_ 
Tedegraphic Determinations of Longitude. i 
A foundation was made, of timbers about six inches square, mor- 
tised together 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 instru- 
ments, while a table in one corner, supported the chronometer, 
and offered a convenient place for an assistant to record observa- 
tions, ete. 
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 24 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 doesnot 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 instru- 
ment, could be raised slightly, when it was easily revolved hori- 
zontally 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. 
