Page:The National geographic magazine (IA nationalgeograph21890nati).pdf/27

 A foundation was made, of timbers about six inches square, mortised at the ends which could be placed in position and leveled before the observatory was set up, rendering this operation much easier and giving greater stability. A floor was laid upon joists supported by this foundation. Shelves were put up at various points, affording resting places for tools and small instruments, while a table in one corner, supported the chronometer, and offered a convenient place for an assistant to record observations, etc.

The principal instrument used was the transit. Those furnished for the use of the expedition were designed by Mr. J. A. Rogers, and constructed under his supervision in the repair shop of the Hydrographic office. The object glasses, made by the Clarks at Cambridge, were of 2½ inches clear aperature with a focal length of thirty inches. The instruments were of the prismatic or "broken" form in which the eye piece is at one end of the axis, and the light is reflected from the object glass to the eye by a prism placed at the junction of the telescope tube with the axis. The observer does, not have to change the position of his eye, no matter what the zenith distance of the star may be. This renders observation much less fatiguing and conduces to accuracy. The eye-piece was furnished with the usual spider line reticle and also with a filar micrometer for the measurement of zenith distances for latitude. A vertical finding circle was on the eye-piece end of the axis, and the instrument was provided also with a horizontal circle, fourteen inches in diameter, graduated to ten seconds. Other necessary parts were the striding and zenith telescope levels, and the illuminating lamps. The ends of the axis were supported by Ys at the ends of a transverse arm which in its centre was screwed to the top of a vertical axis supported in a socket surmounting the tripod. This vertical axis was slightly conical in shape and accurately fitted into its socket. A screw was so placed underneath, that the axis, and with it the instrument, could be raised slightly, when it was easily revolved horizontally into any desired position, a reverse movement of the screw then lowered the axis into its seat, when the instrument was held firmly by the friction. For supporting the instrument there was used at first, a portable pier made in the shape of the frustrum of a cone, of strong oak staves, firmly bound with iron hoops, and when set up, filled with sand or earth. Subsequently a brick pier was found to be more stable and the wooden ones were discarded.