THE RECENT PROGRESS OF SOLAR PHYSICS. 9 



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 con- 

 cerns 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 Lawi'ence, 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 immen- 

 sity of the force thus placed at our disposal, or to see what the util- 

 ization 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 thei'e are regions of the earth at present as entirely worth- 

 less as that great African desert which it is now proposed to partly 

 reconvert to an inland sea, a sunburned area now apparently hopeless- 

 ly 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 con- 

 sumption 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 be- 

 lieve our researches already indicate, such engines may become an 

 economical as well as a mechanical success, and in a larger sense it is 

 stijl only a question of time when the rapidly consuming coal-beds of 

 Great Britain yield their last, and her manufacturing empire is trans- 

 ferred to countries which have not exhausted their supply. But these 

 will exhaust their own in turn ; the stock, though great, is finite and 



