was towards the north. With these data he set to work to account for this particular arrangement, which he explains in nearly the following terms:-When a little gravel, from any cause, accumulates behind an elevation upon the surface of a glacier, it by degrees sinks down in the ice, and this produces a small hollow, or a miniature kind of basin. But as gravel absorbs much more caloric than ice, it follows, that it will be on the side where the sun's rays strike with the greatest intensity, and for the longest time, that this basin will most enlarge, and the gravel sink the deepest. Now, this side must necessarily be the northern, and hence the reason that every basin has its convexity turned to the north.

These small basins prove not a little useful to the mountain traveller, in affording him facilities for piloting himself with considerable certainty in spite of fogs, and even of ascertaining the hour of the day, however obscured the sun may be. For these objects, he has only to place his pole, or walking-staff, over the cavity, in such a way that it touches, on one side, the summit of the small icy elevation, and, on the other, the summit of the arc, and the line which his staff forms, will lie precisely north and south. A line at right angles to this, will consequently lie east and west. The meridian being thus known, it will be easy for him approximately to learn the hour, from the angle which the sun forms with the meridian line. It is, accordingly, on account of the facilities which these small excavations present on this point, that we designated them Meridian cavities. In Germany, they are called Kellerlöcher, in honour of our friend M. Keller.

Observations upon Temperature.-The experiments upon temperature were those which M. Agassiz prosecuted most zealously during his sojourns in the years 1841 and 1842. Not only were the variations of the external temperature made the subject of daily observations, often repeated from hour to hour, but he was especially anxious to observe what was the state of the temperature in the interior of the glacier. We have, on a former occasion, remarked, that many shafts or bores were cut with this object in view, to the depths, one of 100 feet, and another of 50, and several to the depths of 15, 20, and 25 feet. The bore to the depth

of 200 feet, whereby we had hoped to reach the bottom, in the year 1842, was also exclusively used for this purpose, during the last eight days of our sojourn. Every evening we introduced a self-registering thermometer to the bottom of each of these bores, respectively 200, 100, 50 feet deep, and occasionally into a fourth 20 feet deep. There they were allowed to remain during the night, every care being taken, by means of such things as sheep-skins, blankets, &c. covered over again with plates of ice, to prevent the ingress of the external cold. Next morning, usually about six o'clock, MM. Agassiz and Girard went to examine the thermometers, both those on the surface, and the others in the ice. These experiments were repeated in 1841, for a period extending to nearly six weeks, and in 1842, during two consecutive months. Often also we reintroduced the instruments, leaving them during the day, and examining them in the evening, before the temperature fell below 32° F. We will not here, of course, enumerate the details. Suffice it to state, that the general result has been confirmatory of what the observations of the year 1840 had shewn, namely, that the temperature is nearly constant in the interior of the glacier. We have scarcely seen it oscillate beyond the limits of three-tenths of a degree; in other words, the self-registering thermometers have never indicated a higher temperature than 32° F., or a lower temperature than -0°.3 (31°.46 F.) and this when the external temperature was as low as 5° (23° F.), and even 6° C. (21°.2.) Generally, the index stood precisely at 32° F.

But M. Agassiz was unwilling to limit himself to daily observations; for however important these might be, they supplied the temperature of the glacier only at a determinate epoch, namely, at that corresponding to the hottest season of the year. But where were the grounds for supposing that the temperature was the same at the other seasons of the year ? And admitting that the night cold did not exercise any marked influence upon the interior of the glacier, did this authorize us to conclude, that the colds of winter, so severe and prolonged in these high regions, have not a greater influence upon the temperature of the glaciers? These objections had been foreseen in 1841, and M. Agassiz has publish

ed an account of the placing of the thermometers, in Jameson's Journal, vol. xxxiii. p. 277.

*

Made wise by previous experience, we determined to be more careful in our use of means for withdrawing the second thermometrograph, which was twelve feet beneath the first. Having noticed the quantity of ice which daily disappeared from the surface, M. Agassiz determined to postpone any attempt, until near the termination of our stay. In fact, when we again thought of the undertaking, three weeks later, the surface of the glacier had experienced a sinking of many feet of ice, and our thermometrograph was so much nearer the surface. We now set to work, using the most cautious and sure means, and carefully superintending the workmen who were engaged in the service. M. F. Keller of Zurich, who at the time was of the party, imagined, that by heating iron bars, which might be introduced into the tin case, we might possibly easily melt the cylinder of ice. Two iron bars, therefore, an inch in diameter, were at the same time introduced into the tube, and around them was constructed, by means of large slabs, a kind of fire-place, in which a fire was lighted. The two bars were thus in the midst of a brazier ; but although they were thereby heated to a very considerable temperature, it was found that they sank exceedingly slowly, so that at the close of the day, they had not melted even two inches of ice. This proceeding, therefore, was much too tedious, as well as very costly, from the quantity of wood which was required. We were forced, then, to return to the use of boiling water; and with this, it required no less than five days to melt the ice in the ice-cylinder, until we reached the thermometrograph. At last, however, we succeeded in detaching it, and it was with the keenest impatience that we waited for its being brought up. Our satisfaction was great when it reached us unhurt. The index was at — 0°.3. C. (31°. 46 F.) We immediately set to work to verify the zero point of our instrument, which we found perfectly correct. It is worthy of remark, that this indication — 0°.3, corresponds to the lowest which was given during the observa

* The experiment with the first thermometrograph failed, as stated in this Journal, vol. xxxiii. p. 402.

tions which were made during the night, at the bottom of the shaft; so that it really appears that this is truly the lowest temperature to which the glacier is subjected.

That this observation may not rest isolated, M. Agassiz has taken measures whereby it may be verified. Before quitting the glacier in 1842, he again introduced two of Bunten's thermometrographs, and a small horizontal alcohol minimum thermometrograph into the glacier, at different depths. These two thermometrographs are in the same bore,—the one at the depth of fifteen feet, the second eight; the thermometer is at seven feet below the surface. We may hope thus completely to confirm next summer the observation of the last.

Observations upon the presence of Water in the interior of the Glacier. The presence of water in the interior of the glacier constitutes one of the essential postulates of the theory of infiltration. It is clear that a quantity of water is continually engulphed within the glacier, as is proved by the numerous streams and rivulets which are nearly all lost in the crevasses previous to their having attained the extremity of the glacier. This, however, does not amount to a proof that this water quite penetrates throughout the mass, as it would do through< a sponge. On the contrary, it appears more natural, at the first glance, to suppose that it would always flow straight on, and especially when we consider how exceedingly compact the ice everywhere is, when defended against the action of external agents. It was, therefore, a matter of great importance to determine, by direct experiments, that water was to be found in the interior of all ice; and the more so, as many scientific men, maintaining that the water did not penetrate beyond a certain depth, founded on this belief an objection against the theory of infiltration. Mr Hopkins was one of these, supporting his opinion in his learned memoir, entitled, Theoretical Investigations on the motions of Glaciers. The bores which were opened in the ice by M. Agassiz supplied a rare and happy occasion for making direct experiments upon this subject, previous to his determining to make experiments by means of the infiltration of coloured liquids. The results obtained by M. Agassiz, and detailed by him in Jameson's Journal, vol. xxxiii. p. 262, have been confirmed in the most satis

factory manner, by the account given of the experiments on infiltration in a former article.* Hence, it has now been demonstrated, that during the summer the whole mass of the glacier is thoroughly imbued with water at all depths to which we have hitherto reached; but in a manner somewhat unequal, according to the character of the ice, the state of the atmosphere, and the epochs and hours at which the observations are made.

After this, is it not astonishing to observe that a naturalist has recently affirmed that he has satisfied himself by direct experiment, not only that the glacier does not imbibe water, but, also, that it does not even contain moisture? The following is the experiment: M. Hugi, for he is the individual in question, caused a canal or tunnel, 10 feet long, 3 inches high, and from 7 to 9 inches wide, to be excavated beneath a pool of water which he found on the surface; into this tunnel he introduced several tin-boxes, filled with the chloride of lime and other matters, which have a great attraction for water, the lids of the boxes being pierced with holes, and the mouth of the tunnel being carefully closed. After remaining about fourteen. hours, it was found that the boxes had scarcely increased in weight, whilst similar ones exposed in the open air had become much heavier. Hence it was that M. Hugi concluded that there was no moisture in the glacier? But M. Vogt has already answered this argument, by remarking, that it is quite natural that water, at the temperature of zero (32° F.), enclosed within a narrow compass, should not disengage much vapour; and that if M. Hugi had introduced a sponge instead of the chloride of lime into his tunnel, he would have found it soaked with water derived from its icy walls. I should not have alluded to this experiment, had it not been made by a naturalist who has travelled extensively over the glaciers. Moreover, it is the only experiment which M. Hugi mentions in his last work.

Observations regarding the advance of the Glacier.—The measures which were begun by M. Hugi at the glacier of the Aar, and prosecuted by M. Agassiz during the earlier years of his

* See Bibl. Univ.; Mars 1843 (Tome xliv.), p. 131; and this Journal, vol. xxxv. p. 290.