Water mill is the name by which all mills are designated that receive their motion from the impulse of the water.

As each of these mills will come under their respective heads,we shall, in the present article, confine ourselves to a minute description of the different kinds of water-wheels, by whose axis the force with which they have been impressed may be transmitted to move any species of machinery, however simple
or complex.

In the same treatise of Hero mentioned previously, there is a drawing and description of another Steam Machine, more ingenious than the preceding, and forming one of the various forms of an ancient apparatus called an aeolipile, which is still used by lecturers on Natural Philosophy, to illustrate the nature and the properties of steam.

All bodies in nature are presented to us in one or other of the three differernt forms or states- viz. the solid, the liquid and the gaseous. Most bodies can be made, by changes in temperature, to assume thses different forms. The simplest and best know example is that of water. At the ordinary temperature of out climate, this is a perfect liquid; at a considerably lower temperature, it becomes solid, and then it is called ice; at a very high temperature, it becomes gaseous, and is then called steam.

A liquid, under the influence of heat, generates an aeriform fluid; this fluid, when free to expand in a given space, is called a vapor, but when it completely fills that space, and prevents the futher vaporization of that fluid, it is called a gas or an elastic fluid. Although simple variations of temperature and volume will cause a gas to pass into the state of vapor, and vice versa, yet the physical properties of the fluid, in these two states, are essentially different. Thus, by heating a given volume of air, or any other vapor, from 32 degrees to 212 degrees Fahrenheit, it's elastic force is increased only in the ratio of 1 to 11, or of 8 to 11, nearly; whereas, the elastic force of the vapor of water, while in contact with the generating liquid, is increased in the ratio of about 1:150. It is this enormous development of force, produced by the application of heat, which accomplishes in modern times, the amazing mechanical effects of steam, when employed as a moving power.

The Ancients had no such clear notions of the nature of steam as those which have just been stated; although, in the writings of Plato, we find speculations concerning the nature and properties of the four elements, as they were called, Earth, Air, Fire and Water, which show that they were partially acquainted with the subject. It is true that only about a century and a half have elapsed, since steam began to be usefully employed as a moving power; and yet, the knowledge of it's expansive force can be traced to remote antiquity. This discovery must, indeed have forced itself on the notice of those who first made use of a pot, with a lid on it, in culinary operations.