i88 
Mr. Wollaston’s description of the 
Fig. 3, within the collar, by screws passing through the flanch 
P, at the back of the scale. It would be well, that a piece 
of thick leather, or soft wood, should be screwed between 
these two, if it can be done with sufficient firmness, for the 
purpose of preventing the scale getting inconveniently hot. 
The adjusting screw, which carries the Vernier, is raised 
by the standards above the scale, and is placed opposite the 
centres of the plates in fig. s and 3, by which means the 
milled head Q goes better into the case hereafter described. 
The tube of the thermometer, when passed through the cen- 
tral holes in the two plates, turns by its bend to the left hand, 
and up the side of the scale, being slightly fastened to it 
at the top only, with a small piece of cork under it, to keep 
it clear from the scale. The Vernier has fixed to it, by means 
of a screw head, two pieces of thick paper laid upon each 
other, the one black and the other white, half of the outer 
paper being cut away straight, makes a line between the 
black and white, better than any that can be drawn for ad- 
justment to the top of the mercurial thread. 
Were I wanting another of these instruments, and employ- 
ing a workman to construct it for me, I would have the whole 
length of the adjusting rod square, with a small piece sliding 
by hand on it for the larger alterations, and carrying a short 
screw for the finer movements, or the whole scale and move- 
ments might, I think, conveniently be made with tubes in the 
manner of the mountain barometers, and the thermometer 
would not in that case require the bend, but would run up 
the centre. 
To the Vernier Mr. Cary has attached for me, with a joint, 
a small lens of one inch focus, which assists in observing the 
