1822.) Specific Gravity of Gases. 31 
the quantity of heat being as the temperature ; when the fact is, 
that while the temperature has been advancing by 30°, the real 
quantity of heat is doubled; and it will be found that a cubic 
foot of steam of 60 inches pressure in mercury, although only 30° 
in temperature above a cubic foot of 30 inches pressure will 
heat twice the quantity of water to the same temperature, or 
melt twice the quantity of ice, which is the clearest proof that 
their respective quantities of heat are as 2 to 1. 
The remaining part of this paper which is applied to calculate 
the correction for the specific gravity of gases, as affected by the 
presence of aqueous vapour, is very valuable. If the force of 
aqueous vapour at different temperatures be correctly taken in 
order to get the specific gravity of the same, nothing can be 
more simple than the formula given by Dr. Thomson for finding 
the allowance to be made for the presence of vapour in any gas. 
This is the same formula which is contained in my paper sent to 
your journal for finding the proportions of mixed inflammable 
gases. I remain, dear Sir, yours very truly, 
C. SyLvesTex.. 
ArTICLE XI. 
Extracts from the “ Journal * of a Survey to explore the Sources 
of the Rivers rea and Jumna.” By Capt. J. A. Hodgson, 
10th Reg. Native Infantry. 
As I have had it in my power to explore and survey the course 
of the Ganges within the Himalaya mountains to a considerable 
distance beyond Gangautri, and to the place where its head is 
concealed by masses of snow which never melt, I hope, that an 
account of my journey may be acceptable to the Asiatic Society. 
J must premise that, as Capt. Raper’s account of Capt. Webb’s 
survey in 1808, has already appeared in the eleventh volume of 
the Researches, I have nothing to add to that officer’s able and 
fzithful description of the mountainous country, passed through 
in the route of the survey from the Dan Valley to Cajani, near 
Reital, where the survey towards Gafigautri was discontinued in 
consequence of the serious obstacles which impeded it. I shall, 
therefore, only give an account of the course of the river above 
* The Editor is favoured with these extraets from almost the only copy of Captaia 
Hodgson’s Journal, which has reached England, by Mr. Edmonstone, of Newcastle ; 
who observes, that in order to shorten the communication, a number of minute and inte- 
resting details have been necessarily omitted. This circumstance will serve to explain 
the breaks which the narrative occasionally assumes, and we should hope will be 
received as a sufficient apology for our not doing all the justice that we could wish to the 
labours of Capt. Hodgson, who has since been appointed to the important situation of 
Surveyor-General of India. 
