4 THEORY OF HEAT. 



is the changes of temperature which periodically displace every 

 part of the atmosphere. 



The waters of the ocean are differently exposed at their 

 surface to the rays of the sun, and the bottom of the basin 

 which contains them is heated very unequally from the poles 

 to the equator. These two causes, ever present, and combined 

 with gravity and the centrifugal force, keep up vast movements 

 in the interior of the seas. They displace and mingle all the 

 parts, and produce those general and regular currents which 

 navigators have noticed. 



Radiant heat which escapes from the surface of all bodies, 

 and traverses elastic media, or spaces void of air, has special 

 laws, and occurs with widely varied phenomena. The physical 

 explanation of many of these facts is already known ; the mathe 

 matical theory which I have formed gives an exact measure of 

 them. It consists, in a manner, in a new catoptrics which 

 has its own theorems, and serves to determine by analysis all 

 the effects of heat direct or reflected. 



The enumeration of the chief objects of the theory sufficiently 

 shews the nature of the questions which I have proposed to 

 myself. What are the elementary properties which it is requisite 

 to observe in each substance, and what are the experiments 

 most suitable to determine them exactly? If the distribution 

 of heat in solid matter is regulated by constant laws, what is 

 the mathematical expression of those laws, and by what analysis 

 may we derive from this expression the complete solution of 

 the principal problems ? Why do terrestrial temperatures cease 

 to be variable at a depth so small with respect to the radius 

 of the earth ? Every inequality in the movement of this planet 

 necessarily occasioning an oscillation of the solar heat beneath 

 the surface, what relation is there between the duration of its 

 period, and the depth at which the temperatures become con 

 stant ? 



What time must have elapsed before the climates could acquire 

 the different temperatures which they now maintain; and what 

 are the different causes which can now vary their mean heat ? 

 Why do not the annual changes alone in the distance of the 

 sun from the earth, produce at the surface of the earth very 

 considerable changes in the temperatures ? 



