But if the whole available mechanical power, laid in store in the coal mines, in addition to all the unimproved wind and water power, should seem to any one insufficient to work out this world's manifest destiny, the doctrine of the essential unity or conservation of force is not exhausted of consolation. All the coal of which we have spoken is but the result of the action of sun-light in past ages, decomposing carbonic acid in the vegetative process. The combustion of the carbon reproduces a force exactly equivalent to that of the sun-light which was absorbed or consumed in its vegetative separation. Supposing the whole estimated stock of coal in the world to be consumed at once, it would cover the entire globe with a stratum of carbonic acid about seventy-two feet deep. And if all the energy of sun-light which this globe receives or encounters in a year were to be devoted to its decomposition, according to Pouillet's estimate of the strength of sunshine,--and he probably knows, if any one does,-- deducting all that would be wasted on rock or water, there would be enough to complete the task in a year or two. A marvellous growth of forest, that would be! But the coal is not to be burned up at once. When we get our steam-engines in motion to the amount of two or three thousand millions of horse-power, and are running off the coal at the rate of one tenth of one per cent per annum, the simple and inevitable consequence will be that the wood will be growing enough faster to keep good the general stock of fuel. Doubtless the forests are now limited in their growth and stunted from their ante-Saurian stature, not so much for want of soil, moisture, or sunshine as for want of carbonic acid in the air, to be decomposed by the foliage, the great deposition of coal in the primitive periods having exhausted the supply. Our present havoc of wood only changes the locality of wood-lots, and our present consumption of coal, rapid enough to exhaust the entire supply in about seventy-seven thousand years, is sure to increase the aggregate cordage of the forests. By the time we have brought our locomotive steam-cultivators to such perfection as to plough up and pulverize the great central deserts, we may see trees flourish where it would have been useless to plant the seed before we had converted so much of the earth's entrails into smoke.