Nov. 25, 1875] 



NATURE 



67 



Through the kindness of the Hydrographer of the Admiralty, 

 I have been favoured with all the observations made in the 

 Challenger of the specific gravities of the Atlantic at intermediate 

 depths between surface and bottom. From these observations it 

 will be seen that there is scarcely any sensible difference between 

 the mean specific gravity of the equatorial and the two Atlantic 

 columns. 



The following Table shows the mean specific gravities of the 

 three columns : — 



The mean specific gravity of the equatorial column as proved 

 by the two soundings III. and IV. in the Table is I "026202 ; 

 and I "02621 1 of sounding I. of the Table may be regarded as 

 the mean specific gravity of the North Atlantic Column A, for 

 the observations were made at a place on the same latitude, and 

 only about two degrees to the east of that column. Conse- 

 quently the specific gravity of Column A exceeds that of the 

 equatorial by only "000009, * quantity which does not amount 

 to one inch in 1,500 fathoms ! Sounding No. II. of the Table, 

 made at a place a few degrees to ths east of Column B of the 

 section, gives i"02623, which may be regarded as the mean 

 specific gravity of that column, and the more so as another 

 sounding made in this region gives identically the same mean 

 value. The difference between the Equatorial Column and 

 Atlantic Column B in lat. 23° N. therefore amounts to only 

 •000028, or 3 inches in 1,500 fathoms. It must of course be 

 observed that as the specific gravities in the table are not taken 

 at equal intrrvals the mean of the figures does not represent the 

 mean specific gravity of a column. The number of fathoms 

 represented by each separate value must be taken into account in 

 determining the mean value of a column. 



My result is, therefore, not materially affected, even after I 

 have thus taken into account difference of salinity, and computed 

 the amount of expansion according to Prof. Hubbard's Table. 

 The surface of the North Atlantic in lat. 38" to be in static equi- 

 librium must be 3 feet 3 inches above that of the equator, and in 

 lat 23", 2 feet 3 inches above it. 



It is perfectly true that according to the gravitation theory 

 the ocean is never in a state of static equilibrium, but it must 

 be observed that as the surface-flow according to this theory is 

 from the equator i)olewards, it is the equatorial column that is 

 kept constantly b:loA7 the level necessary to static equilibrium ; 

 hence, were I to make allowance for want of static equilibrium, 

 I should make the slope greater than 3 feet 6 inches. Dr. Car- 

 penter's objection that the lorce of my argument rests on the 

 assumption that the sea is in equilibrium is based on a misappre- 

 hension of the problem, for in reality, by not making allowance 

 for want of equilibrium, I give his theory an advantage which it 

 does not deserve. Were the surface-flow from the North Atlantic 

 to the equator, there would then be some force in his objection, 

 for by leaving out of account want of equilibrium I would be 

 making the slope greater than it should be. Dr, Carpenter 

 states that his objection met the approval of Groieral Strachey 



and Sir William Thomson at the British Association meeting. 

 If it did, it shows that they must either have misapprehended my 

 argument or his objection to it. 



I have again to remind Dr. Carpenter that "viscosity" can 

 have nothing to do with the question at issue. The water has 

 to flow up the "gradient," ancl that by means of gravity. This 

 is mechanically impossible, whether water be viscous or not. 



It is needless to quote the opinions of Lenz, Arago, and 

 Pouillet. They were not in possession of sufficient data to 

 enable them to determine the question with certainty. The 

 question, be it observed, is not ' ' Can difference of temperature 

 produce circulation ? " Everyone will admit that were there no 

 other agencies at work but equatorial heat and polar cold, a dif- 

 ference of temperature would soon arise which would induce and 

 ■ustain a system of circulation ; but this condition of things is 

 prevented by the equatorial waters being swept away by the 

 ■winds as rapidly as they are heated. I submit that I have proved 

 that this is the case in reference to the Atlantic. If I am wrong, 

 let it be shown where my error lies. James Croll 



Edinburgh, Nov. xo 



Refraction of Light and Sound through the Atmosphere 

 There is in Upper Thibet a plateau called the " Kyan Chu 

 Plain," on which phenomena ot mirage are frequently seen. 

 The plain is at a height varying from 15,000 to 16,000 feet. A 

 cold wind comes down from the surrounding mountains, while 

 an exceedingly hot sun heats the ground. While marching 

 through this plain on Aug. 19 I saw the mirage in perfection, 

 A mountain in front of us, at a distance of about five miles, 

 appeared to be situated on the border of a lake of a deep and 

 rich blue. A shepherd with a flock of sheep seemed to wade 

 through the water, and the reflection of each sheep was most 

 distinct and sharp. The effect was so complete that one of my 

 companions proposed to leave the pool of water at the side of 

 which we had encamped for breakfast, in order to go to the 

 borders of the lake. 



I measured the temperature of the air at various heights from 

 the ground. The fo lowing readings were obtained : — 



Height above ground. Dry Bulb. Wet Bulb. 



5 feet 49'' 32° 



4 inches 55=' 38° 



I inch 56^ 39" 



The groini at that place was stony, and no accurate measure- 

 ment of its temperature could be taken. A few mdes further on, 

 however, a sandy ground was found to have a temperature of 90^. 



The difference between the temperature of the ground and 

 that of the air was painfully striking to me, as, owing to blisters, 

 I had to walk with bare feet. My feet felt burning hot, while 

 the remainder of the body was unpleasantly cold. The mirage 

 was seen in its greatest perfection at about 9 o'clock a. m. 



Such a condition of the atmosphere must, according to Prof. 

 Reynolds, prevent any sound from being heard at a great distance, 

 owing to Its refraction upwards. Such was really the case. A 

 rifle fired by the above-mentioned companion at a short distance 

 rem lined almost unheard. 



With regard to the question whether our better hearing at 

 night is due to the absence of disturbing noises, or to the cause 

 suggested by Prof. Reynolds, I wish to remark that the Upper 

 Himalayas are particularly free from any disturbing noises, yet 

 the increase in our power of hearing at night is most marked. 



Sunnyside, Upper Avenue Arthur Schuster 



Road, N.W., Nov. 20 



Evidences of Ancient Glacier Action in Central France 



Having read with much interest Dr. Hooker's contribution to 

 Nature on " Evidences of Ancient Glaciers in Central France," 

 I am tempted to send yoa a few remarks which may interest 

 those who look out for glacial phenomena wherever they travel. 



When travelling in Auvergne with Sir William Guise m 1S66, 

 we unfortunately missed ihe t'ansported erratics in the Tranteine 

 Valley, described by Dr. Hooker. We saw, however, examples 

 of what we believed to be ice-bome erratics, on more than one 

 occasion, and consulted M. Lecoq on the subject at his residence 

 at Clermont Ferrand. He had observed travelled boulders in 

 certain localities, but, as mentioned in the note-book of Sir 

 William Guise, "attributed to transport by snow many of the 

 effects generally assigned to glacial action." 



I would also ask attention to a subject which appears to me ot 

 considerable interest with regard to the age of the most modem 



