[Illustration: Fig. 75.—Irrigation wheel in Egypt.] The Water Ram is a machine used for raising water to a great height by means of the momentum of falling water. The Hydraulic Lift is familiar to us all, as it acts in our hotels, and we need only mention these appliances here; full descriptions will be found in Cyclopædias. We have by no means exhausted the subject of Water in this chapter. Far from it. But when we come to Chemistry and Physical Geography we shall have more to tell, and our remarks as to the application of science to Domestic Economy, in accordance with our plan, will also lead us up to some of the uses of water. So for the present we will take our leave of water in a liquid form, and meet it again under the application of Heat, which subject will take us to Ice and Steam,—two very different conditions of water. FOOTNOTES: [9] The curved surface of a column of liquid is termed a “meniscus,” from the Greek word _meniskos_, meaning “a little lens.” CHAPTER VII. HEAT—WHAT IT IS—THEORY OF HEAT—THE THERMOMETER—EXPANSION BY HEAT—EBULLITION AND DISTILLATION—LATENT HEAT—SPECIFIC HEAT. What is Heat?—We will consider this question, and endeavour to explain it before we speak of its effects on water and other matter. Heat is now believed to be the effects of the rapid motion of all the particles of a body. It is quite certain that a heated body is no heavier than the same body before it was made “hot,” so the heat could not have gone into it, nor does the “heat” leave it when it has become what we call “cold,” which is a relative term. Heat is therefore believed to be a vibratory motion, or the effects of very rapid motion of matter. The Science of Heat, as we may term it, is only in its infancy, or certainly has scarcely come of age. Formerly heat was considered a chemical agent, and was termed caloric, but now heat is found to be motion, which affects our nerves of feeling and sight; and, as Professor Stewart tells us, “a heated body gives a series of blows to the medium around it; and although these blows do not affect the ear, they affect the eye, and give us a sense of light.” Although it is only within a comparatively few years that heat has been really looked upon as other than matter, many ancient philosophers regarded it as merely a _quality_ of matter. They thought it the active principle of the universe. Epicurus declared that heat was an effluxion of minute spherical particles possessing rapid motion, and Lucretius maintained that the sun’s light and heat are the result of motion of primary particles. Fire was worshipped as the active agent of the universe, and Prometheus was fabled to have stolen fire from heaven to vivify mankind. The views of the ancients were more or less adopted in the Middle Ages; but John Locke recognized the theory of heat being a motion of matter. He says: “What in our sensation is _heat_, in the object is nothing but _motion_.”