tumn of the valve by degrees; the stroke of the valve on its disk or guard would be very great, and the parts would soon work out of order. The pipe which carries away the water that is pumped out of the condenser by the air pump, is shown near the top of the hot well, on the side farthest from the cylinder. The way in which the fire is regulated, is as follows:When the steam gets too strong, the water in the boiler rises in the feed pipe, and carries up the float W; and as the float is connected by a chain and a pulley with the damper V, the damper descends into the flue, and reduces the draught in the furnace, and the force of the steam. Again, if the steam gets too low, the float falls and raises the damper, to increase the draught. The two pulleys which form the connexion between the damper and the float are both fixed on one shaft; on account of the one being placed exactly behind the other, one of them only can be seen. As the balls YY are carried round along with the rod Z, when the engine is going too quick, the balls by their centrifugal force fly out, and the rods and levers in connexion with them shut more or less a valve at A', in the steam pipe; if the engine goes too slow, the balls fall down, and open this valve to give the engine more steam to bring up its motion. The rod B', and the lever C', form part of the connexion with the valve in the steam pipe and the go vernor. It is clear, that the power of the steam engine will dcpend upon the energy of the steam,-1st. Steam of two atmospheres will, other things being equal, produce double the effect of steam of one atmosphere.-2d. the force of the steam remaining the same, the power of the engine will depend on the extent of surface acted upon, that is, on the area of the piston.-3d. these two circumstances remaining the same, the power of the engine will depend on the velocity with which the piston moves. For the sake of illustration, let us suppose that steam is admitted into the cylinder, so as to press down the piston with the force of one hundred pounds, and that the length of the stroke is five feet; and suppose that the end of the piston rod is attached to a beam whose fulcrum is in the centre, and that to the other end of the beam there is attached a weight of any thing less than one hundred pounds, there being no friction. By the descent of the piston, the weight at the end of the beam will be raised 5 feet; therefore it follows, that 100 pounds raised 5 feet during one descent of the piston, will express the mechanicai effect of the engine. The reader will easily perceive that the weight at the end of the beam must be somewhat less than 100 pounds, for as it acts contrary to the power of the piston, if they were equal the machine would be at rest. If we suppose the area of the piston double of what t was before, other things being the same, the engine would raise 200 pounds through the same space of 5 feet in the same time: and the same effect would evidently ensue if we supposed the area of the piston to remain as it was at first, but the force of the steam to be doubled. the area of the piston and force of steam be the same as at first, but the length of stroke doubled, then the mechanical effect of the engine will be 100 lbs. raised 10 feet high during one descent of the piston; and if the descents be performed in the same time, this engine will be double the power of the first. If Let us proceed now to actual cases. In the common low-pressure steam engine of Watt, steam is admitted into the cylinder whose elastic force is somewhere about that of the atmosphere, which we have all along supposed to be 15 lbs. to the square inch; but friction and imperfect vacuums tend to diminish this pressure, and the effective pressure may be reckoned only four-fifths of this. If the pressure of the steam is diminished by its one-fifth part, which is 3 lbs. to the square inch, then will the effective pressure be 12 lbs. to the square inch. The working pressure is generally reckoned at 10 lbs. to the circular inch, and Smeaton only makes it 7 lbs. The effective pressure we have taken is between these extremes, being equivalent to 9.42 lbs. to the circular inch. Mr. Tredgold gives the following table, which will show how the power of the steam, as it issues from the boiler, is distributed. In an engine which has no condenser: The pressure on the boiler being................. 1. The force necessary for producing ......... •2000 10.000 4. The force required to expel the steam 5. The force expended in opening the 6. By the steam being cut off before the Amount of deductions Effective pressure••• In one which has a condenser: The pressure on the boiler being...... ⚫0069 ⚫0622 ..... •1000 3920 6080 1000 007 1. By the force required to produce motion ..... .... 125 2. By the cooling in the cylinder and pipes 016 007 5. By the force required to open and close ........ 063 6. By the steam being cut off before the end 100 7. By the power required to work the air pump..... 050 368 632 If we now suppose a cylinder whose diameter is 24 inches, the area of this cylinder, and consequently the area of the piston in square inches, will be, = 242 x 7854 = 452.39. Let us also make the supposition that steam is admitted into the cylinder of such power as exerts an effective pressure on the piston of 12 lbs. to the square inch; therefore, 452.39 x 12 5428 68 lbs., the whole force with which the piston is pressed. If we now suppose that the length of the stroke is five feet, and the engine makes 44 single or 22 double strokes in a minute, then the piston will move through a space of 22 × 5 × 2 220 feet in a minute; and from what has been said before, it will not be difficult to see, that the power of the engine will be equivalent to a weight of 5428 lbs. raised through 220 feet in a minute. = This is the most certain measure of the power of a steam engine. It is usual, however, to estimate the effect as equi valent to the power of so many horses. This method, however simple and natural it may appear, is yet, from differences of opinion as to the power of a horse, not very accurate; and its employment in calculation can only be accounted for on the ground, that when steam engines were first employed to drive machinery, they were substitutea instead of horses; and it became thus necessary to estimate what size of a steam engine would give a power equal to so many horses. There are various opinions as to the power of a horse. According to Smeaton, a horse will raise 22,916 lbs. one foot high in a minute. Desaguliers makes the number 27,500; and Watt makes it larger still, that is, 33,000 There is reason to believe that even this Lumber is too small, and that we may add at least 11,000 to it, which gives 44,000 lbs. raised one foot high per minute. Now, in the case above, we found that the engine of 24 inch cylinder, would raise 5428 lbs. through the space of 220 feet in one minute; and it is easily seen that it could raise 220 × 5428 lbs. through one foot in the same time, therefore, 220 × 5428 = 1194160 lbs. raised through one foot in one minute, is the effective power of the engine; and from these considerations it will be easy to find the power according to the different estimates of a horse's power. For, according to the usual estimate. The reader will have no difficulty in forming a general rule for estimating the power of a steam engine. (The effective pressure on each square inch × the area of pistor in square inches x length of stroke in feet X number of strokes per minute) 44000 = the number of horses' power of the engine. What is the power of a low-pressure engine, whose cylinder is 30 inches diameter, length of stroke 6 feet, making 16 double strokes in the minute? NOTE. An easy rule to find the area of the piston in square inches, is this, The diameter × circumference equa the area of the piston in square inches; and 12 the effective pressure, 6 the length of stroke, 16 the number of double strokes in a minute? If the cylinder of a high-pressure, steam engine has a piston of 5 inches diameter, with a twelve inch stroke, making 32 double strokes in a minute; steam being admitted of an elastic force equivalent to 7 atmospheres on the inside of the cylinder. Its effective pressure will be 7 X 15105 lbs. to the square inch without friction; but allowing one-fifth for friction, the effective pressure will be 105-21 = 84 lbs. to the square inch. horses' power. 44000 105530.88 44000 A convenient rule for finding the power of a high-pressure engine, is-let P be the force of the steam in the boiler, A the area of the piston, and V the velocity of the piston in feet per minute, then, 0.9 P6 x A x V horses' power. The pressure of the steam in a boiler is 30 lbs. per square inch, the diameter of cylinder 12 inches, length of |