heat applied to it. If it were otherwise, thaw and evaporation must be instantaneous; at the first touch of warmth, all the snow which lies on the roofs of our houses would descend like a water-spout into the streets: all that which rests on the ground would rush like an inundation into the water courses. The hut of the Esquimaux would vanish like a house in a pantomime: the icy floor of the river would be gone without giving any warning to the skater or the traveller: and when, in heating our water, we reached the boiling point, the whole fluid would "flash into steam," (to use the expression of engineers,) and dissipate itself in the atmosphere, or settle in dew on the neighbouring objects.

It is obviously necessary for the purposes of human life, that these changes should be of a more gradual and manageable kind than such as we have now described. Yet this gradual progress of freezing and thawing, of evaporation and condensation, is produced, so far as we can discover, by a particular contrivance. Like the freezing of water from the top, or the floating of ice, the moderation of the rate of these changes seems to be the result of a violation of a law: that is, the simple rule regarding the effects of change of temperature, which at first sight appears to be the law, and which, from its simplicity, would seem to us the most obvious law for these as well as other cases, is modified at certain critical points, so as to produce these advantageous effects:—why may we not say in order to produce such effects?

VIII. Another office of water, which it discharges by

means of its relations to heat, is that of supplying our springs. There can be no doubt that the old hypotheses, which represent springs as drawing their supplies from large subterranean reservoirs of water, or from the sea by a process of subterraneous filtration, are erroneous and untenable. The quantity of evaporation from water and from wet ground is found to be amply sufficient to supply the requisite drain. Mr. Dalton calculated* that the quantity of rain which falls in England is thirty-six inches a year. Of this he reckoned that thirteen inches flow off to the sea by the rivers, and that the remaining twenty-three inches are raised again from the ground by evaporation. The thirteen inches of water are of course supplied by evaporation from the sea, and are carried back to the land through the atmosphere. Vapour is perpetually rising from the ocean, and is condensed in the hills and high lands, and through their pores and crevices descends, till it is deflected, collected, and conducted out to the day, by some stratum or channel which is watertight. The condensation which takes place in the higher parts of a country, may easily be recognised in the mists and rains which are the frequent occupants of such regions. The coldness of the atmosphere and other causes precipitate the moisture in clouds and showers, and in the former as well as in the latter shape, it is condensed and absorbed by the cool ground. Thus a perpetual and compound circulation of the waters is kept up; a narrower circle between the evaporation and precipita tion of the land itself, the rivers and streams only

* Manchester Memoirs, v. 357

occasionally and partially forming a portion of the circuit; and a wider interchange between the sea and the lands which feed the springs, the water ascending perpetually by a thousand currents through the air, and descending by the gradually converging branches of the rivers, till it is again returned into the great reservoir of the ocean.

In every country, these two portions of the aqueous circulation have their regular, and nearly constant, proportion. In this kingdom the relative quantities are, as we have said, 23 and 13. A due distribution of these circulating fluids in each country appears to be necessary to its organic health; to the habits of vegetables, and of man. We have every reason to believe that it is kept up from year to year as steadily as the circulation of the blood in the veins and arteries of man. It is maintained by machinery very different, indeed, from that of the human system, but apparently as well, and therefore we may say as clearly, as that, adapted to its purposes.

By this machinery we have a connection established between the atmospheric changes of remote countries. Rains in England are often introduced by a south-east wind. "Vapour brought to us by such a wind, must have been generated in countries to the south and east of our island. It is therefore, probably, in the extensive valleys watered by the Meuse, the Moselle, and the Rhine, if not from the more distant Elbe, with the Oder and the Weser, that the water rises, in the midst of sunshine, which is soon afterwards to form our clouds, and pour down our thunder-showers." "Drought

G

and sunshine in one part of Europe may be as neces sary to the production of a wet season in another, as it is on the great scale of the continents of Africa and South America; where the plains, during one half the year, are burnt up, to feed the springs of the mountains; which in their turn contribute to inundate the fertile valleys, and prepare them for a luxuriant vegetation."* The properties of water which regard heat make one vast watering-engine of the atmosphere.

CHAP. X.-The Laws of Heat with respect to Air.

We have seen in the preceding chapter, how many and how important are the offices discharged by the aqueous part of the atmosphere. The aqueous part is, however, a very small part only: it may vary, perhaps, from less than 1-100dth to nearly as much as 1-20th in weight of the whole aërial ocean. We have to offer some considerations with regard to the remainder of the mass.

I. In the first place we may observe that the aërial atmosphere is necessary as a vehicle for the aqueous vapour. Salutary as is the operation of this last element to the whole organised creation, it is a substance which would not have answered its purposes if it had been administered pure. It requires to be diluted and associated with dry air, to make it serviceable. A little consideration will show this.

We can suppose the earth with no atmosphere except the vapour which arises from its watery parts: and if

* Howard on the Climate of London, vol. ii., pp. 216, 217.

we suppose also the equatorial parts of the globe to be hot, and the polar parts cold, we may easily see what would be the consequence. The waters at the equator, and near the equator, would produce steam of greater elasticity, rarity, and temperature, than that which occupies the regions further polewards; and such steam, as it came in contact with the colder vapour of a higher latitude, would be precipitated into the form of water. Hence there would be a perpetual current of steam from the equatorial parts towards each pole, which would be condensed, would fall to the surface, and flow back to the equator in the form of fluid. We should have a circulation which might be regarded as a species of regulated distillation.* On a globe so constituted, the sky of the equatorial zone would be perpetually cloudless; but in all other latitudes we should have an uninterrupted shroud of clouds, fogs, rains, and, near the poles, a continual fall of snow. This would be balanced by a constant flow of the currents of the ocean from each pole towards the equator. We should have an excessive circulation of moisture, but no sunshine, and probably only minute changes in the intensity and appearances of one eternal drizzle or shower.

It is plain that this state of things would but ill answer the ends of vegetable and animal life: so that even if the lungs of animals and the leaves of plants were so constructed as to breathe steam instead of air, an atmosphere of unmixed steam would deprive those creatures of most of the other external conditions of their well-being.

* Daniell. Meteor. Ess., p. 56.