Bois, had risen laterally against a neighbouring mountain, designated the Chapeau, which it enclosed on its northern aspect, threatening to make inaccessible a spot which is often visited by the curious traveller who visits Chamouni. During the spring of the year 1818, although the winter had been much more mild than severe, the glaciers had not ceased to advance, especially at their lower portions, in a way which alarmed the occupiers of the nearest dwellings; and this phenomenon occurred also at the glaciers both of the Tyrol and at those of Chamouni. Far, however, from continuing their progressive advance, in the following year, all these glaciers, after having undergone a process of arrest, actually retrograded to an extent which could not be mistaken, and resumed their ordinary dimensions, at all points; as prolonged observation teaches us to expect. The extension and the succeeding retreat depended upon this,--that the years 1816 and 1817, after having been extraordinary years, in respect of the enormous quantity of rain and snow which fell, had been followed by seasons which were rather dry than wet, or which, at least, were below the average, as to the quantity of rain. And, if the occurrence of two consecutive years more humid than ordinary sufficed to produce such a vast increase of the existing glaciers, it is not difficult to comprehend, that if a certain number of similar years had succeeded each other without interruption, then the glaciers of Chamouni, filling up the valley of the Arve, would have advanced, and finally would have penetrated to the valley of the Rhone. In fact, in circumstances analogous to those which produced the two consecutive years of 1816 and 1817, all we may say that is desiderated, so far as the mountains which produced them. are concerned, is, that winters should regularly succeed each other, and without intermediate summers; this result then would follow, that the accumulations of snow would soon become so enormous as to produce a most rapid progressive advance of glaciers descending from their summits.

After having thus explained the appearance and production of great glaciers, let us now consider how we may account for their disappearance. At the first glance it appears that the enormous humidity which had accompanied

the emersion of the formations which most recently appeared, should unceasingly perpetuate itself, evaporation being insufficient to disperse it, inasmuch as it would in its turn. produce as much more rain as it, the humidity, was itself considerable; and since, moreover, the winds could not in this matter change the condition of the atmosphere, seeing that the humidity alike prevailed throughout. It results, then, as a necessary consequence, that ere the conditions in which the vast amount of ice has been produced undergoes any modification, a certain absolute quantity of meteorological water should disappear. Well, then, it is not difficult to discover in a novel phenomenon which developed itself upon the whole of the emerged portion of the surface of the earth, which was unoccupied by ice and snow, a portion too, by much the most extensive, a cause of a gradual absorption of the water which impregnated these strata, and thereby saturated the neighbouring strata of the superimposed atmosphere. This phenomenon is that of the vegetation, which, of necessity would by degrees manifest itself upon the new soil which would be exposed to the light, and which would develope itself with a rapidity which would increase to the epoch of man's appearance, who, in spreading over the surface of the earth, would again contend with this extension, which hitherto had been unchecked throughout the vegetable kingdom.

According to the experiments and the calculations of M. Chevandier, one hectare (about two acres) of forest annually absorbs of oxygen and hydrogen a quantity equivalent to 1800 kilogrammes of water, a result which has been obtained by the chemical analysis of wood which was perfectly dry; for there can be no question concerning the hygrometric water which plants absorb and emit, nor concerning that which the sap, the leaves, and generally the humid portions of trees absorb. On the other hand, a cubic metre (yard) of air at the temperature of 10 degrees, contains, when it is saturated with moisture, 10 grammes of water; consequently, one hectare of forest consumes during one year the quantity of water which would saturate, at the temperature of 10°, a stratum of atmospheric air of one hectare of surface, and of

18 metres of height. Hence, it is easy to perceive that this quantity of water is much larger than that which would be necessary to saturate the whole atmospheric column with a base of one hectare of surface; for if it be true that this column be somewhat higher than 1800 metres, on the other hand, its mean temperature is much below 10°. Thus, in supposing that a portion only of the continents had been covered with forests, it is not less true that the water absorbed by these forests has been more than sufficient to produce a notable diminution in the humidity of the surface of the earth and of the atmosphere, and consequently in the evaporation, as in the cold produced by this evaporation, and in the quantities of rain and snow; effects which, in their combination, must have produced a gradual retreat of the glaciers. This retreat has ceased when, at the termination of a certain time, an equilibrium has been established between the action of the causes which determined the absorption of the meteorological water and the action of those which produced its formation. At the same time, there is no doubt that if, from any cause, the vegetation were to disappear from the whole surface of the earth at the same time, the existing phenomena would assimilate themselves to those of the epoch which immediately preceded and accompanied the formation of glaciers, and would still reproduce themselves, although with less intensity, the soil being devoid of that humidity which resulted from its recent emergence.

It is almost unnecessary to observe that this general influence which the presence of great forests must exert upon the meteorological state of the atmosphere, after the appearance of the most recent formations, is not to be confounded with the local influence which the actual clearance and establishment of forests produces, and which is of an opposite nature with the foregoing; they are, in fact, contrary phenomena, whose discrepancy it is easy to account for. Finally, whilst assigning to the cause which I have pointed out the principal agency in the retirement of glaciers, and in the determination of the existing meteorological condition of the atmosphere, I am far from pretending that it is the only one; and I am far from denying the influence of those causes

which M. Constant Prevost has pointed out. I only conclude as I began, by maintaining, that the existing causes amply suffice, without its being necessary to have recourse to a glacial era or period, to explain the fact generally admitted by geologists, namely, the successive appearance and disappearance of the ancient glaciers.

Meteorological and Astronomical Notices for December 1851. By Professor C. PIAZZI SMYTH, Astronomer-Royal for Scotland.

The Appendix to the second volume of the Observations of the National Observatory at Washington has recently been received here, and contains further interesting and important particulars with reference to Lieutenant Maury's important generalization on the motions of the atmosphere.

The most crucial and convincing proof of the truth of the Lieutenant's idea, that the trade-winds cross over into opposite hemispheres at the pole, instead of returning to their own poles, was mentioned in our last Notices, as being the fact of Ehrenberg having ascertained that in rain falling on the west coast of North Africa, and in Spain, certain dust had been dropped, which consisted mainly of infusorial animalcules of South American origin; this being precisely the direction which the trade-winds of the southern hemisphere, after getting up exhalations of all kinds in their passage over South America, should take on passing to the north of the equator.

It now appears that Ehrenberg had had sent to him specimens of this dust which had fallen at various periods between 1803 and 1849, and over various parts of NW. Africa, Spain, Portugal, France, and Italy, with the adjacent seas. The dust was almost always sent with the statement, that it was volcanic dust, or that it was the fine sand of the African deserts; while mere chemical analysis gave the following comparatively uninteresting and uninstructive table of contents :

To microscopic analysis, however, the contents were invariably found to be mainly organic, partly flint-shelled polygastric infusoria, partly fragments of flint earth plants (Phytolitharia), partly carbonaceous but uncarbonised fragments of other plants, and partly chalk-shelled Polythalamia; the Polygastrica and Phytolitharia evidently giving the flint earth, the Gallionella the iron and probably the manganese, and the Polythalamia the carbonic acid chalk earth.

In nine specimens of dust gathered at very different places and times, but of very similar constituents, there were found the following number of species:

Evidently, then, this was not volcanic dust, and it had moreover no symptoms of having been exposed to volcanic action.

And again, it could not be fine sand from African deserts, for there was so great a preponderance of fresh-water forms; and while there were many of them which were common to many parts of the world, there were none which were peculiar to Africa, but there was a large proportion which were peculiar to South America.

Of the above-mentioned 119 forms, 8 Polythalamia, 7 Polygastrica, and 2 Phytolitharia, only, belong to sea-water, the rest appertaining to fresh-water; among which the following are forms indigenous and peculiar to South America :

:

Arcella constricta.
Desmogonium Guayanense.
Eunotia Camelus.

depressa.

pileus.
quaternaria.
quinaria.

Gomphonema Vibrio.
Himantidium papilio.
Zygodon.

Navicula undosa.
Stauroneis dilatata.

Suririella Peruana.

Synedra Entomon.

From all these circumstances, Ehrenberg had been led to conclude, altogether independently of Lieut. Maury, that there is some current in the atmosphere, tending from tropical South America to South-Western Europe, gathering dust and moisture in the abovementioned regions, bringing them into the northern hemisphere, over the NE. trades, and dropping them on reaching the surface of the earth in its course to the NE. Further, the sources of the Orinoco, the "dried upland swamp regions," to which Ehrenberg was led for the habitat of some of his infusoria, have been separately described