NATUKAL PHILOSOPHY. 1 G7 



ence of the solar heat upon the ground, and the consequent circulation of 

 small- atmospheric currents, it affords a very satisfactory confirmation of the 

 trustworthiness of the photographical method of registi'ation. 



Prof. H. also showed, that the decrease of temperature in ascending through 

 the atmosphere depended not only on height above the sea level, but also 

 upon the absolute height above the nearest surface of solid land. In this 

 way the decrease of temperature over plains, mountains, and plateaus, would 

 be necessarily very different ; and we cannot immediately infer the state of 

 the phenomena in the two latter instances from what may exist in a former. 



Admiral FitzRoy thought that one circumstance was too much over- 

 looked by Prof. Hennessy in these researches, namely, that along with these 

 ascending currents the whole body of the air was carried along by horizontal 

 currents, so that it could not be assumed that it was the very same air which 

 gave some of the indications which afforded the others. Again, it had been 

 clearly shown that a thermometer placed upon the ground, or close to it, fre- 

 quently fell 17 or 18 below one placed a few feet or inches above it, while 

 somewhat higher up still, the indications of the thermometer again fell; thus 

 clearly indicating a spot at which there was a maximum temperature. As to 

 the latter part of what he stated, it was so commonly observed, that if you 

 placed a thermometer in the lower window of a house, and another in the 

 window immediately above it, in nine cases out of ten you would find the 

 latter indicate a lower temperature than the former. Prof. Stevelly said 

 that, besides what Admiral FitzRoy had pointed out, there were two other 

 circumstances of much importance to be attended to in such observations as 

 Prof. Hennessy had been making. First, that evaporation was going on 

 more or less rapidly according to the circumstances of the locality Avhcre the 

 observations were conducted. Secondly, that the air, w r hen having either 

 gradually, as in some cases, or abruptly, as in others to ascend in its course 

 very elevated ground, was compelled to contract in volume, become con- 

 densed, and yet, in some cases, part w r ith a portion of its vapor, and thus 

 form the cloud which we so often saw capping the hills, as well as giv- 

 ing origin to the high winds and storms which so frequently prevailed 

 there. Dr. Tyndall said that in Switzerland, on the tops of high mountains, 

 he had a full opportunity of witnessing these phenomena on a scale of 

 grandeur truly sublime. The snow in these regions was naturally as dry as 

 dust, and he had frequently an opportunity of witnessing columns of it 

 whirled up to an immense height by the ascending currents of air, into 

 regions where it was soon dissipated or melted, and dispersed into vapor. It 

 was also to be observed that the sun's heat had a power of penetrating water 

 and other screens, such as the clouds formed, far surpassing that possessed 

 by heat derived from less intensely ignited or heated sources, as, for instance, 

 from bodies heated red-hot, or from vessels filled with hot water and the like. 

 Hence the sun's rays, though they penetrated the clouds and the earth, yet 

 there they totally lost their former powers, and when radiated back possessed 

 no such power as before of penetrating clouds or other screens ; and thus the 

 earth and its atmosphere became a kind of trap for the solar rays. 



OX THE DISTRIBUTION OF HEAT IN THE INTERIOR OF THE EARTH. 



In a paper submitted to the British Association, 18-38, b} 7 Dr. Siljestrom, of 

 Stockholm, on the distribution of heat in the interior of the globe, the author 

 stated as his belief, that the interior of the earth was occupied by currents of 



