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Worth marking as epochs of the nineteenth century were such dates as October 10, 1846, when the first determination of difference of longitude of two places was made by the use of the telegraph wire. Sears C. Walker, in Washington, and E. Otis Kendall, in Philadelphia, compared their clocks by interchanging telegraphic signals, and thus found their respective longitudes.

In 1850, Professor William C. Bond, of Harvard College, invented the chronograph. Through the urgency of Sir David Brewster, it was shown in the great exhibition of that year in London, where a medal was awarded for it. The chronograph was speedily adopted throughout Europe, and together with other apparatus made by Bond constituted what there became known as the “American method” of recording observations. Through it the errors for which the “personal equation” is a partial remedy are largely eliminated, and a superior definiteness of record is obtained.

On August 7, 1869, the first application of the spectroscope to the examination of the corona of the sun was the beginning of the revelation of the inner mysteries of the constitution and activities of the great luminary. The transit of Venus that occurred on December 6, 1882, was fruitful in measurements, by which the estimates of the distance of the sun were reduced from the long-accepted figures, 95 to 92 millions of miles. Yet this loss of three millions of miles resulted from the apparently trifling change of reckoning the sun’s parallax at 8.82″, instead of 8.57″. An occurrence of vast practical advantage to the whole nation was that of November 18, 1883, when the four standard meridians of railroad time were adopted and put into use. From that day the clocks of the Union were set to keep either Eastern, Central, Mountain, or Pacific Coast time.

Professor Edward E. Barnard had used the magnificent telescope of thirty-six inches aperture, belonging to the Lick Observatory in California, but a short time before he astonished the world by discovering a fifth satellite of Jupiter, although it appeared as but a faint speck of light. Besides other honors for this achievement, in 1894 the French Academy of Sciences awarded him the Arago medal, of the value of a thousand francs, a distinction given but twice before, first to Le Verrier, for the discovery of Neptune in 1846, and to Asaph Hall, for finding the two moons of Mars in 1877.

“Personal equation” is the name given to the amount of error to which any person is habitually liable in attempting to note the time of a fixed occurrence. When the astronomer looks at a star passing the cross-wires of his transit, he is likely to make the record one or two tenths of a second after the true time, or possibly a like small amount of time before the actual occurrence, by anticipation. This is not a matter of wrong intention, nor due to willfulness. But in precise observations, especially where comparisons are to be made between the records of several persons, the “personal equation” must be determined, if possible, and allowed for. Various methods of correcting this inaccuracy have been used. But the best is that of Frank H. Bigelow, of the Nautical Almanac Office, Washington, who, in 1890, devised a process of taking star transits by photography. It entirely does away with this source of error, and has proved of great value.