196 SOLAR RADIATION. 



power of solar radiation in a comparatively limited space ; but the many 

 thousands of tons raised from that space do not represent the full power 

 of the sun's rays. They merely represent weight raised, without our 

 taking into account the force exerted in converting the water into 

 vapour, and in that form elevating it hundreds, or it may he thousands 

 of feet, notwithstanding the pressure of the atmosphere. In a commu- 

 nication on the Mechanical Action of JRadiant Seat or Light, by 

 Professor William Thomson, Philosophical Magazine, fourth series, 

 vol. iv., p. 256, it is stated that "mechanical effect of the statical 

 kind might be produced from the solar radiant heat, by using it as the 

 source of heat in a thermo-dynamic engine. It is estimated that 

 about 556 foot-pounds (that is, so many pounds raised one foot high) 

 per second of ordinary mechanical effect, or about the work of ' one- 

 horse power,' might possibly be produced by such an engine exposing 

 1800 square feet to receive solar heat during a warm summer 

 day in this country ; but the dimensions of the moveable parts of 

 the engine would necessarily be so great as to occasion practical 

 difficulties in the way of using it with economical advantage that might 

 be insurmountable.'' This is more than twenty times the power we 

 have assigned to the raising of water, |iiid even this appeared so vast 

 that until the data were thoroughly examined, the statement appeared 

 incredible. 



The same author proceeds to state that " the deoxidation of carbon 

 and hydrogen from carbonic acid and water, effected by the solar light 

 on the green parts of plants, is a mechanical effect of radiant heat. In 

 virtue of this action, combustible substances are produced by plants, 

 and its mechanical value is to be estimated by burning them, and 

 multiplying by the mechanical value of the thermal unit. Taking from 

 Liebig's Agricultural Chemistry the estimate, 2,600 poimds of dry 

 Fir-wood, for the annual produce of one Hessian acre, or 26,910 square 

 feet of forest land, which is at the rate of 4208 pounds or nearly 2 tons 

 per English acre, and assuming, as a very rough estimate, 4000 thermal 

 units centigrade as the heat of combustion of dry Fir-wood, the author 

 finds 650,000 foot-pounds, or the work of a horse power, for 1000 seconds, 

 as the mechanical value of the mean annual produce of a square foot of 

 the land ; and taking 50° 34', that of Giessen, as the latitude of the 

 locality, he estimates the mechanical value of the solar heat, which, 

 were none of it absorbed by the atmosphere, would fall annually on each 

 square foot of the land, at 630,000,000 foot-pounds ; and infers that 

 probably t-oW of the solar heat which falls on growing plants is con- 

 verted into mechanical effect. 



"When the vibrations pf light thus act during the growth of plants, 

 to separate, against forces of chemical affinity, combustible materials 

 from oxygen, they must lose vis viva to an extent equivalent to the 

 statical mechanical effect thus produced, and therefore quantities of 



