This average daily descent gives for the whole month of August a descent of 863288. If the average daily variation of temperature of the month of August had continued throughout the year, the lead would have descended 10-19148 inches every year. And in the two years from 1851 to 1853 it would have descended 20-38296 inches. But the daily variations of atmospheric temperature are less in the other months of the year than in the month of August. For this reason, therefore, the calculation is in excess. For the following reasons it is in defect:-1st., The daily variations in the temperature of the lead cannot but have been greater than those of the surrounding atmosphere. It must have been heated above the surrounding atmosphere by radiation from the sun in the day-time, and cooled below it by radiation into space at night. 2ndly., One variation of temperature only has been assumed to take place every twenty-four hours, viz. that from the extreme heat of the day to the extreme cold of the night; whereas such variations are notoriously of constant occurrence during the twenty-four hours. Each cannot but have caused a corresponding descent of the lead, and their aggregate result cannot but have been greater than though the temperature had passed uniformly (without oscillations backwards and forwards) from one extreme to the other. These considerations show, I think, that the causes I have assigned are sufficient to account for the fact observed. They suggest, moreover, the possibility that results of importance in meteorology may be obtained from observing with accuracy the descent of a metallic rod thus placed upon an inclined plane. That descent would be a measure of the aggregate of the changes of temperature to which the metal was subjected during the time of observation. As every such change of temperature is associated with a corresponding development of mechanical action under the form of work *, it would be a measure of the aggregate of such changes and of the work so developed during that period. And relations might be found between measurements so taken in different equal periods of time -successive years for instance-tending to the development of new meteorological laws. Mr. JOULE has shown (Phil. Trans., 1850, Part I.) that the quantity of heat capable of raising a pound of water by 1° Fah. requires for its evolution 772 units of work. THE DESCENT OF GLACIERS. The following are the results of recent experiments* on the expansion of ice : Linear Expansion of Ice for an Interval of 100° of the Centigrade Thermometer. Ice, therefore, has nearly twice the expansibility of lead; so that a sheet of ice would, under similar circumstances, have descended a plane similarly inclined, twice the distance that the sheet of lead referred to in the preceding article descended. Glaciers are, on an increased scale, sheets of ice placed upon the slopes of mountains, and subjected to atmospheric variations of temperature throughout their masses by variations in the quantity and the temperature of the water, which, flowing from the surface, everywhere percolates them. That they must from this cause descend into the valleys is therefore certain. That portion of the Mer de Glace of Chamouni which extends from Montanvert to very near the origin of the Glacier de Léchaud has been accurately observed by Professor James Forbes. † Its length is 22,600 feet, and its inclination varies from 4° 19′ 22′′ to 5° 5′ 53′′. The Glacier du Geant, from the Tacul to the Col du Geant, Professor Forbes estimates (but not from his own observations, or with the same certainty) to be 24,700 feet in length, and to have a mean inclination of 8° 46′ 40′′. According to the observations of De Saussure, the mean daily range of Reaumur's thermometer in the month of July, at the Col du Geant, is 4257, and at Chamouni 10°.092. The resistance opposed by the rugged channel of a glacier to its descent cannot but be different at different points, and in respect to different glaciers. The following passage from Professor Forbes's work contains the most authentic information I am able to find on this subject. Speaking of the Glacier of la Brenva he says::-"The ice removed, a layer of fine mud covered the rock, not composed, however, alone of the clayey limestone mud, but of sharp sand derived from the granitic moraines of the glacier, and brought down with it from the opposite side of the valley. Upon examining the face of the ice removed from contact with the rock, we found it set all over with sharp angular fragments, from the size of grains of sand to that of a cherry, or larger, of the same species of rock, and which were so firmly * Vide Archir, f. Wissenchaftl Kunde v. Russland, Bd. vii. s. 333. fixed in the ice as to demonstrate the impossibility of such a surface being forcibly urged forwards without sawing any comparatively soft body which might be below it. Accordingly, it was not difficult to discover in the limestone the very grooves and scratches which were in the act of being made at the time by the pressure of the ice and its contained fragments of stone" (Alps of the Savoy, pp. 203-4). It is not difficult from this description to account for the fact that small glaciers are sometimes seen to lie on a slope of 30° (p. 35.). The most probable supposition would indeed fix the limiting angle of resistance between the rock and the under surface of the ice set all over, as it is described to be with particles of sand and small fragments of stone at about 30°; that being nearly the slope at which smooth surfaces of calcareous stone will rest on one another. If we take then 30° to be the limiting angle of resistance between the under surface of the Mer de Glace and the rock on which it rests, and if we assume the same mean daily variation of temperature (4.257 Reaumur, or 5.321 Centigrade) to obtain throughout the length of the Glacier du Geant, which De Saussure observed in July, at the Col du Geant; if, further, we take the linear expansion of ice at 100° Centigrade to be that (00524) which was determined by the experiments of Schumacher, and, lastly, if we assume the Glacier du Geant to descend as it would if its descent were unopposed by its confluence with the Glacier de Léchaut; we shall obtain, by substitution in equation (2.) for the mean daily descent of the Glacier du Geant at the Tacul, the formula The actual descent of the glacier in the centre was 15 feet. If the Glacier de Léchaut descended, at a mean slope of 5°, singly in a sheet of uniform breadth to Montanvert without receiving the tributary glacier of the Taléfre, or uniting with the Glacier du Geant, its diurnal descent would be given by the same formula, and would be found to be '95487 feet. Reasoning similarly with reference to the Glacier du Geant; supposing it to have continued its course singly from the Col du Geant to Montanvert without confluence with the Glacier de Léchaut, its length being 40,420 feet, and its mean inclination 6° 53', its mean diurnal motion 7 at Montanvert would, by formula (2.) have been 2.3564* feet. The actual mean daily motion of the united glaciers, between the 1st and the 28th July, was at Montanvert †, * On the 1st of July the centre of the actual motion of the Mer de Glace at Montanvert was 2.25 feet. † Forbes'"Alps of Savoy," p. 140. U U |