LIMITS OF HUMAN POWER. 45 



wliicli tlie action of tlie elements puts forth \ritli such astonishing 

 energy.* Could we gather, and bind, and make subservient to 

 our control, the power which a TVest Indian hurricane exerts 

 through a small area in one continuous blast, or the momentum 

 expended by the waves, in a tempestuous winter, upon the break- 

 water at Cherbourg,f or the lifting power of the tide, for a month, 



* Some ■well-known experiments show that it is quite possible to accumulate 

 the solar heat by a simple apparatus, and thus to obtain a temperature which 

 might be economically important even in the cUmate of Switzerland. Saus- 1 

 sure, by receiving the sun's rays in a nest of boxes blackened within and cov- 

 ered with glass, raised a thermometer enclosed in the inner box to the boiling 

 point ; and under the more powerful sun of the Cape of Good Hope, Sir John 

 Herschel cooked the materials for a family dinner by a similar process, using, 

 however, but a single box, surrounded with dry sand and covered with two 

 glasses. "Why should not so easy a method of economizing fuel be resorted to 

 in Italy, in Spain, and even in more northerly climates ? 



The unfortunate John Davidson records in his journal that he saved fuel in 

 Morocco by exposing his tea-kettle to the sun on the roof of his house, where 

 the water rose to the temperature of one hundred and forty degrees, and, of 

 course, needed little fire to bring it to boil. But this was the direct and sim- 

 ple, not the concentrated or accumulated, heat of the sun. 



On the utilizing of the solar heat, simply as heat, see the work of MorcHOT, 

 L(i Ghaleur solaire et ses applications industrielles. Paris, 1869. 



The reciprocal convertibihty of the natural forces has suggested the possi- 

 bility of advantageously converting the heat of the sun into mechanical power. 

 Ericsson calculates that in all latitudes between the equator and 45°, a hundi-ed 

 square feet of surface exposed to the solar rays develop continuously, for nine 

 hours a day on an average, eight and one-fifth horse power. 



I do not know that any attempts have been made to accumulate and store 

 up for use at pleasure, force derived from this powerful source. 



f In heavy storms, the force of the waves as they strike against a sea-wall 

 is from one and a half to two tons to the square foot, and Stevenson, in one 

 instance at Skerryvore and in another at the Bell Rock lighthouse, found this 

 force equal to nearly three tons per foot. 



The seaward front of the breakwater at Cherbourg exposes a surface of 

 about 2,500,000 square feet. In rough weather the waves beat against this 

 whole face, though, at the depth of twenty -two yards, which is the height of 

 the breakwater, they exert a very much less violent motive force than at and 

 near the surface of the sea, because this force diminishes in geometrical, as 

 the distance below the surface increases in arithmetical, proportion. The 

 shock of the waves is received several thousand times in the course of twenty- 

 four hours, and hence the sum of inipulse which the breakwater resists in one 

 stormy day amounts to many thousands of millions of tons. The breakwater 

 is entirely an artificial construction. If then man could accumulate and con- 

 trol the forces which he is able effectually to resist, he might be said to be 

 physically speaking, omnipotent. 



