same. That is, if I be the latent heat, and t the temperature, ltc a constant quantity, whatever t may be. I differ with Dr. Thomson as to any practical advantage derived from the variable quantity of latent heat at different temperatures either in distillation or in its agency in steam engines. Suppose in the former application that vapour is distilled over at the temperature of 70°, and condensed in a temperature of 50°, a constant succession of liquid will be formed by condensation, which is the practical effect desired, and it must be admitted from the law quoted by Dr. Thomson, that the stock of vapour at 70° constantly passing from the still to the receiver will hold more latent heat than the same quantity at a higher temperature. The difference will consist in having a small excess of latent heat, which is "in the uncondensed vapour at every period of the process without any disadvantage," as to the ultimate quantity of liquid condensed. I am inclined to think that if the size of the apparatus be increased so that the same weight of vapour may come over in the same time, the advantage would be in favour of the low temperature, owing to the quantity of heat lost in all processes carried on at high temperatures by radiation and the conducting power of contiguous bodies.

For the same reasons there is no advantage in using steam for engines at a high pressure. Whatever may be the fuel consumed to make a given volume of steam equal to one atmosphere, it will take twice the quantity to give twice that volume, or the same volume of a density to give a pressure equal to two atmospheres. I should think therefore, that the increased temperature of the volume equal to two atmospheres would lose more heat to surrounding bodies than the two volumes of one atmosphere, but the mechanical advantage of the two will be obviously the same. The boasted advantage of the Cornish engines has chiefly arisen from their inventor assuming some erroneous data respecting the power of steam, and many others, even Mr. Herapath, seem to have fallen into the same mistake. In the range of temperature commonly used for high pressure steam, it will be found that from an increase of temperature of every 30° degrees, the density and elasticity of the steam become doubled; that is, at 2120, its elasticity is equal to about 30 inches of mercury, and a cubic foot of such steam would weigh about 253 grains. At 212 + 30 = 242 degrees, the volume remaining the same, it will support 60 inches of mercury, and a cubic foot will contain 253 x 2 506 grains. Hence it will appear that the temperature is increasing in arithmetical proportion while the power of the steam increases in geometrical proportion, and hence the apparent advantage by working with high pressure. *

The source of this fallacy will be found in the assumption of

The force of steam has not strictly a geometrical ratio to the temperature. The ratio for 10° below 212° is about 1.23. And this ratio for every ten degrees above will decrease by 01, while steam, for every ten degrees below, has a similar increase,

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. SYLVESTER.

ARTICLE XI.

*

Extracts from the "Journal of a Survey to explore the Sources of the Rivers Ganges 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. I 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 faithful description of the mountainous country, passed through in the route of the survey from the Dun Valley to Cajani, near Reital, where the survey towards Gangautri 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 extracts from almost the only copy of Captain 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 interesting 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.

the village of Reital, where I halted to make arrangements for my progress through the rugged regions before me, in which I found I had no chance of getting any supplies of grain for my followers: I was consequently obliged to buy grain, and to send it off before me, so as to form little magazines at the places I intended to halt at; and as I learned that several of the sangas or spar bridges over the river had been destroyed by avalanches of snow, I sent a large party of labourers to re-establish them.

Considering Reital as a point of departure, it will be satisfactory to know its geographical position. By a series of observations with the reflecting circle of Troughton, and also by his astronomical circular instrument, I found the latitude to be 30° 48′ 28′′ N.; and having been so fortunate as to get two observations of immersions of the first satellite of Jupiter, and one of the second, I am able to give a good idea of the longitude of the place; and the more satisfactorily, as two of the immersions are compared with those taken at the Madras Observatory on the same night, and with which I have been favoured by Mr. Goldingham, the astronomer there.

The telescope used by me in observing the satellites was a Dollond's 42 inches achromatic refractor, with an aperture of two and three-quarter inches, and power of about 75 applied, having a tall stand, and rack work for slow motion. The watch was a marine chronometer, made by Molineux, of London, and went with the greatest steadiness on its rate, as nightly determined by the passage over the meridian of fixed stars observed with a transit instrument. The time of mean noon when required was always found by equal altitudes.

By a mean of several observations taken at Madras about, the time of four emersions of the first satellite, which I observed at Mr. Grindall's house near Seharanpur. (Mr. Goldingham finds 5h 10' 24" for the longitude of Seharanpur.) A snowy peak called Sri Canta is visible both from Reital and Seharanpur, its position is détermined by means of a series of triangles instituted by me for the purpose of taking the distances and heights of the snowy peaks. find the angle at the pole or difference of longitude between Seharanpur station and Sri Canta to be 1° 14′ 47′′, the peak being east, and at Reital, the difference of longitude of that village, and the peak is found to be 12′ 6′′; the peak being east, consequently the difference of longitude of Seharanpur and Reital is 1°2′ 41′′. On the whole, I think 5h 14' 20'6", or 78° 35′ 60-7" may be safely taken for the longitude of Reital, east of Greenwich.

Reital contains about 35 houses, and is esteemed a considerable village; as is usual in the upper mountains where timber is plentiful, the houses are large, and two or three stories high. When a house has three stories, the lowest serves to shelter the cattle by night; the second is a sort of granary, and in the upper the family dwells; round it there is generally a strong

wooden gallery, or balcony, which is supported by beams that project from the walls. The roofs of the houses are made of boards or slates; they are shelving, and project much beyond the top of the walls, and cover the balcony, which is closed in bad weather by strong wooden shutters or pannels. These houses are very substantial, and have a handsome appearance at a distance, but they are exceedingly filthy within, and full of vermin. The walls are composed of long cedar beams, and stone in alternate courses, the ends of the beams meet at the corners, where they are bolted together by wooden pins. Houses of this construction are said to last for ages; for the Deodar or Cailon pine, which, I suppose, to be the cedar of Lebanon,* is the largest, most noble and durable, of all trees.

1

The situation of this village on the east side of a mountain, the summit of which is covered with snow, and the foot washed by the Bhagirathé is very pleasant. It commands a noble view of the Srī Čanta, and other adjoining peaks of the Himalaya, on which the show for ever rests. Snow also remains until the rains, on all the mountains of the second order, which are visible hence, both up and down the river. Many cascades are formed by the melting of the snows on the foot of the surrounding mountains. One in particular descends in repeated falls of several hundred feet each, from the summit of a mountain across the river, and joins it near Batheri.

In the following account of my progress up the river, I have put down such remarks as occurred at the time, and they were written on the spot, and are here inserted with very little alteration. Though I am aware that such minute descriptions of localities must appear tedious, I hope they will be excused by those who, feeling interested in the subject, may have the patience to read the detail. To give general descriptions of such rude regions is difficult, if not impossible, and I trust that particular ones, though often tedious, will be found more faithful, and to give more precise ideas of those remote recesses of the Himalaya, which I visited.

On the 19th of May, I was joined at Reital by Lieut. Herbert, of the 8th Reg. N. I. who had been appointed my assistant, and from his skill and zeal, the survey has received much benefit.

Mr. Herbert came direct from Calcutta, and brought me a pair of mountain barometers, but the tubes filled in England had been broken before they arrived in Calcutta: there were some spare empty tubes which we filled and used as hereafter mentioned, but we could not succeed in boiling the mercury in the tubes to free it entirely of air. The height of Reital above the sea, as indicated by our barometers, is 7108 feet.

Having received reports that the sanghas were repaired, and that the grain I sent forward was lodged in the places I directed,

*It is the pinus Deodára of Roxburgh; the Dévadáru of Sanscrit writers.-H. H. W, New Series, VOL, IV,

D

I left every article of baggage I could possibly do without, and having given very light loads to the coolies that they might proceed with less difficulty, we marched from Reital on the 21st of May.

On the 27th we reached the Soar river, from whence to immediately above Tawarra, the path is exceedingly rugged, over broken masses of rock. The whole is an ascent; and in some places, very steep open precipices to the right, and high rocks above to the left; precaution is required in the footing, and some places are very unpleasant to turn, where it is adviseable to go bare footed.

The mountains are of granite, with various proportion of quartz and feldspar, of which I have specimens. Heavy rain both on going and returning; could not get a latitude. Water boiled at 1980, the temperature of the air being 67°.

At the village of Tawarra, direction of the small lake called Cailae Tal, whence the Dinni Garh river issues 71°. It is said to be 50 yards in diameter, but deep, and is formed by the melting snow; there is a small piece of level ground near it to which the villagers drive their sheep to pasture in August.

Descent through the fields and down the Dell steep and slippery. Rhoh (or Rhai) pines and the mohora, a species of oak, grow here..

*y,

Descent to the Elgie Garh torrent; cross it by a sangha 15 feet long. Granite rock in large blocks, with quartz nodules and bands in the bed of the stream.

Cross Camaria Gadh (rivulet), eight paces wide.

Down the narrow glen of the rivulet to its junction with the Ganges; the whole a descent, and in many places bad and difficult, over large blocks of rock which have fallen from above, and overturned and shattered all the trees in their course. The granite precipices which confine the river at this place have split and fallen in large masses into the bed of the stream. ..Path along the side of the Ganges, but above it a cascade opposite, falls 800, but not in one sheet, river up to 6°; path rocky. VA

Across the river and on its steep bank is a range of hot springs; they throw up clouds of steam, and deposit a sediment of a ferruginous colour; these are the first hot springs I have observed on the Ganges; the river not being fordable, we cannot go to them.

Huge blocks of rock fallen to left.

Climb over and under the ruins of a most tremendous fall of the precipices; blocks of granite from 100 to 150 feet in diameter are thrown on each other in the wildest and most terrific confusion; the peak whence they fell is perpendicular, and of solid rock. This fall took place three years ago.

Cross the Ganges by a sangha made of two stout fine spar laid from rock to rock. It is a good bridge of the kind, and