barus.] METHODS OF PYROMETRY. 47 
the air thermometer itself. The form of apparatus most easily used 
experimentally, viz, the platinum transpiration tube, is based on prin- 
ciples not quite as direct as Maxwell's law. Nevertheless Meyer 1 has 
succeeded in interpreting Graham's 2 data, has discussed his experi- 
mental methods, and has more recently shown that both Graham's and 
Coulomb's vibration methods lead to the same results. Work of this 
kind has also occupied Stefan. 3 In Meyer's deduction the volume of 
gas transpiring per unit of time under given conditions, besides de- 
pending on the pressures, the internal friction, the length of tube, 
involves an expression containing the fourth power of the radius 
of the capillary tube and the ratio of internal to external gaseous 
friction coefficiented by the third power of radius. Hence .in such a 
pyrometer the coefficient of heat expansion of platinum must be some- 
what carefully predetermined. According to Nichols (1. c.) this is by 
no means seriously difficult. Supposing a capillary platinum spiral to 
terminate in two larger platinum tubes (of which one may wholly en- 
velop the other), we have given at once the effective part of the mech- 
anism of a thermometer based on the viscosity of gases. Such a ther- 
mometer may be used as far as the melting point of platinum. For 
temperatures beyond this, porous fire-clay plugs in an impervious tube 
suggest themselves. 
Acoustics. — The next year after Pouillet's fundamental research on 
pyrometry, his brilliant and ingenious countryman, Cagniard-Latour, 4 
acting in concert with Demonferrand, proposed an acoustic air ther- 
mometer. Inasmuch as the velocity of sound in dry air is proportional 
to the square root of the absolute temperature, Latour and Demonfer- 
rand easily wrought out a formula in which temperature is expressed 
in terms of the vibrations of the fundamental note of their apparatus at 
the high temperatures and at normal temperatures. They estimate that 
the error of a comma would not exceed 30° at 1,000°. This apparatus 
was afterwards reinvented by Mayer, 5 who discusses its principle ex- 
haustively. Mayer calculates tables for temperature, velocity of sound 
and wave length, between —300° and +2,000°, and suggests many 
devices of measurement. After Mayer the same principle was empha- 
sized by Chautard, 6 who simplified the apparatus necessary, but he 
expresses some doubt as to its efficiency. 
1 O. E. Meyer: Fogg. Ann. ? vol. 127, 1866, pp. 253, 353 ; ibid., vol. 125, 1865, pp. 177, 
401; ibid., vol. 143, 1871, p. 14; Wied. Ann., vol. 32, 1887, p. 642; cf. Konig, ibid., p. 
193. 
2 Graham's original researches. See Philos. Trans., London, 1846, p. 573; 1849, 
pt. 2, p. 349. The suggestion of using platinum capillary tubes at high temperatures 
is my own. 
3 Stefan: Wien. Ber., vol. 46, 1862, p. 495. 
4 Cagniard-Latour et Demonferrand : C. R., vol. 4, 1837, p. 28. 
6 Mayer: Pogg. Ann., vol. 148, 1873, p. 287. 
6 Chautard: C. R., vol. 78, 1874, p. 128; Pogg. Ann., vol. 153, 1874, p. 158. 
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