with us two hours longer still. There is thus not only a great direct increase, but a great accumulation of heat. The mode in which this effect is produced, may be shortly mentioned. The rays of the sun, or whatever the influence may be which generates heat, in passing through a perfectly transparent medium, do not increase the temperature of that medium. They seem to require resistance to produce this effect. It is not therefore till they reach the earth, that their power is very sensibly exerted. In striking upon the opaque surface of our globe, they give out their qualities. Light and warmth are produced and reflected. The earth and the atmosphere are thus both subjected to their influence. These become heated, the one by conduction, the other by reflection. Now, it is obvious, that while the intensity must be in proportion to the directness with which the globe is struck by the sun's rays, the accumulation must be in proportion to the length of time during which the influence continues. Hence, there is a double cause for the summer's heat, the height to which the luminary rises in the heavens, and the length of the day compared with the night. These causes operate in an increasing ratio. Day after day the accumulated heat receives fresh accessions. Every time the sun's influence is repeated, it penetrates deeper below the surface, and is more intensely reflected into the already heated atmosphere. This effect continues even after the direct solar heat has begun to be diminished; and it is not till several weeks after the sun has begun to take a retrogade motion, that the temperature is at its maximum. In June, the sun reaches his greatest height, and begins to decline, but the heat continues to increase till the middle or end of July.

But there are various circumstances besides warmth, which constitute summer weather. The mechanism of the atmosphere is very complicated, and the adjustments which it requires are exceedingly nice, and considering merely the nature of the powers employed, we may well

add, hazardous. Any change in the relative proportion of one of the principles, is calculated to produce a powerful effect on all the rest, and were there not a regulating power of consummate wisdom, it might be expected that the balance would be overset, and that the most disastrous consequences would ensue. Let us look for a moment at the constituents of the atmosphere, and this will become apparent. The air, which forms the chief part of the atmosphere, is composed of two substances, held together merely by mechanical admixture, which are of very different properties, and which require to continue united in the precise proportion they actually bear to each other, in order to be capable of sustaining animal and vegetable life. Were that proportion destroyed even in a slight degree, the air we breathe would be instantly converted into a deadly poison. Now, it is well worthy of remark, that, although in the functions both of animal and vegetable life, and in the process of combustion, a great and apparently unequal consumption of these two substances takes place, the proportion between them is always maintained, and that notwithstanding any difference of temperature. Heat expands, and cold contracts them, but they are not thus disunited, or in any way disturbed in their proportions. On the contrary, it is probably in some degree owing to the alternations of heat and cold, which keep up a constant motion in this wonderful fluid, that the necessary balance is maintained.

Another ingredient in the atmosphere is moisture. This is very sensibly acted on by heat. It is the principle of heat which evaporates the moisture from the earth, and causes it to mix with the air, and to float in it, sometimes as an invisible fluid, sometimes in the form of clouds, and which at other times causes it to be precipitated in the form of rain. Now, the remarkable circumstance is, that although heat is the agent in these operations, the change of temperature does not so affect the process as to cause the operations to cease, or very

materially to disturb them. Evaporation goes on both at a low and a high temperature, and in both states clouds are formed and rain falls. This is owing to a very peculiar provision, obviously imposed by consummate wisdom. The air is made capable of containing vapour in a certain proportion to its temperature, and it is not till it be saturated that the evaporation from the surface of water ceases, or that deposition takes place. The temperature of the air in winter does not indeed admit of the same quantity, being held in solution, as in summer, but up to a certain point it is equally capable of sustaining it in the one case as in the other. Evaporation, therefore, takes place in very cold weather, even from ice and snow, and the water thus infused into the air is carried up into the higher regions, till it reaches the point where the temperature is such as to correspond with the quantity of moisture. Precisely the same process takes place in summer, with this difference, that the evaporation is much more abundant, and the air, owing to its increased temperature, is capable of containing a far greater quantity in solution. Again, the point of deposition is regulated by a similar law, with a similar difference. Deposition does not take place either in winter or summer, till the air is more than saturated; but this effect is produced at very different temperatures, according to the quantity actually held in solution, so that a very slight degree of cold will form clouds and cause rain in summer, compared with what is necessary to occasion the same phenomena in winter. Hence the processes of evaporation and deposition are made, by this very peculiar law, always to bear a relation to the actual temperature of the season, and such a balance is kept up between these processes, as is admirably suited to the wants of vegetable and animal life.

I may add to all this the properties of the atmosphere, by means of which it is made the vehicle of light and sound, and the means of respiration. The changes which

the air undergoes by the operation of heat and cold, might easily be supposed, and might even perhaps, reasoning without the aid of experience, be expected to produce a material alteration on such properties. But although these changes are so considerable in different seasons, and in different climates, we do not find that the laws either of vision or of acoustics, are in any material degree affected by them,- -or that the action of the lungs either in man or the lower animals is impeded or deranged.

a

In attending to the complicated nature of the atmosphere, and the various important functions it has to perform, and in considering the diversified modifications it must necessarily undergo by the alteration of its temperature, both in the various latitudes of the globe, and in the different seasons of the year, it does seem impossible to doubt that the uniformity of its properties, and of its salutary influences under all these modifications, has been provided for, by what Whewell, considering that subject in a more extended view, justly calls " most refined, far-seeing, and far-ruling contrivance." So many opposing forces, and the mingling of such subtle and fearfully active elements, appear in the most quiescent state to require amazing prospective skill for their regulation and control; and when we find them, even under the influence of extensive changes, still harmoniously combining their powers for the general good, we cannot but perceive that all this could not be effected but by the same paternal Hand which originally called their powers into action.

BESIDES the phenomena already alluded to, relative to heat, there is another of very extensive operation, which, although it cannot be considered as entering immediately in any perceptible degree, into the formation of climate, is yet too remarkable to be passed over without some notice. I allude to the existence of that heat in the interior of our globe, which cannot be referred to external causes, and which has produced, and is continually producing, great and important changes on its surface.†

Indications of the action of a powerful heat in the inner strata of the earth, meet the geologist at every step of his investigation, and it is only by this agency that he is able to account for various existing phenomena at or near the surface, from which, instead of stopping to enumerate them all, I shall select a few of the most striking instances.

Two kinds of springs exist in Nature, the common, and the thermal or warm. The temperature of the common spring is found to be nearly that of the medium

*For several of the facts stated in this paper, I am indebted to a valued friend.

"The evidence in proof of great and frequent movements of the land itself, both by protrusion and subsidence, and of the connexion of these movements with the operations of volcanoes, is so various and so strong, derived from so many quarters on the surface of the globe, and every day so much extended by recent inquiry, as almost to demonstrate that these have been the causes by which those great revolutions were effected; and that, although the action of the inward forces which protrude the land has varied greatly in different countries, and at different periods, they are now, and ever have been, incessantly at work in operating present changes, and preparing the way for future alteration in the exterior of the globe."— Fitton's Geological Sketch: See also, Lyell's Geology, vol i. passim. This author observes, that the subsidence occasioned by volcanic agency, is somewhat greater than the elevation, and that the climate has been much affected indirectly by the changes thus produced in the relative position of sea and land.