per second per square centim. of surface of the copper, per degree of difference between its temperature and that of the walls of the enclosure. These latter were blackened internally, and were kept at a nearly constant temperature of 14° C. The air within the enclosure was kept moist by a saucer of water. The greatest difference of temperature employed in the experiments (in other words, the highest value of t) was 50° or 60° C. The following Table contains the values of x calculated from the above formulæ, for every fifth degree, within the limits of the experiments. 137. Professor Tait has published ('Proc. R. S. E.' 1869-70, p. 207) observations by Mr. J. P. Nichol on the loss of heat from blackened and polished copper, in air, at three different pressures, the enclosure being blackened internally and surrounded by water at a temperature of approximately 8° C.* Professor Tait's units are the graindegree for heat, the square inch for area, and the hour for time. The rate of loss per unit of area is heat emitted area × time The grain-degree is 0648 gramme-degree. Hence Professor Tait's unit rate of emission is of our units. Employing this reducing factor, Professor Tait's Table of Results will stand as follows: Pressure 1'014 × 106 [760 millims. of mercury]. 62.5 57'5 53°2 47'5 43 28.5 Pressure 136 x 105 [102 millims. of mercury]. * This temperature is not stated in the "Proceedings," but has been communicated to me by Professor Tait. Mechanical Equivalent of Heat. 138. The value originally deduced by Joule from his experiments on the stirring of water was 772 foot-pounds of work (at Manchester) for as much heat as raises a pound of water through 1° Fahr. This is 1389.6 footpounds for a pound of water raised 1° C., or 1389.6 footgrammes for a gramme of water raised 1° C. As a foot is 3048 centims., and the value of g at Manchester is 9813, this is 1389⋅6 × 30°48 × 9813 ergs per grammedegree; that is, 4'156 × 107 ergs per gramme-degree. A later determination by Joule ('Brit. Assoc. Report,' 1867, pt. i., p. 522, or 'Reprint of Reports on Electrical Standards,' p. 186) is 25187 foot-grain-second units of work per grain-degree Fahr. This is 45337 of the same units per grain-degree Centigrade, or 45337 foot-grammesecond units of work per gramme-degree Centigrade; that is to say, 45337 × (3048)2 = 4212 × 107 ergs per gramme-degree Centigrade. At the meeting of the Royal Society, January, 1878 ('Proceedings,' vol. xxvii., p. 38), an account was given by Joule of experiments recently made by him with a view to increase the accuracy of the results given in his former paper. (Phil. Trans., 1850.') The result he has now arrived at from the thermal effects of the friction of water, is, that taking the unit of heat as that which can raise a pound of water, weighed in vacuo, from 60° to 61° of the mercurial Fahrenheit thermometer; its mechanical equivalent, reduced to the sea-level at the latitude of Greenwich, is 772.55 foot-pounds. To reduce this to water at o° C. we have to multiply by 1'00089,* giving 773 24 ft. lbs., and to reduce to ergs per gramme-degree Centigrade we have to multiply by 139. Some of the best determinations by various experimenters are given (in gravitation measure) in the following list, extracted from Watt's 'Dictionary of Chemistry,' Supplement 1872, p. 687. The value 429'3 in this list corresponds to 4'214 × 107 ergs :— We shall adopt 42 × 107 ergs as the equivalent of * This factor is found by giving t the value 15.8 (since the temperature 60.5 Fahr. is 15.8 Cent.) in formula (3) of art. 98. I gramme-degree; that is, employing J as usual to denote Joule's equivalent, we have The numbers in the last column are the products of the numbers in the preceding column by 42 millions. The authorities for these determinations are indicated by the initial letters A (Andrews), F (Favre and Silbermann), T (Thomsen). Where two initial letters are given, the number adopted is intermediate between those obtained by the two experimenters. |