It is a universal property of matter, that by the application of heat, so as to raise its temperature, it suffers an increase in its magnitude. Also in different substances, when certain temperatures are attained by the application of fire, or other methods of heating, they undergo a change of form. Solids, at certain temperatures, are converted into liquids; and liquids, in like manner, when heated to certain degrees, become aeriform fluids or gases. These changes are familiar to every one in the ordinary phenomena attending water. Below the temperature of 32° of the common thermometer, that substance exists in the solid form, and is called _ice_. Above that temperature it passes into the liquid state, and is called _water_; and when raised to the temperature of 212°, under ordinary circumstances, it passes into the aeriform state, and is called _steam_. It is to this last change that we wish at present principally to call the attention of the reader. In the transition of water from the liquid state to the state of vapour or steam, an immense change of bulk takes place. In this change, a solid inch of water enlarges its size about 1,700 times, and forms 1,700 solid inches of steam. This expansion takes place accompanied with a certain force or pressure, by which the vapour has a tendency to burst the bounds of any vessel which contains it. The steam which fills 1,700 solid inches at the temperature of 212°, will, if cooled below that temperature, return to the liquid form, and occupy only one solid inch, leaving 1,699 solid inches vacant; and, if it be included in a close vessel, leaving the surfaces of that vessel free from the internal pressure to which they were subject before the return of the water to the liquid form. If it be possible, therefore, alternately to convert water into vapour by heat, and to reconvert the vapour into water by cold, we shall be enabled alternately to submit any surface to a pressure equal to the elastic force of the steam, and to relieve it from that pressure, so as to permit it to move in obedience to any other force which may act upon it. Or again, suppose that we are enabled to expose one side of a movable body to the action of water converted into steam, at the moment that we relieve the other side from the like pressure by reconverting the steam which acts upon it into water, the movable body will be impelled by the unresisted pressure of the steam on one side. When it has moved a certain distance in obedience to this force, let us suppose that the effects are reversed. Let the steam which pressed it forwards be now reconverted into water, so as to have its action suspended; and at the same moment, let steam raised from water by heat be caused to act on the other side of the movable body; the consequence will obviously be, that it will now change the direction of its motion, and return in obedience to the pressure excited on the opposite side. Such is, in fact, the operation of an ordinary