316 Intelligence and Miscellaneous Articles. 



by a dark band contiguous to the moon's limb. No such appearance 

 was visible in this instance. The circumstances of the reappearance 

 of Saturn at the moon's bright limb on May 8, were very similar to 

 those of the reappearance of Jupiter, excepting that there was no 

 depression of the limb where Saturn reappeared such as that which 

 was noticed at the i)lace of Jupiter's reappearance. The comparison 

 of the two occultations seems, therefore, to indicate that the phaeno- 

 menon seen in the case of Jupiter was in some way connected with 

 the indented form of the moon's limb. 



XLVIII. Intelligence and Miscellaneous Articles. 



DENSITY OF STEAM. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



THE first to deduce, from the principles of thermodynamics, a law 

 of relation between the temperature, pressure, density, and latent 

 heat of vapours, was Professor Clausius, in PoggendorfF's Annalen 

 for 1850. 



By combining an equation substantially identical with that of 

 Professor Clausius, with a formula expressing the relation between 

 the pressure and boiling-point of a vapour, a formula is obtained 

 for computing the density of a vapour from its latent heat, and the 

 rate of increase of its pressure with elevation of the boiling-point ; 

 as to which see the Philosophical Transactions for 1854, part 1, and 

 the Number of this Magazine for December 1854. 



In the absence of direct experimental data as to the density of 

 steam, I used that formula to compute tables of that density for 

 practical use, which were first published to the extent of a few 

 copies in 1855, and have since been revised and reprinted. 



On the 20th of September, 1859, the results of the first series of 

 experiments by Mr. Fairbairn and Mr. Tate, on the density of steam, 

 were laid before the British Association. So far as I am aware, this 

 is the first series of experiments on the absolute density of steam at 

 high pressures that has ever appeared ; the experiment of M. Adolphe 

 Hirn having been a detached one, and those of M. Siemens having 

 reference to the rate of expansion of steam rather than to its abso- 

 lute density. 



The following Table shows a comparison between the results of 

 the formula already mentioned, and those of Messrs. Fairbairn and 

 Tate's experiments. The differences range from — to 1 of the 

 quantities compared, and are all in one direction, the theoretical vo- 

 lumes being the greater. But this may probably be accounted for by 

 a difference in the value assumed for the density of water ; for in the 

 theoretical calculation, the unit of volume is the volume assumed by 



