THERMOMETER AND PYROMETER. 



moves round the circle h, divided into 

 300. The consequence of this arrange- 

 ment is, that if the expansion of the 

 metal were to push the rack e one inch 

 forward, it would turn the pinion d 4| 

 times round; and the wheel/, moving 

 at the same rate, will carry the pinion e, 

 and consequently the index (10 x 44) 

 = 41 1 round. Hence the index would 

 have moved over (4 1 x 300), or 12,500 

 divisions of the scale ; or each degree 

 of the instrument is equivalent to T s|o5 

 of an inch of the expansion of the prism 

 a. Similar prisms of different metals 

 applied in like manner to the instru- 

 ment.enabled Musschenbroek to measure 

 the different expansibility of steel, iron, 

 copper, brass, and lead, with considera- 

 ble accuracy :* but there is always some 

 uncertainty in the movements of so 

 many loosely connected teeth and pini- 

 ons ; and this pyrometer was improved 



by 



2. Desaguliers,t who instead of prisms 

 substituted cylinders, as wires are 

 more easily procured than prisms of 



equal dimensions. For the first pinion 

 he employed steel slightly roughened 

 by the file in the same direction as the 

 teeth. Thus a more equable motion 

 was given to the instrument. The 

 toothed wheel and second pinion were 

 supplied by a wheel and roller, having 

 grooves in their circumference for re- 

 ceiving a watch-chain, by which motion 

 was communicated to the index. The 

 dial plate was square and movable, in 

 order to stretch the watch-chain as 

 there might be occasion. A thin plate 

 of rough steel ^ inch wide, slightly 

 convex towards the first roller, was 

 substituted for the rack ; and this last, 

 which in Musschenbroek's pyrometer 

 was made to travel lightly over a small 

 bit of fine watch-spring, moved in 

 Desaguliers over a well constructed fric- 

 tion wheel, or roller. 



These changes improved the delicacy 

 of the instrument very considerably ; 

 but it soon underwent other modifica- 

 tions. 



3. The pyrometer of Mr, John Ellicot, 



Fig. 9. 



m 



of London, is seen in fig. 9, a a is a 

 flat plate of brass screwed 1o 



*Tentanu Acad. del Cimento. 

 t Desagulier's Experimental Philosophy,!, 421, 



mahogany S. >IQ , A : which the thiee brass 

 a thick uprights bbb are firmly attached. 



The pyrometric pieces consist of two 

 metallic bars : the flat on c c is of 



