THERMOMETER AND PYROMETER. 



27 



which may readily be seen by the eye, 



= 640-00- 



Such is the microscopic pyrometer of 

 Kamsden ; an instrument not indeed 

 suitable for ordinary purposes, but ad- 

 mirably adapted for obtaining an accu- 

 rate estimate of the comparative expan- 

 sibility of different solids ; an object of 

 the highest importance, not only in 

 bringing to perfection the delicate in- 



struments required by the refinements 

 of modern philosophical investigations, 

 but essential to the perfection of diffe- 

 rent kinds of machinery in daily use, 

 and even to a successful investigation 

 of the laws and nature of heat itself. 

 With this instrument Roy determined 

 the expansion of the seven solids in the 

 annexed table. 



Expansion of 



By 180. 

 Revolutions. Parts. 



By F. 



Parts. 



Dutch brass 35.69 = 19^5 



English plate brass, a rod . 36.41 = 20 T 2 o 3 o 

 Ditto, in the form of a trough 36.45 = 20 JL% 



Steel rod 22.02=12 T ^_ 



Cast iron prism . . . . 21 34 = 11 T */ 



Glass rod 15.54= 8 T <y_ 



Ditto tube 14.93 = 



13. The instruments hitherto noticed 

 are inapplicable to very high tempera- 

 tures, or to ascertain the heat of closed 

 fire-places ; an object, in many processes 

 in the arts, of the utmost importance. To 

 supply this deficiency, our celebrated 

 Wedgwood took advantage of the pro- 

 perty which clay has of contracting 

 by heat, and remaining afterwards in 

 that state of contraction. This property 

 is not, strictly speaking, an exception to 

 the general law of expansion by increase 

 of temperature : clay is not a homoge- 



Inch. 

 on 5 feet. 



0.111323 

 0.113568 

 0.113693 

 0.(J68684 

 0.066563 

 0.048472 

 0.046569 



Inch. 

 on 1 foot. 



0.0222646 

 0.0227136 

 0.0227386 

 0.0137368 

 0.0133126 

 0.0096944 

 0.0046569 



neous body, but a mechanical mixture 

 of argil and silex, which by the influence 

 of heat are brought into more intimate 

 union, and therefore diminish in bulk ; 

 until a temperature sufficiently high to 

 melt them, that is, to convert them into 

 a homogeneous mass, is applied : after 

 which the product obeys the general law 

 of expansion by heat. Availing himself 

 of this property, Mr. Wedgwood em- 

 ployed as pyrornetric pieces cylinders of 

 dain clay, slightly flattened on 

 as seen in A B, Jig. \ 9,* 



formed by pressing the clay into an 

 iron tube, and baked in a potter's fur- 

 nace. It was found, after repeated 

 trials, that the pieces of clay contracted 

 more and more in an uniform ratio 

 to the degree of heat communicated to 

 them, and permanently retained this 

 contraction ; so that by applying them 

 when cold to a scale, an indication of 

 the degree of heat was obtained. 



The scale employed by Wedgwood 

 consisted of two brass rods \ inch 



square, and two feet in length, fixed on 

 a brass plate converging ly, so that they 

 were distant at one end just 0.5, and at 

 the other 0.3 inch. For convenience 

 the rods are usually divided and fixed as 

 in the figure on the plate, forming two 

 nearly parallel grooves- With the 

 above -stated convergence the whole 



Phil. Trans, vol. Ixxii. Ixxiv. Ixxvi. 

 f The degree of convergence bi'injr only one-tenth 

 of an inch in a foot, is not perceptible in the tigure. 



