NATURAL PHILOSOPHY. 167 



He remarks that if this difference appears small to some readers, they will, 

 at least, admit that the solar rays have equal power on mountain-tops as in 

 the valleys beneath. The results of a scries of observations on the tempera- 

 ture of the soil on mountains lead to the conclusion that its mean tempera- 

 ture was greater than that of the air; while at "Brussels, and other low 

 situations, the mean temperature of the air was a little Higher than that of 

 the earth. He found that this warming of the earth on mountains was not 

 confined to the summer, but that between the twenty-first of September and 

 the first of October it was even greater. This relative heating of the earth on 

 mountains exercises an important influence on Alpine vegetation, and like- 

 wise drives the permanent snow-line to a higher region. But although the 

 soil of mountains is warmed, as well as that of the plains, the air is much 

 colder, and the nocturnal radiation much greater, so much so as to constitute 

 a proof that the earth must receive more warmth during the day, or its 

 temperature would fall lower than is observed. M. Martins remarks, that 

 on a plain the earth is only in contact with the lower stratum of the atmos- 

 phere, while an isolated peak, like the Faulhorn, is plunged into the aerial sea, 

 and radiates, not only towards the zenith, but in every direction, and the 

 process is favored by the rarefaction of the air. When mountains are 

 covered with snow their radiation is still more considerable, especially at 

 altitudes at which it never melts, and where it remains as a fine powder or 

 dust. Flocculent snow does not exhibit this great radiating power. Another 

 cause of the cooling of the earth and air on mountains is the great evapora- 

 tion which takes place, and which, other things being equal, is more active 

 than in the plains. Another cause is the dilatation of ascending currents, 

 owing to the diminution of atmospheric pressure; a subject upon which M. 

 Martins made numerous experiments, of which he tabulates the results, and 

 in which he imitated, as far as possible, natural conditions. Passing from 

 the question of thermometrical cold, M. Martins considers the reasons of the 

 sensation of cold experienced by travellers. Among these he reckons the 

 agitation of the air, which, he says, is never quiet on isolated peaks. He 

 likewise notices the effect of walking through the intensely cold, deep, pow- 

 dery snow, and the deficient supply of oxygen, through breathing rarefied air. 



ON THE EXISTENCE OF A LUNAR TIDAL-WAVE ON THE GREAT 



AMERICAN LAKES. 



The following letter has been addressed to the Editor of the Annual of 

 Scientific Discovery, by Lieutenant Colonel J. D. Graham, of the Topograph- 

 ical Engineers, U. S. A., Superintendent of Lake and Harbor Improve- 

 ments for the Northern Lakes. 



Until the announcement to the Topographical Bureau of the War Depart- 

 ment, contained in my annual report of November 15, 18 38, 1 and to the 

 Chicago Historical Society at its annual meeting on the 30th of that month, 2 

 the existence of a lunar tidal- wave on our great North American fresh-water 

 lakes had never been demonstrated ; but, on the contrary, it had very gene- 

 rally been denied. That announcement was based upon a series of daily 



1 See pp. 1107 and 1108 of vol. ii. Part 2 of Congressional (Executive) Doc., No. 2, 

 of the 3oth Congress, 2d Session. 



2 See Historical Magazine, vol. iii., p. 39, published monthly, at New York, by 

 C. B. Richardson. 



