*1
Dearee, West Bank of the Soane River; Main road to Benares. Feb[ruary]. 15th 1848
My dear Prof[essor]. Wheatstone
The kindness you showed me while I was preparing my outfits for India, encourages me to address you, when anything, which I think may be of consequence in Physics, occurs during my travels. You are aware that Botany is my first object; & that my observations on other branches of science must be somewhat desultory: Still, I have been able to keep a tolerable Meteorological Register, since leaving Calcutta *2; the results of which may be of use, as coming from parts of the country, whose climate has never been investigated. I quitted England, you may remember, in the 2nd week of November in HMS "Sidon" with Lord Dalhousie & arrived in Egypt on the 4th Dec[embe]r. During our 3 days' stay at Cairo, I made a few observations on the effects of the sun's rays on the soil, on the depth to which the heat penetrated, & also on the power of nocturnal radiation & thickness of soil through which the heat is radiated. In all these investigations, I find the great difficulty to consist in choosing a position where the instrument itself is screened from radiation. Limestone, Sand, and Sandstone-rock, in the Desert, have all different temperatures; & except a brisk wind be stirring, they give very different results. At the Great Pyramid I selected two stations, which I thought unexceptionable, one at the N[orth]. face of the N[orth].E[ast]. angle, the other at the W[est]. face of the N[orth].E[ast]. angle, & I was mortified to find as much as 5 1/2° difference in the temperature, & several in the Dew point, &c. At each angle I shifted the Instruments from one to the other face, with the same results. At the summit there was considerably more vapour in the atmosphere than at the base. The temperature of the two chambers agreed (78°).
In the Desert, midway between Cairo and Suez, I found

a little before sunrise after a very cold night, the Dewy surface to be cooled down to 44° (Therm: in shade 47°) & the increase of the temperature to be 1° an inch, down to 10 inches. Similar soil, on the previous & succeeding days, was heated, (at 3 P.M.) to about 80°, & the power of the suns rays penetrated to more than that distance.
At Suez we embarked onboard H[onorable]. E[ast]. I[ndia]. Co[mpany]. St[ea]m Frigate, "Moozuffer" for Calcutta: & I noted three times a day the temperatures, Dew point, &c, but not the Barometer; for my "Newman's Portable" jumped so much as to render impossible to observe within 2/10 of an inch: this was owing to the great power of the engines. Some of the phenomena are very curious. In the first place, the waters of this gulf are salter than those of any other sea having a free communication with the greater Oceans, & contain 3/10 more salt in an equal bulk than those of the Indian Ocean. This high specific Gravity decreases on the passage down to Mocha, where the increase is diminished to 2/10ths, & suddenly to the usual standard of sea-water. My attention was first drawn to this by the chief Engineer, with whom I conducted such experiments as the motion of the vessel allowed. During Ross' voyage I had frequently examined the water (with Capt. Ross); &, whether from the various oceans over which we passed through, or different depths (down to 800 fathoms) in those oceans, I always obtained a very constant quantity of salts. I also enquired about the water of the Persian Gulf, & am assured that they do not differ from those of the Indian Ocean. From the Straits of Babelmandel to Cape Comorin, I perceived no difference.
There are three classes of winds in the Red Sea, very remarkable in their distribution. During all June, July, Aug & Sept. a north wind prevails throughout the sea; produced I suppose, by the heated continents of Arabia & especially Africa; & the same wind continues all the year round from Suez to the Straits of Jibbel,

& with particular force down the gulf of Akabar. During the remainder of the year the winds in the middle part of the sea, from Jibbel Teer 15° " 30" to 19° or 20° 21° North, are light and variable. In South part again from Jibbel--Teer to the Straits: the S[outh]E[ast] wind is constant from October till May: increasing in violence as you approach the straits. This we experienced ourselves; for we carried N[orth]. & N[orth].E[ast]. winds from Suez to Latitude 20° variables from Latitude 20 to Jibbel--Teer, & Suez south from Jibbel Teer to the Straits. I do not know how far the accompanying phenomena may account for the great saltnes & well known depression (below the level of the Mediterranean, & of the Straits amounting if I remember aright to 35 odd feet,) of the upper part of the sea: my observation gives the following results:
Mean temperature Wet bulb Thermometer D[ew].P[oint]. vapour in cub[ic]. feet calculated evaporation
air sea
Suez to Lat[itude]: 20 76 -- 1 78 -- 0 68 -- 3 64 -- 1 6 -- 841 1 -- 56
Lat[itude]. 20 -- Jibbel--Teer 81 -- 6 80 -- 4 74 -- 5 71 -- 4 8 -- 478 1 -- 38
Jibbel Teer to Straits 80 -- 3 76 -- 0 70 -- 2 65 -- 0 4 -- 311 2 -- 61

The perennial N. wind of the upper potion, may of itself reduce the level, it is further a drier wind & effects more evaporation from the surface, than do the winds of the middle portion, at which it arrived loaded with vapor & increased in elasticity. Whatever evaporation takes place at the South portion, during the dry South wind, maybe compensated by an indraught from the Indian Ocean. The central portion again, during the same season, receives the loaded currents from either quarter, which its high temperature enables it to retain, its elasticity also being very considerable.
I fear, however, that this may be a familiar subject to you but my library is limited: the maps in the 2nd Edition of Daniell being my only guide; & there are very inaccurate as regards the Red Sea.

Few other phenomena of importance occurred to me during the voyage; except a curious variety, as I suppose it, of the crepuscular arch, which I witnessed for two nights after leaving Madras *3 Roads. The first I saw on Jan[uar]y 9th at 6 3/4 while still in view of land: it lasted hardly a minute after I first observed it & appeared like a broad lunar rainbow, over the sun's position, & about 70° altitude. On the following evening I looked out for it: we were then some 150 miles on our course to Calcutta. At 3/4 hour after sunset, a pale milk--white arch, with the faintest tinge of purple, appeared at 60° altitude. It was about 8° broad: the north end rested on a very faint Cirrhus altitude 30°. The southend descended lower, but did not touch the horizon: its limits were not clearly defined: it rose rapidly & disappeared in about 3" or 5" on reaching the Zenith. The days had in both cases been very fine & clear, the sky at the time deep blue grey, with a peach--blossom tinge (or twilight) resting on a yellow horizon. This peach--color is a very common tropical sunset, & for delicacy of tint unequalled. A t Aden, where it contrasted with the stern pitchy dark crags of that Peninsular, & deep blue of the Ocean, it produced the finest sunset effect I ever witnessed.
All this time, however, I have not told you where I am, & what doing. After a fortnight in Calcutta, I started to join Mr Williams, who is on his way to survey a coal--field on the Soane River, whose banks we have reached, & up which we immediately proceed. My objects are purely Botanical; but hope, by the careful use of good instruments, to obtain data for calculating the effects of climate on the vegetation of large areas. We have travelled W[est]. from Burdwan, over an elevated table--land gradually rising: its culminate point is Paris--Nath [Parasnath] *4, the highest mountain

in lower Bengal: this I ascended, but have no table or rule for calculating the heights.
At the Dunwar pass, some 90 miles east, there is a rapid descent of about 800 feet, to a level plain which reaches to the Soane. We now follow up the Soane, from whence I go to Mirzapore & return to Calcutta, for a few days, previous to starting for Sikkim in the East Himalayah. I take observations as nearly as I can at 9 A.M., 3 P.M., 9 P.M. & between 2 A.M. & sunrise; but in the morning I cannot be exactly punctual being often up till midnight, & always on the march long before dawn. The barometer I observe on all occasions, "Newman's Portable" is excellent; but I must carry it everywhere myself to be comfortable about it. When the weather is very hot a few globules of mercury escape at the cistern.
Terrestrial radiation I observe in various way, night & morning, on the surface, Earth, grass, leaves & at such depths as I can conveniently plunge the Thermometer guarded with Pith. I have twice had bores made of 3 or 4 feet at places 14' apart, & in both cases had a constant temperature of 72° for 15 hours of afternoon and night: but this alluvium is often too hard to bore with common tools; it always takes 6 hours & 6 men to work the jumper. I guard the bulb with pith, & sink it in a brass tube. The dryness of the upper plains we travelled is wonderful, during these N[orth]. W[est]. winds. I have been very careful with the wet--bulb observations. Solar radiation is all but impracticable: I persevere in the Black bulb & wedge of glass Photometer, made, as you recommended, by "Darker"[.]
Last night I saw the best developed aurora I ever witnessed, taking brightness, extent of surface covered, & length of continuity of beams, into account. Never in Scotland, where I have

observed many, or the South Polar regions, where also these phenomena were frequent, have I noted one, so altogether good as this. The moon spoiled it sadly; though its beams were brilliantly defined within 8° of her orb on each side. I send you the observations I took of it, with a good quadrant & compass, from my first seeing it, till it had nearly disappeared at midnight. I have also transmitted an account, to be published in Calcutta, & hope it has attracted notice elsewhere.
There is no change since in the weather, but much cirrhus since noon today, which is unusual, though possibly owing to the hills we are now near, & which are new features on our landscape.
Monday, Feb 14, Barroon, East bank of Soane River. 9 P.M. Bar. 29. 924. A.H.T. 68. Tp. Air 62. Wet Bulb 51.5. Grass 53. Blue sky & clear horizon. Moon & stars clear, milky way invisible. Zodiacal light D[itt]o moon by Photometer 3.07 inch: inch: (sun at 3 P.M. being 4.17 inches).
Observed the Auroral Arch well defined, 12° broad, altitude of upper limb (best defined) 20°. Extremes bearing W[est]. 20 S[outh]. & N[orth]. 50 E[ast]. Light pale, but bright, resting on an arch no darker than sky at zenith. Beams crowded, from 20--30, linear & lancet shaped, crossing the zenith & converging in opposite horizon towards S[outh]. 15 E[ast]. All beams bright, clear, & well defined, moving slowly, forked at the apices, or split from apex to zenith, almost obscuring stars of 1st magnitude. Longest beams point to S[outh]. 10 E[ast]. descending to 25 altitude. Middle beam broad, crosses zenith, points S[outh]. 50 E[ast]. & descends to 40° N[orth].W[est]. beams almost parallel to horizon, point S[outh]. 70 E[ast]. & descend to 20°[.]
10 P.M., General appearance more diffused, upper limb of arch less defined. No beams cross the zenith. Two detached ones 15° above horizon at S[outh]. 15 E[ast].: after a few minutes one beam

appeared on zenith.
10:15, Appearance to W[est]. of N[orth]. as before. one beam on zenith, two cross the meridian, one to S[outh]. 30 E[ast]. at 15° above horizon, which disappears towards the arch in S[outh]. E[ast].. Arch more diffused & descending to horizon, forming a pale mass altitude 25°. Beams broader, shifting & splitting more frequently. Soon a dark horizontal band 4° broad, crosses the arch, extending from N[orth]. 55 W[est]. to N[orth]. 10 W[est].: upper limb 12° altitude: it appears as a break in the auroral arch. Whole horizon, all round, covered with a pale diffused light, strongest towards arch & in opposite quarter. Beams still clear; the lateral broadest & best defined. Dark band becomes broader, breaking up the arch.
10:30. Beams from arch still clear, linear, 2°6° broad, about 12 in number; none reach the zenith: a few lateral ones cross the moon's meridian; the upper approaching within 8° of her orb, & still well defined. N[orth]. E[ast]. beams most crowded: N[orth]. W[est]. best defined & broadest. Dark band broader, severing the arch. Whole Phenomena fading: longest & brightest & most numerous beams stretching along N[orth]. E[ast]. horizon. 10:50 still fading. Beams & arch all disappearing to W[est]. of N[orth].: 18 narrow beams between N[orth]. & W[est]. 20 E[ast]. from remains of arch. Cold southerly breeze sprung up.
10:55. Breaking up, as before. 11 P.M. Diffused light over all horizon, (possibly reflection of the moon's light on ground mist, which, however, is not discernible). Scattered beams like cirrhus here & there: linear along the N[orth]. & N[orth]. E[ast]. horizon.
Midnight. Two faint beams to the N[orth]. E[ast]. & strongly define lance shaped, a parallel ones to S[outh]. W[est].
Should you think this account of any value, perhaps you

will kindly communicate it to the Philosophical Club & the Athenaeum or make whatever use of it you deem proper. I shall not fail to collect any notices of the Phenomena that may occur in India.