466 Major-Genera/ Roy'!? Account of the 



helical fleel fpring ; which a(fling upon abroad fhoulder of thsr: 

 rod prepared for the purpofe, thereby prelies its inward end, 

 which enters- the boiler^ againft the perpendicular furface of 

 the end-piece of the. fhort flide. Thus, the farther endof the 

 rod in experiment, lupppfed now to be in its contra6led ftate, , 

 is conftautly made to bear againft the furface that \s under the 

 fixed microfcope. But on the application of heat, the irre- 

 liftible force of expanfion in the rod obliges, the fpring to give- 

 way ; the fhort. flide changes its place, and with it the object 

 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 fhewn, 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 refls on 

 the furfaces of three rollers, about an inch in diameter; and 

 by means of- three pair of milled-headed nuts i| inch in dia- 

 3Tieter, which move on axes that are formed into fcrews,. until 

 they almoft touch the fides of the rod, this is kept in its true 

 €entrical pofition, whatever may be its form or lateral 

 dimenfions.- 



The microfcope towards the left-hand has been denominated 

 £xed, becaufe it correfponds with the firfl 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 confe- 

 quence, that the expanfion of the ftandard brafs fcale, which 

 is not quite forty- three inches long, fliould 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 necefi[ary to move the marks and 

 (E?ye-pieces of the fixed microfcope, along their refpedive prifms, 



to 



