936 AN ACCOUNT OF THE 
brass rod is firmly fastened: in the others, it plays loosely, and is free to ex- 
pand or contract. The end b has a pin ¢ passing through it vertically, 
which presses against an index of brass d, that moves over a graduated arc, 
and thus points out the alteration in the relative lengths of the wood and 
brass, from time to time. ‘The absolute change of length in the brass be- 
ing known by the thermometer, and the received rates of expansion, it fol- 
lows; that the actual change of length in the wood becomes also known. 
It is hardly necessary to mention, that the wooden piece A B is only fasten- 
ed at one end, being free to contract or expand between wooden studs that 
‘ 
prevent its warping. 
14, ‘Tue index dis kept up to the pin, when the brass is contracting, 
by means of a small spring, which in every situation keeps it in accurate 
contact with the pin. | The point where the pin presses, is within 3 an inch 
of the centre of motion, while the index extends 12 inches beyond it. By 
this means the minutest changes are discovered, being mcreased in a ratio 
of 24 to 1, and such was the sensibility of the instrument, that scarcely for 
40 minutes did the index ever remain stationary. This instrument I called 
a comparator, because it served to compare the length of wood, with that of 
brass, and therefore to detect any changes in the former. As the wooden 
rod of eight feet (A B fig. 8), was cut out of the same piece of timber 
as the measuring rods, I did at first imagine that it would prove a very 
_ Satisfactory means of doing away the objections to wooden rods, arising’ 
from the effects of the weather in altering their length. ‘The result was 
not however answerable to my expectation. 
