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 precipitation 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 necessary 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 fürther 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 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. 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. We should 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. The real state of things which we enjoy, the steam being mixed in our breath and in our sky in a moderate quantity, gives rise to results very different from those which have been described. The machinery by which these results are produced is not a little curious. It is, in fact, the machinery of the weather, and therefore the reader will not be surprised to find it both complex and apparently uncertain in its working. At the same time some of the general principles which govern it seem now to be pretty well made out, and they offer no small evidence of beneficent arrangement. The Besides our atmosphere of aqueous vapour, we have another and far larger atmosphere of common air; a permanently elastic fluid, that is, one which is not condensed into a liquid form by pressure or cold, such as it is exposed to in the order of natural events. pressure of the dry air is about 29 inches of mercury; that of the watery vapour, perhaps, half an inch. Now if we had the earth quite dry, and covered with an atmosphere of dry air, we can trace in a great measure what would be the results, supposing still the equatorial zone to be hot, and the temperature of the surface to decrease perpetually as we advance into higher latitudes. The air at the equator would be rarefied by the heat, and would be perpetually displaced below by the denser portions which belong to cooler latitudes. We should have a current of air from the equator to the poles in the higher regions of the atmosphere, and at the surface a returning current setting towards the equator to fill up the void so created. Such aërial currents, combined with the rotatory motion of the |