32 



THERMOMETER AND PYROMETER. 



moves therefore three times round for 

 every revolution of i. The action of 

 the spiral spring m draws round the 

 wheel * and the index, when the ex- 

 pansion of the platina rod permits it to 

 act. The dial is divided into 360 de- 

 grees. By experiment, Daniell ascer- 

 tained that each degree of his scale 

 = 7 degrees of Fahrenheit's : and he has 

 published an account of some well con- 

 ducted experiments on the fusing points 

 of some of the metals with this instru- 

 ment, which very widely differ from the 

 results obtained by Wedgwood, but 

 nearly agree with those of Morveau, 

 Mr. Daniell found, that after being 

 exposed to high temperatures, the py- 

 rometer did not fall to the point from 

 which it set out ; a circumstance which 

 he attributes, with justice, to changes 

 in the form of the tube induced by a 

 high temperature. This is certainly an 

 imperfection in the principle of the in- 

 strument ; but if the degrees of heat 

 be marked by the ascending series, its 

 indications seem tolerably correct, and, 

 although perhaps little to be depended 

 on in nice investigations, it may become 

 an useful instrument to manufacturers 

 who make use of high temperatures. 

 The tube should not be exposed to 

 a naked fire, except it be of wood char- 

 coal ; because the foreign ingredients 

 of fossil coal will adhere or incorporate 

 with the blacklead ware of the tube. 

 On these grounds we should feel more 

 inclined to recommend the pyrometer 

 of Morveau, which, besides, is extremely 

 portable ; and, being wholly exposed to 

 the heat, is less liable to be affected by 

 extrinsic circumstances in its indications. 

 The following table exhibits some of 

 Daniell's comparative results. 



50 Fahrenheit = 7. 2' Daniell. 



100 =140 



150 = 22.5 



200 = 30.5 



250 = 38.5 



300 = 45.4 



350 = 51.5 



400 = 58.5 



450 = 66.9 



50J = 73 5 



550 =770 



580 = 840 

 600 by calculation he estimates at 86.4. 



I>. F. 



Melting point of tin 63 441 



bismuth 66 462 



lead 87 609 



Boiling point of mercury 92 644 



Melting point of zinc 94 658 



Red heat in full daylight 140 980 



Heat of a parlour fire 163 1141 



Melting point of brass 267 1869 



^ silver 319 2233 



copper 364 2543 



gold .. 370 2590 



cast iron... ,. 497 3479 



18. Messrs. Breguet, the celebrated 

 chronometer-makers, have lately con- 

 structed a most elegant and delicate 

 pyrometer, or metalline thermometer, 

 of which we give a figure. (See fig. 

 25.) 



Fig. 25. 



It consists of a helix formed of three 

 metals of unequal expansibility. The 

 exterior plate of this delicate helix 

 is of silver, the interior of platina, and 

 between them is one of gold. Two only 

 are necessary to the perfect action of 

 the instrument ; but from the difference 

 of expansibility between silver and pla- 

 tina, they would be liable to separate by 

 sudden changes of temperature ; and a 

 thin plate of gold, which is of interme- 

 diate expansibility, is interposed. The 

 whole form a single flat plate or wire 

 about isoth of an inch in thickness. 

 The upper extremity of the helix is fixed 

 to the brass support b, which by its 

 form insulates the helix, and permits its 

 coiling and uncoiling freely. To its 

 lower extremity is attached a gold needle 

 e, kept horizontal by a small counter- 

 poise. This needle moves round a gra- 

 duated circle representing degrees of 

 the centigrade scale. When the am- 

 bient air is heated, the expansion of the 

 metals carries round the needle in the 

 direction of the coils of the helix, and a 

 diminution of temperature moves it in 

 the opposite direction by relaxing the 

 coils. Experiment has proved that 

 equal increments of temperature move 

 the needle over equal spaces of the 

 scale, so that it is comparable with other 

 thermometers. 



