TERKESTEIAL HEAT. 



4th. That of the heat radiated by the earth, ninety per cent, 

 is intercepted by the atmosphere, and ten per cent, dispersed 

 in space. 



5th. That the heat evolved on the surface of the sun in a day 

 would liquefy a shell of ice 10^ miles thick, enveloping the sun, 

 and the intensity of the solar fire is seven times greater than that 

 of the fiercest blast furnace. 



6th. That the temperature of space outside the atmosphere of 

 the earth is 256 below that of melting ice. 



7th. That the solar heat alone, constitutes only two-thirds of 

 the entire quantity of heat supplied to the earth to repair its 

 thermal losses by terrestrial radiation ; and that without the heat 

 supplied by stellar radiation, the temperature of the earth would 

 fall to a point which would be incompatible with organic life. 



54. No meteorological phenomenon has had so many observers, 

 and there is none of which the theory is so little understood, as 

 the winds. The art of navigation has produced in every seaman 

 an observer, profoundly interested in the discovery of the laws 

 which govern a class of phenomena, upon the knowledge of which 

 depends not only his professional success but his personal security, 

 and the lives and property committed to his charge. 



The chief part of the knowledge which has been collected 

 respecting the causes which produce these atmospheric currents 

 is derived, nevertheless, much more from the comparison of 

 the registers of observatories than from the practical experience 

 of mariners. 



55. Winds are propagated either by compression or by rarefac- 

 tion. In the former case they are developed in the same direction 

 in which they blow ; in the latter case they are developed in the 

 contrary direction. To render this intelligible, let us imagine a 

 column of air included in a tube. If a piston inserted in one end 

 of the tube be driven from the mouth inwards, the air contiguous 

 to it will be compressed, and this portion of air will compress the 

 succeeding portion, and so on ; the compression being propagated 

 from the end at which the piston enters toward the opposite end. 

 The remote end being open, the air will flow in a current driven 

 before the piston in the same direction in which the compression is 

 propagated. 



If we imagine, on the other hand, a piston inserted in the tube 

 at some distance from its mouth, to be drawn outwards toward 

 the mouth, the air behind it will expand into the space deserted 

 by the piston, and a momentary rarefaction will be produced. The 

 next portion of air will in like manner follow that which is next 

 the piston, the rarefaction which begins at the piston being pro- 

 pagated backwards through the tube in a direction contrary to the 

 $6 



