466 Major-General Roy’j Account of tBe 
helical fleel fpring ; which a cling upon abroad fin odder of the- 
rod prepared for the purpofe, thereby preifes its inward end, 
which enters the boiler,, again!! the perpendicular furface of 
the end-piece of the fhort Aide. Thus, the farther end of the' 
rod in, experiment, fuppofisd now to be in its contradied fate,, 
is conftantly made to bear, again!! the furface that is under the- 
fixed microfcope. But on the application of heat, the irre— 
fiftible force of expanlion in the rod obliges the fpring to give, 
way; the fhort Aide changes its place, and with it the objedt 
lens of the micrometer microfcope moves on a fpace propor- 
tionable to the degree of heat that is applied ; and it is this 
diftance,. meafured by means of the micrometer, as hereafter 
will be (hewn, that determines the quantity of expanfion, or 
the fpace by which the rod has lengthened.. From the plate 
It will be further obferved, that the rod in experiment refts on 
the furfaces of three rollers, about an inch in diameter ; and 
by means.of three pair of milled-headed: nuts if inch in dia- 
meter, which move on axes that are formed into ferews, until 
they almofl touch the fides of the rod, this is kept in its, true 
centrical pofition, whatever may be its form or lateral, 
dimenfons. 
The microfcope towards the left-hand has been denominated 
Axed, becaufe it correfponds with the fir A or fixed end of the: 
rod in experiment, and never changes its place while thefe are 
of the length of five feet. But it appearing to be of confer 
quence, that the expanfion of the flandard brafs fcale, which, 
is not quite forty-three inches long, fihould be determined, the 
pyrometer has therefore been adapted for the reception of any 
rods lefs than five feet, whereby it is made more univerfally ufeful. 
For this purpofe it becomes necefiary to move the marks and 
eye-pieces of the fixed microfcope, along their refpedtive prifms, 
to 
