Made for University of Pennsylvania by Warner & Swasey.]
As applied to geodesy, astronomy has added a process of ascertaining geographical latitude with marvelous accuracy and speed by the use of the zenith telescope, an instrument devised by Major Talcott in 1835. This instrument can be set in a vertical direction with ease, and be pointed alternately to two stars that cross the meridian at a brief interval of time, the one north and the other south of the zenith. Difficulties that arise from refraction are avoided, and the resulting latitude is quickly computed. This method is largely employed in the surveys of the public lands, as also in establishing the boundary between the United States and British America.
XIV. NOTABLE EPOCHS IN THE NINETEENTH CENTURY.
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.