing ^{Ref. 007}; yet still they continue to bless the new bells, and jangle the old ones whenever it thunders. One would think it was now time to try some other trick; and ours is recommended (whatever this able philosopher may have been told to the contrary) by more than twelve years’ experience, wherein, among the great number of houses furnished with iron rods in North America, not one so guarded has been materially hurt with lightning, and several have evidently been preserved by their means; while a number of houses, churches, barns, ships, &c., in different places, unprovided with rods, have been struck and greatly damaged, demolished, or burnt. Probably the vestries of our English churches are not generally well acquainted with these facts; otherwise, since as good Protestants they have no faith in the blessing of bells, they would be less excusable in not providing this other security for their respective churches, and for the good people that may happen to be assembled in them during a tempest, especially as those buildings from their greater height, are more exposed to the stroke of lightning than our common dwellings.

I have nothing new in the philosophical way to communicate to you, except what follows. When I was last year in Germany, I met with a singular kind of glass, being a tube about eight inches long, half an inch in diameter, with a hollow ball of near an inch diameter at one end, and one of an inch and a half at the other, hermetically sealed, and half filled with water. If one end is held in the hand, and the other a little elevated above the level, a constant succession of large bubbles proceeds from the end in the hand to the other end, making an appearance that puzzled me much, till I found that the space not filled with water was also free from air, and either filled with a subtile, invisible vapor continually rising from the water, and extremely rarefiable by the least heat at one end, and condensable again by the least coolness at the other; or it is the very fluid of fire itself, which parting from the hand pervades the glass, and by its expansive force depresses the water till it can pass between it and the glass, and escape to the other end, where it gets through the glass again into the air. I am rather inclined to the first opinion, but doubtful between the two.

An ingenious artist here, Mr. Nairne, mathematical instrument-maker, has made a number of them from mine, and improved them; for his are much more sensible than those I brought from Germany. I bored a very small hole through the wainscot in the seat of my window, through which a little cold air constantly entered, while the air in the room was kept warmer by fires daily made in it, being winter time. I placed one of his glasses, with the elevated end against this hole; and the bubbles from the other end, which was in a warmer situation, were continually passing day and night, to the no small surprise of even philosophical spectators. Each bubble discharged is larger than that from which it proceeds, and yet that is not diminished; and by adding itself to the bubble at the other end, the bubble is not increased, which seems very paradoxical.

When the balls at each end are made large, and the connecting tube very small, and bent at right angles, so that the balls, instead of being at the ends, are brought on the side of the tube, and the tube is held so that the balls are above it, the water will be depressed in that which is held in the hand, and rise in the other as a jet or fountain; when it is all in the other, it begins to boil, as it were, by the vapor passing up through it; and the instant it begins to boil, a sudden coldness is felt in the ball held; a curious experiment this, first observed and shown by Mr. Nairne. There is something in it similar to the old observation, I think mentioned by Aristotle, that the bottom of a boiling pot is not warm; and perhaps it may help to explain the fact; if indeed it be a fact.

When the water stands at an equal height in both these balls, and all at rest, if you wet one of the balls by means of a feather dipped in spirit, though that spirit is of the same temperament as to heat and cold with the water in the glasses, yet the cold occasioned by the evaporation of the spirit from the wetted ball will so condense the vapor over the water contained in that ball, as that the water of the other ball will be pressed up into it, followed by a succession of bubbles, till the spirit is all dried away. Perhaps the observations on these little instruments may suggest and be applied to some beneficial uses. It has been thought, that water reduced to vapor by heat was rarefied only fourteen thousand times, and on this principle our engines for raising water by fire are said to be constructed; but, if the vapor so much rarefied from water is capable of being itself still farther rarefied to a boundless degree, by the application of heat to the vessels or parts of vessels containing the vapor (as at first it is applied to those containing the water), perhaps a much greater power may be obtained, with little additional expense. Possibly, too, the power of easily moving water from one end to the other of a movable beam (suspended in the middle like a scale-beam) by a small degree of heat, may be applied advantageously to some other mechanical purposes.

The magic square and circle, I am told, have occasioned a good deal of puzzling among the mathematicians here; but no one has desired me to show him my method of disposing the numbers. It seems they wish rather to investigate it themselves. When I have the pleasure of seeing you, I will communicate it.

With singular esteem and respect, I am, dear Sir,

Your most obedient humble servant,

B. Franklin.