20 



THERMOMETER AND PYROMETER. 



Hence the value of each division of 

 the index-plate will = 7rr^^ s f an 

 inch ; and as, when the instrument was 

 well adjusted, the difference of contact 

 was very perceptible when the screw 

 was moved through of a division, the 

 53T43th of an inch of expansion was de- 

 terminable by this pyrometer, with which 

 Mr. Smeaton ascertained the expansi- 

 bility of many solids. 



The following table is the result of 

 his experiments, showing in 10,000dths 

 of an inch the expansion of rods of 

 different kinds of matter, in passing 

 from the freezing to the boiling point of 

 water. 



White glass barometer tube . .100 

 Martial regulus of antimony . .130 



Bistered steel 138 



Hard steel 147 



Iron 151 



Bismuth 167 



Copper hammered 204 



Alloy, 8 copper, and 1 tin . . .218 



Cast brass 225 



Alloy, brass 16, tin 1 229 



Brass wire 232 



Telescope speculum metal . . . 232 

 Alloy, 2 brass, 1 zinc .... 247 



Fine pewter 274 



Grain tin 298 



Soft solder, 2 lead, 1 tin . . . .301 

 Alloy, 8 zinc, 1 tin, slightly ham- 

 mered 323 



Lead 344 



Zinc 353 



Zinc hammered out 1 inch per foot 373 

 These experiments correspond as 

 nearly with the results obtained by 

 Ellicot, as the difference of the instru- 

 ments admit. They introduced a pre- 

 cision hitherto unknown in the law of 

 expansion of solid bodies ; and are still 

 quoted with approbation in those nice 

 disquisitions which have paved the way 

 to the perfection of horology, and the 

 modern refinements in geodesical ope- 

 rations, while they have extended our 

 knowledge of the effects of heat. 



7. The metalline thermometer of Mr. 

 Keane Fitzgerald comes next in order 

 of time ; but it is chiefly applicable to 

 mark the alterations of atmospheric tem- 

 perature. Its general construction will 

 be readily learnt from fig. 13.* 



The basis of the instrument is a piece 

 of well seasoned deal, on which a system 

 of levers is fixed ; a a is the pyrometric 

 bar, 2 feet long, the upper extremity of 



* Phil. Trans, vol. li. p. 523. 



which bears against the fulcrum z 

 Fig. 13. 



while its other end rests on a small 

 hemisphere of metal on the short arm of 

 the lever b. The long arm of this lever 

 is 2| times as long as the other ; b is 

 joined by a pivot to the rod c, 2 feet 



2 inches in length, which bears against 

 the short arm of d, and the long arm of 

 d is 2| times as long as the former. 

 The rod e is 2 feet 4 inches long, and 

 is jointed to /, as in the figure. The 

 long arm of fis 4 times the length of 

 its short arm, and terminates in a 

 slender arch-head, which is attached to 

 the lower end of the rod g by a watch- 

 chain, as in the figure. This last rod is 



3 feet long, and is kept perpendicular 

 by sliding between two friction rollers 

 p, v, its connection with the arch-head, its 

 suspension from the lever y, and its 

 friction on the pulley h. The weight of 

 the levers, &c. is counterbalanced by 

 the springs m and o, and the spring of 

 y is nearly neutralized by the pressure 

 of x. The pulley h is fixed at 2 feet 

 6 inches from the lower end of g, and 

 is 3 inches in diameter. Two cords 

 fixed to the spring q, pass twice round 



