and the Mode of its Communication. 105 
to it, farther from it, or to bring them nearer to it ; in order that 
this may be done easily, and expeditiously, by one person, and 
without its being necessary for him to remove his eye from the 
bubble, (which he should constantly have in his view,) I make 
use of a simple machine, which I have found to be very useful. 
It is a long and shallow wooden box, open at both ends. It 
is 6 feet long, 12 inches wide, and 5 inches deep, measured on 
the outside: its vertical sides are made of i-|-inch deal; its 
bottom and top, of inch deal. A part only of the top or cover 
of this box is fixed down on the sides, and is immoveable. The 
part of the cover which is fixed, and on which the thermoscope 
is placed, occupies the middle of the box, and is 33 inches in 
length. On the right and left of this fixed part, the top of the 
box is covered by a sliding board, 2 feet 3 inches long, which 
passes in deep grooves, made to receive it, in the sides of the 
box. A rack is fixed to the under side of each of these sliding 
boards; and there is a small cog wheel in the box, the axis of 
which passes through the sides of the box, and is furnished with 
a winch in the front of the box. By turning round these wheels, 
by means of their winches, (both of which can be managed by 
the same person, at the same time,) the sliders may be moved 
backwards and forwards, at pleasure. 
In order to ascertain with facility and dispatch, the distances 
of the hot bodies from their respective balls, the top of the front 
side of the wooden box is divided into inches, on each side of 
the fixed part of the cover of the box ; and there is a nonius 
belonging to each of the sliders, which is placed in such a manner 
as to indicate, at all times, the exact distance of the hot body 
from its corresponding ball. 
The level of the upper surface of that part of the cover 
mdccciv. P 
