Page:Popular Science Monthly Volume 7.djvu/447

Rh Yale College. These instruments were so constructed that the eyepiece was at one end of the horizontal axis, a prism at the junction of the axis and telescope-tube reflecting at a right angle the rays from the object-glass, thereby enabling the observer to direct the instrument upon stars of any elevation above the horizon without change of position.

The command of the expedition was given to Lieutenant-Commander F. M. Green, U. S. N., and it was intended that the work should be commenced in the winter of 1873-'74; but the non-completion of the instruments and the probability of trouble resulting from the Cuban outrages interfered with this plan, and the Fortune was temporarily employed as a tender to the squadron at Key West.

Upon the dispersion of the assembled squadron in April, 1874, Lieutenant-Commander Green was directed to complete a survey of the Mexican Gulf coast, commenced by the United States steamship Wyoming. This work employed the time till the following August, when the Fortune returned to Washington, and was at once refitted for the prosecution of the original design.

Fortunately for the success of the work, the services of Mr. Miles Rock, formerly of the observatory at Cordova, were secured as principal astronomical assistant; and the Fortune sailed on the 24th of November, 1874, from Hampton Roads for Jamaica.

Upon arrival at Kingston, definite arrangements were made with the manager of the telegraph cables, the gratuitous use of which had been offered very promptly and courteously by the London board of directors.

As it had been decided to commence the work by measuring between Panama and Aspinwall, the Fortune sailed for the latter place on the 9th of December, arriving on the 12th.

Portable observatories had been constructed, to shelter the instruments and observers, and were immediately set up at Panama and Aspinwall upon obtaining permission from the local authorities.

Throughout the work the same general system was pursued, and was briefly as follows: As soon as practicable after the establishment of a party at each station, the work was commenced by observing stars on five clear nights, from 8 to 10, and from 11 to 1  for determining the errors of the chronometers, and during the hour from 10 till 11  exchanging time-signals between the two stations. This was effected as follows:

Telegraphic communication being established between the observatories, the senior observer sent a preparatory signal at ten seconds before the completion of a minute by tapping his key several times in quick succession; then exactly at the even minute, pressing his key again for about a quarter of a second, and repeating this signal at intervals of five seconds till the completion of the next even minute. The hour and minute when the first signal was sent were then