•56 WRITINGS OF JOSEPH HENRY. [1855- 



of the same vapor is probably carried by the upper current 

 far beyond the tropics, and deposited in fertilizing rains even 

 at the extremities of the polar circles. 



The condensation of the vapor which ascends in the equa- 

 torial regions evolves an astonishing power, in the form 

 of heat, accelerating the upward motion of the air, and 

 modifying in a greater degree than almost any of the causes 

 we have heretofore mentioned, the primary motion due 

 simply to the difference of heat between the poles and the 

 equator. To understand this, it is sufficient to refer to the 

 great amount of heat contained in a given amount of steam ; 

 and for illustration let us suppose the following simple ex- 

 periment: A quantity of water at the temperature of melting 

 ice is placed in a vessel over a lamp, which is so adjusted as 

 to impart one degree of heat to the water in each minute of 

 time. If the process is properly conducted, the heat will con- 

 tinue to increase, and, in accordance with the supposition 

 we have made, the water at the end of about twelve hundred 

 minutes will be all converted into vapor. If the process has 

 been so conducted that a degree of heat has been given to 

 the liquid in each minute of time, the steam will evidently 

 contain about twelve hundred degrees of heat above the zero 

 of Fahrenheit's scale. The greater portion of this will be in 

 what is called a "latent" state; but it will all re-appear, as 

 is well known from abundant experiments, when the vapor 

 is re-converted into water. From these data it is easy to 

 prove mathematically that every cubic foot of water which 

 falls on the surface of the earth in the form of rain leaves in 

 the air whence it descended sufficient heat to produce at 

 least 6,000 cubic feet of expansion of the surrounding at- 

 mosphere beyond the space which the vapor itself occupied. 

 The ascensional force evolved by this process must evidently 

 be immense, when we consider the great amount of rain 

 which falls within the tropics. A similar power is evolved 

 whenever rain falls; and this principle, which has been so 

 ably developed by Mr. Espy, is undoubtedly a true and suf- 

 ficient cause of most of the violent and fitful agitations of 

 the atmosphere which have so long puzzled the scientific 



