concerned, none of us are yet prophets, or more able to tell from our knowledge of the sun what the weather will be next week than what the harvest will be next year.
There is another utilitarian aspect of our study about which there is less public interest, but more real promise—I mean that which concerns the direct application of solar heat to arts and manufactures. These are now all using it indirectly—by the water, for instance, which it lifts into the clouds to turn the mills of Lowell or Lawrence, as it flows back to the sea, or by the coal which it stored in former ages to drive our engines to-day. These indirect means use but the feeblest portion of the solar heat, which is in theory capable of furnishing nearly one horse-power for each square yard of the earth's surface under full sunshine.
What we have actually realized in experiment is still considerable.
The visitor to the last Paris Exposition may have seen upon its grounds a machine of strange appearance, in the open air, pointing sunward the axis of an immense reflector, shaped like a truncated cone, which gathered the rays to a linear focus upon the boiler of a working steam-engine, which it drove thus by direct solar heat. Many not dissimilar solar engines have been built in this country and in India, the particular one of which I speak, due to M. Mouchot, having actually realized about one horse-power to ten feet square of surface.
We are startled when we make the computation, to find the immensity of the force thus placed at our disposal, or to see what the utilization of the waste places of the earth would bring us. Upon the limited area of the Adirondack wilderness to the north of us, for instance, the daily wasted sun-power actually realizable, and after every allowance for loss, is many times that of all the estimated steam-power at present in use in the whole world. I am not myself so far utilitarian as to wish to see this use made of our pleasant summer haunts, but there are regions of the earth at present as entirely worthless as that great African desert which it is now proposed to partly reconvert to an inland sea, a sunburned area now apparently hopelessly useless to man, and yet on which an amount of power is every year poured in utter waste which could not be made good by the consumption of all the coal known to underlie the soil of Great Britain.
Such machines as those of M. Mouchot, owing to the expense of construction and attendance, cost more than an engine driven by coal, though the sun supplies its power gratis; but it is simply, it seems to me, a question of time when, with another form which I believe our researches already indicate^ such engines may become an economical as well as a mechanical success, and in a larger sense it is still only a question of time when the rapidly consuming coal-beds of Great Britain yield their last, and her manufacturing empire is transferred to countries which have not exhausted their supply. But these will exhaust their own in turn; the stock, though great, is finite and