time less, as will probably happen, then the motion will be alternately accelerated and retarded. This variation in the speed of the body moved will not, however, affect the mechanical effect produced by the power, provided that the momentum imparted to the moving mass be allowed to expend itself at the end of the stroke, so that the piston may be brought to rest as nearly as possible by the resistance. of the load, and not by any shock on any fixed points in the machine. This is an object which, consequently, should be aimed at with a view to the economy of power, independently of other considerations connected with the wear and tear of the machinery. So long as the engine is only applied to the operation of pumping water, great regularity of motion is not essential, and, therefore, the variation of speed which appears to be an almost inevitable consequence of any extensive application of the expansive principle, is of little importance. In the patent which Watt took out for the application of the expansive principle, he specified several methods of producing a uniform effect upon a uniform resistance, notwithstanding the variation of the energy of the power which necessarily attended the expansion of the steam. This he proposed to accomplish by various mechanical means, some of which had been previously applied to the equalisation of a varying power. One consisted in causing the piston to act on a lever, which should have an arm of variable length, the length increasing in the same proportion as the energy of the moving power diminished. This was an expedient which had been already applied in mechanics for the purpose of equalising a varying power. A well-known example of it is presented in the main-spring and fuzee of a watch. According as the watch goes down, the main-spring becomes relaxed, and its force is diminished; but, at the same time, the chain by which it drives the fuzee acts upon a wheel or circle, having a diameter increased in the same proportion as the energy of the spring is diminished.

Another expedient consisted in causing the moving power, when acting with greatest energy, to lift a weight which should be allowed to descend again, assisting the piston when the energy of the moving force was diminished.

Another method consisted in causing the moving force, when acting with greatest energy, to impart momentum to a mass of inert matter, which should be made to restore the same force when the moving power was more enfeebled. We shall not more than allude here to these contrivances proposed by Watt, since their application has never been found advantageous in cases where the expansive principle is used.

(92.) The application of the expansive principle in the engines constructed by Boulton and Watt, was always very limited, by reason of their confining themselves to the use of steam having a pressure not much exceeding that of the atmosphere. If the principle of expansion, as above explained, be attentively considered, it will be evident that the extent of its application will mainly depend on the density and pressure of the steam admitted from the boiler. If the density and pressure be not considerable when the steam is cut off, the extent of its subsequent expansion will be proportionally limited. It was in consequence of this, that this principle from which considerable economy of power has been derived, was applied with much less advantage by Mr. Watt than it has since been by others, who have adopted the use of steam of much higher pressure. In the engines of Boulton and Watt, where the expansive principle was applied, the steam was cut off after the piston had performed from one half to two thirds of the stroke, according to the circumstances under which the engine was worked. The decreasing pressure produced by expansion was, in this case, especially with the larger class of engines, little more than would be necessary to allow the momentum of the mass moved to spend itself, before the arrival of the piston at the end of the stroke.

Subsequently, however, boilers producing steam of much higher pressure were applied, and the steam was cut off when the piston had performed a much smaller part of the whole stroke. The great theatre of these experiments and improvements has been the mining districts in Cornwall, where, instead of working with steam of a pressure not much exceeding that of the atmosphere, it has been found advantageous to use steam whose pressure is at least four times as great as

that of the atmosphere; and instead of limiting its expansion to the last half or fourth of the stroke, it is cut off after the piston has performed one fourth part of the stroke or less, all the remainder of the stroke being accomplished by the expansive power of the steam, and by momentum.

PROPERTIES OF STEAM.

CHAP. VII.

STEAM.

- COMMON STEAM. - SUPERHEATED LAW OF DALTON AND GAY LUSSAC.-LAW OF MARIOTTE. -RELATION BETWEEN TEMPERATURE AND PRESSURE OF COMMON STEAM. -EFFECTS OF THE EXPANSION OF COMMON STEAM. MECHANICAL EFFECTS OF STEAM.-METHOD OF EQUALISING THE EXPANSIVE FORCE.

HORNBLOWER'S ENGINE. - WOOLF'S ENGINE. -WATT'S ATTEMPTS TO EXTEND THE STEAM ENGINE TO MANUFACTURES.-PAPIN'S PROJECTED APPLICATIONS OF THE STEAM ENGINE.-SAVERY'S APPLICATIONS OF THE ENGINE TO MOVE MACHINERY.-JONATHAN HULL'S APPLICATION TO WATER WHEELS. STEWART'S APPLICATION OF THE ENGINE TO MILL WORK.-WASHBOROUGH'S APPLICATION OF THE FLY WHEEL AND CRANK.-WATT'S SECOND PATENT.-DOUBLE-ACTION VALVES.

(93.) SINCE the application of the expansive action of steam involves the consideration of its properties when it ceases to be in contact with the water from which it was produced, and likewise the variation of its pressure in different states of

density and at different temperatures, it is necessary here to explain some of the most important of these properties of vapour. Steam may exist in two states, distinguished from each other by the following circumstances :

1st. It may be such that the abstraction from it of any portion of heat, however small, will cause its partial condensation.

2d. It may be such as to admit of the abstraction of heat from it without undergoing any other change than that which air would undergo under like circumstances, viz. a diminution of temperature and pressure.

(94.) We shall call, for distinction, the former Common Steam, and the latter Superheated Steam.

Fig.29.

A

To explain the circumstances out of which these properties arise, let B (fig. 29.) be imagined to be a vessel filled with water, communicating by a pipe and stopcock with another vessel A, which in the commencement of the process may be conceived to be filled with air. Let D be a pipe and stopcock at the top of this vessel. If the vessel в be heated, and the two cocks be opened, the steam proceeding from the water in в will blow the air out of the vessel A through the open stopcock D, in the same manner as air is blown from a steam engine. When the vessel A by these means has been filled with pure steam, let both stopcocks be closed, If the steam in A, under these circumstances, have a pressure of 15 lbs. per square inch, its temperature will be found to be 213°. Now, if any heat be abstracted from this steam, its temperature will fall, and a portion of it will be reconverted into water.

B

Again, suppose the vessel A to be filled with pure steam which has been produced from the heated water in B, the stopcock c being open. Let the stopcock c be then closed, and the water in в be heated to a higher temperature, the temperature and pressure of the steam in a being observed. If the stopcock c be now opened, the steam in a will be immediately observed to rise to the more elevated temperature which has been imparted to the water in B, and at the same time it will acquire an increased pressure.