To understand the intensity of this force, we must conceive a steam engine, which, instead of working at 200 or 300 horse-power, like the engines of our larger steamers, should work at 4,163 horse-power. And this, we must bear in mind, refers only to the small space above mentioned. If we calculate the entire surface of the earth, we arrive at the astounding total of 217,316,000,000,000 horse-power. In order to conceive such a force, we must picture to ourselves 543,000,000,000,000 steam engines each working without relaxation day and night, at 400 horse-power. That is the amount of work which the heat of the sun does for our globe alone. The physical and mechanical actions which take place on our planet, vegetation, the phenomena of animal life, industrial and agricultural operations absorb only a very small quantity of this enormous mass of forces. Professor Tyndall says on this subject, in the book we have already quoted:-- "Look at the integrated energy of our world--the stored power of our coal-fields; our winds and rivers; our fleets, armies, and guns. What are they? they are all generated by a portion of the sun's energy which does not amount to 1/2300000000th of the whole. This, in fact, is the entire fraction of the sun's force intercepted by the earth, and in reality we convert but a small fraction of this fraction into mechanical energy. Multiplying all our powers by millions of millions, we do not reach the sun's expenditure."--p. 433. In this chapter we have analyzed the different physical and vital effects produced upon our globe by the light and heat given out by the sun. We have considered its action upon animate and inanimate nature. We have seen that the sun is really the great cause of physical action on our globe, and that he is also the first principle of both vegetable and animal life. Without the sun life would be banished from the terrestrial globe; as we have already said, life is the offspring of the sun.