18 



THERMOMETER AND PYROMETER. 



feet long, its upper extremity termi- 

 nating in a hardened steel point one 

 inch more in length, and entering a 

 hole in a steel plate on the under 

 side of the lever e, while its lower 

 end rests on a point attached to the 

 metallic plate at d. c, d are plates of 

 iron joined at d, and at different other 

 points, as in the figure : at x, x they 

 are turned half round, to allow the 

 application of heated bodies, as sand 

 or water, to the bar, which is immersed 

 in the heated bodies to a certain mark 

 as at b. In the original instrument 

 this mark was at 1^ inch from the 

 bottom: e,f is a lever moving round 

 an axis in g. A string from the end of 

 its longest arm passes twice round the 

 pulley h, and is kept tight by a weight 

 i of lb., while there is another weight 

 I, at the short arm of the lever, suffi- 

 cient to counterbalance the weight of 

 the longer arm, and to keep the point a 

 in close contact with the lever, m, n, o, 

 a dial, of which the face is seen at B, 

 graduated to correspond to Fahren- 

 heit's and Reaumur's degrees, which 

 are indicated by an index fixed on the 

 axis of the pulley h. The frame of the 

 instrument is of oak. The lever from 

 p to a = 4 inches ; from a to g 1.5 

 inch; from g to/= 12 inches; the 

 pulley = 0.5 inch ; the dial = 11 inches 

 in diameter. In the original the melt- 

 ing point of different substances is in- 

 dicdted by their chemical signs in the 

 outer circle of the dial. 



This instrument appears to have been 

 of considerable delicacy, and to have 

 marked minute changes of atmospheric 

 temperature very readily : but the size 

 is inconvenient ; and it must now be 

 regarded rather as an instrument of 

 curiosity than utility. 



5. The pyrometer, figured 11, the 

 invention of Mr. Froteringham, a Lin- 

 coln grazier, combines simplicity with 

 considerable delicacy. It was also in- 

 tended to indicate the changes of at- 

 mospheric temperature, a, a is a bar 

 of iron four feet long and 1 1 inch wide, 

 having a polished brass surface screwed 

 to it with steel screws, which are fitted 

 to short slips in the brass that allow the 

 expansion of the iron bar, without that 

 of the brass ornamental surface, to 

 affect the hardened steel apex b. This 

 apex moves the lever c, which raises 

 the lever d; both turning on well made 

 central disks. A chain from the ex- 

 tremity of the lever d is lapped twice 

 round the pulley/ on the axis of the 



index, which moves round a graduated 

 circle g. The counterpoise i brings 

 back tne index as the levers fall. The 

 screw h is for adjusting the index to 

 the beginning of the scale. It is very 

 obvious that such an instrument would 

 be capable of showing the expansions 

 of the bar in proportion to the differ- 

 ence between the arms of the levers ; 

 and, it is said, that the original in- 

 strument, in the library of a philoso- 

 phical society at Spalding, indicated 

 the changes of the heat of the weather 

 with great precision.* 



6. All these instruments, however, 

 yield in accuracy to the invention of the 

 celebrated Smeaton, which is described 

 in the Philosophical Transactions .t 



In this instrument the expansions of 

 the metallic bars, heated by water, are 

 measured by means of a micrometer 

 screw; a principle which had been 

 before employed by the great chrono- 

 meter-maker Graham, for the adjust- 

 ment of the rods of a rendulum. 



From the principle of its construc- 

 tion, this instrument is called the Mi- 

 crometer- Pyrometer, /g-. 12. 



The basis of this instrument a, b, c, d 

 is of solid brass, which was chosen as 



* Phil. Trans, vol. xlv. p. 125. 

 t Phil. Trans, vol. xlviii. p. 487. 



