153 
a Jioating collhnator. 
inclination to the horizon. I had thus a floating support to 
which I could attach a telescope, a support requiring no ad- 
justment, offering the ready means of extreme accuracy, and 
precluding all fear of those errors which might arise from the 
use of a level. 
For a preliminary experiment I procured a piece of oak, 
seven inches and a half long, four inches and a half wide, 
and one inch thick. Upon this, two wooden uprights in the 
form of Y s, were pinned and glued at the distance of five 
inches. Half way between these a small ring was screwed 
into the oak, and the telescope being laid upon the Y's, was 
firmly secured in its place by a string passing several limes 
round it and through the ring. 
In the middle of the longer sides of the oak support, and at 
right angles to its horizontal surface were inserted two pieces 
of brass, in which very smooth grooves were made, about 
one-tenth of an inch wide. 
A deal box eight inches long, five inches wide, and an 
inch and a half deep, having its bottom just covered with 
mercury, received the float, which was kept in its situation 
in the middle of the box, and prevented from turning hori- 
zontally by two smooth iron pins passing through the sides 
of the box into the grooves. These were carefully regu- 
lated so as to allow the float to adapt itself with perfect free- 
dom to the surface of the mercury. 
The whole of the telescope was above the edges of the 
box, and a screen of black pasteboard, with an aperture equal 
to that of the object glass, was fixed to the end of the box. 
This is indispensably necessary, 'in order to exclude false light. 
A fine achromatic telescope by Dollond, of thirty inches 
MDCCCXXV. X 
I 
