102 PHYSICAL GEOGBAPHY OF THE SEA, AND ITS METEOROLOGY. 



271. We see these majestic streams pomiiig tlieir waters into 



Mt'W^Jur'fol- the ^^® ^®^' ^^^ -^^'^^ ^^^ ^^^ ^^ know tliose waters 

 rivers. must come again, else the sea would be full. AYe 



know, also, that the sunbeam and the sea-breeze suck them up 

 again ; and it is curious to fancy such volumes of water as this 

 mighty company of ten great rivers is continually marching down 

 to the sea, taken up by the winds and the sun, and borne av/ay 

 again through the invisible channels of the air to the springs 

 among the hills. This operation is perpetually going on, yet we 

 perceive it not. It is the work of that ui visible, imponderable, 

 omnipresent, and wonderful agent called heat. This is the agent 

 which controls both sea" and air in their movements "and hi many 

 of their offices. Th© average amount of heat daily dispensed to 

 our planet from the som^ce of light in the heavens is enough to 

 melt a coating of ice completely encasing the earth with a film 1 J in. 

 in thickness.* Heat is the agent that distils for us fresh water 

 from the sea. It pumps up out of the ocean all the water for our 

 lakes and rivers, and gives power to the winds to transport it as 

 vapour thence to the mountains. And though this is but a part of 

 the work which in the terrestrial economy has been assigned to this 

 mighty agent, we may acquire much profitable knowledge by 

 examinmg its operations here in various aspects. To assist in this 

 undertaking I have appealed to the ten greatest rivers for terms 

 and measm-es in which some definite idea may be conveyed as to 

 the magnitude of the work and the immense physico-mechanical 

 power of this imponderable and invisible agent called heat. Calcu- 

 lations have been made which show that the great American lakes 

 contain 11,000 cubic miles of water. This, according to the best 

 computation, is twice as much as is contaiaed in all the other fresh- 

 water lakes, and rivers, and cisterns of the world. The Mississippi 

 Eiver does not, dming a hundred years, discharge into the sea so 

 large a volume of water as is at this moment contamed in the great 

 northern lakes of this continent ; and yet this agent, whose w^orks 

 we are about to study, operating through the winds, has power 

 annually to lift up from the sea and pour down upon the earth in 

 grateful showers fresh water enough to fill the great American lakes 

 at least twenty times over. 



272. That we may be enabled the better to appreciate the power 

 Rain-fall in the Mis- and the majcsty of the thermal forces of the sun, 

 sissippi Valley. ^-^^ Comprehend in detail the magnitude and gran- 



* Deduced from the experiments of Pouillet. 



