common roads at an average rate of ten miles per hour. 2. That at this rate they have conveyed upwards of fourteen passengers. 3. That their weight, including engines, fuel, water, and attendants, may be under three tons. "4. That they can ascend and descend hills of considerable inclination, with facility and safety. 66 6 5. That they are perfectly safe for passengers. 6. That they are not (or need not be, if properly constructed) nuisances to the public. 7. That they will become a speedier and cheaper mode of conveyance than carriages drawn by horses. 66 6 8. That, as they admit of greater breadth of tire than other carriages, and as the roads are not acted on so injuriously as by the feet of horses in common draught, such carriages will cause less wear of roads than coaches drawn by horses.' 401. Still, steam-carriages have not been established anywhere on common roads, and the attempt seems to have been given up. The destruction of the machinery caused by the roughness of the road seems an insuperable obstacle. No system of springs has yet been devised which will at once neutralise the effects of the roughness of the road, so as to save the machinery, and permit a sufficiently rigid connection between the piston and axle, so as to preserve them in harmonious action. It seems probable that a very great change in the construction of common roads, so as to have them much more smooth and level, must precede the profitable introduction of steam-carriages to run upon them. THE STEAM-PLOUGII. 402. AMONG the many purposes for which animal power is employed, ploughing is one to which steam machinery has not yet been successfully adapted; although many attempts have been made to construct an efficient steam-plough. The same difficulties which oppose the use of steam-carriages on common roads, are still more serious impediments to steam-ploughing. Messrs. Galloway and Purkis have taken out patents for improvements in apparatus for ploughing. The ploughing part of their machine consists mainly of an endless chain, supported and driven by two sets of chain wheels, and having the ploughs fastened to it. These are arranged somewhat on the plan of the dredging machines, used in cleaning docks and deepening rivers. They employ a pair of oscillating steam-engines. The frame-work of their steam-plough is eighteen feet long, and it can cut a double row of furrows twenty feet long to any required depth within the range of the machine, which is twenty inches. To plough fresh furrows, the machine is made to advance progressively at right angles, away from the furrows already cut. THE BOILER. 403. The boiler is a large vessel formed of sheet-iron plates hammered together, so as to be water-tight. It is made in many different forms; but these are chiefly of two leading kinds, the wagon-shaped boiler, shown in Fig. 37; and the cylindrical boiler, represented in Fig. 38. The boiler has two principal tubes, one of which conveys to it water (usually water partly heated by being used in condensation) to be turned into steam, while another takes steam from the boiler to the cylinder. The fire is placed below the boiler, as in Fig. 37, or in the interior of it, as in Fig. 38; and the flame and heated air, after coming from the fire, pass along a flue at one side of the boiler, imparting heat to the water, and then along a flue at the other side— which is called the wheel-draft; or, the draft divides on passing from under the boiler into two branches, one to each side flue, which is called the split draft. The boiler has gauge-cocks, to ascertain the height of the water in it; a steam-gauge, to indicate the elastic force of the steam; one safety-valve (or two), to give timely exit to the steam when it is becoming too strong, and prevent explosion; an internal safety valve; and a man-hole, for admission to clean it. By very ingenious contrivances, the feed pipe is made self-regulating, so as to proportion properly the supply of the water to the demand for it within the boiler. 404. GAUGE-COCKS.-The gauge-cocks used to ascertain the height of water in the boiler are the same as in Newcomen's engine, and have been already described. Three are often employed. Another method of ascertaining the height of the water in the boiler will be understood from the annexed figure. There are two apertures in the side or end of the boiler, one near the top, and another near the bottom. These are connected by a glass tube placed outside the boiler, ab, in which, communicating freely with the steam above and the b FIG. 39. water below, the level of the liquid will always be the same as that of the body of water in the boiler; and thus the height of the latter may be ascertained in a moment by a glance at the glass tube. It is now understood that in marine and locomotive engines, these methods, particularly the former, require, at times, great caution in trusting to their indications. 405. STEAM-GAUGE.-This is fixed into the boiler, or some tube freely communicating with it, and is open at both ends, to the interior of the boiler at one end-to the atmosphere at the other. It is curved, in the form of the letter U, and contains a quantity of mercury. The atmospheric pressure acts on the mercury in the limb open to it, with a force of 14-7 pounds per square inch. If the steam act with the same force, the mercury will be at the same level in both limbs. If the steam be of higher elastic power than the air's pressure, it will depress the mercury in the limb on which it acts, and force it up to a corresponding height in the limb open to the air. The difference will indicate the excess of the force of the steam over the air's pressure. The tube may be of glass or iron. In the latter case, a float with a long rod rests upon the surface of the mercury exposed to the air, which rises or falls with the mercury; and the upper extremity of the rod having a scale adjoining, it acts as an index and shows the height of the liquid within the tube. 406. SAFETY-VALVE.-The object of this valve is to permit the escape of steam, should it accidentally become stronger than the boiler is intended to bear, and thus prevent the bursting or explosion of the boiler. It is a valve so loaded as to open with a pressure of steam, a little more than is necessary to work the engine, and considerably less than the utmost the boiler can bear. This valve does not materially differ from that used in Newcomen's engine. It consists of a lever, the joint or fulcrum of which is set on a support at the side of a short tube or pipe, communicating with the boiler. From the lever immediately over the aperture of the tube, a rod descends, having a plug attached, which closes the tube. To the other extremity of the lever, weights may be attached, at different distances from the fulcrum, which will have power in keeping down the valve or plug, in proportion to their distance from the fulcrum. The force of the steam will tend to push up the plug (valve), and permit the escape of the steam; the atmospheric pressure, and the weight attached to the lever, will tend to press down the plug, and prevent the exit of steam. The valve will be open or shut, according to the relative strength of these forces acting on it in opposite directions. Its action will be understood from the figure below (fig. 40). In some steam-boat engines, a conical plug is used, from which a rod rises, |