436 Dr. Rankine on the Expansive Energy of Heated Water. 



at one end, as previously described, the twisted part having a 

 length of 4 inches, I united the loose ends to the electrodes of 

 the galvanometer; and over the twisted portion of the wires 

 forming the pile I doubled a piece of tinfoil 4 inches square. 

 On touching a part of the foil with the finger, the needle exhi- 

 bited a deviation in the same direction as in the previous expe- 

 riments in which the effect was attributed to the heat liberated 

 by condensation ; and on wetting the foil with a sponge having 

 the water squeezed out of it by the hand, a deflection in the same 

 direction occurred ; but as soon as the heat communicated to the 

 water by the hand began to be carried off by evaporation of the 

 fluid from the foil, a contrary deviation was established, lasting 

 as long as any moisture remained upon the foil. If the wet foil 

 was exposed to the sun's rays when these were not too intense, 

 a negative deflection was maintained until evaporation ceased, 

 when the needle experienced a contrary deviation due to direct 

 solar action. 



Since obtaining these results, I have tried to adapt the arrange- 

 ment described to hygrometric purposes, by merely suspending 

 a strip of cotton over the tinfoil with its ends immersed in a 

 cup of water. The evaporation which then takes place from the 

 surface of the cotton of course varies with the dryness or humi- 

 dity of the atmosphere ; but as these latter causes accelerate or 

 retard evaporation from the cotton cloth, so also is a greater or 

 lesser quantity of heat abstracted from the wires, and a corre- 

 sponding variation in the deflection produced. The readings of a 

 wet- and dry-bulb thermometer have been compared with those 

 of the above arrangement (which may be designated a thermo- 

 electrical hygrometer), and their general correspondence observed; 

 but the details of these experiments, and of some other peculiar 

 thermo-electrical phenomena, must be reserved for a subsequent 

 communication. 



Victoria Street, Newcastle-on-Tyne. 



LXII. On the Expansive Energy of Heated Water. 

 By W. J. Macquorn Rankine, C.E., LL.D., F.R.SS.L.fyE.* 



[Supplement to a Paper published in the Philosophical Magazine for 

 November 1863.] 



FOR the sake of more easy comparison with the investiga- 

 tions of the Astronomer Royal and Professor Miller, the 

 formulae and Tables of this paper are here given in metrical 

 measures. 



* Communicated by the Author. 



