STEAM ENGINE. THE law discovered by Mariotte, and confirmed by Arago and Dulong, as far as up to pressures of 27 atmospheres, is the simplest that can be applied to the Steam Engine. be enunciated thus : It may If a given weight of steam or gas be made to vary its volume without changing its temperature, the elastic force of the steam or gas will vary in the inverse ratio of the volume it is made to occupy. B G E F Thus, let A B C D be a cylinder, EF and A HG any two positions of the piston, then the pressure in the position EF is H to the pressure in the position HG as the space HDCG is to the space EDCF. Although this is by far the most simple law that we can apply, yet it is not strictly correct, but sufficiently so for the purpose of showing the great advantages that are to be gained by working steam expansively; the results, however, will be always greater than the truth, since, during the time the expansion is going on in the cylinder, the temperature does not remain exactly the same. We shall hereafter give in detail the various properties of steam given by Arago and Dulong, Gay-Lussac, Tredgold, Biot, Pambour, Pole, and others. D C Work, or dynamical effect, as it is sometimes called, B supposes a body moved, and a resistance overcome: either of these, without the other, is insufficient to constitute work. The work, produced by a pressure moving a body through a certain space, is defined to be the product arising from multiplying the pressure by the space through which this pressure acts. UNIT OF WORK. The unit of work, in this country, in terms of which we measure any amount of work, is the work done where a pressure of one pound is exerted through one foot, the pressure acting in the direction in which the space is described: if, instead of one pound being moved through one foot, it be moved through two feet, it is clear that the work is doubled, or that two units of work have been done. THE UNIT OF WORK IN REFERENCE TO THE UNIT OF TIME. The unit of work has been assumed to be 1 lb. raised 1 foot high. Let this be referred to 1 minute as the unit of time. Considered in this point of view, the unit of work will be represented by 1 lb. raised 1 foot high in one minute. Now, it is assumed that a horse is capable of doing 33,000 such units of work; i. e. he is capable of raising 33,000 lbs. 1 foot high in one minute, or 1 lb. 33,000 feet high; and this is called a horse's power, and is the unit of work in reference to the unit of time commonly used in this country. To determine, then, the number of horses' power consumed in any given work, we have only to divide the amount of that work by 33,000 times the number of minutes in which it is done. Given the length of the stroke, the distance travelled by the piston before the steam is cut off, and the pressure at which the steam is admitted in the cylinder, to find the work done upon each square inch of the piston in one stroke. Rule 1.—Multiply the pressure at which the steam is admitted, by the distance travelled by the piston before the steam is cut off; this gives the work done before expansion begins. Divide the whole length of the stroke by the abovementioned distance, and find the hyperbolic logarithm of the quotient. Multiply this hyperbolic logarithm by the work done before expansion, and the result is the work done after expansion begins. Adding together the work done before to that done after expansion, we obtain the whole work done upon one square inch of the piston in one stroke. To find the Horse Power. Multiply the work done on one square inch in one stroke, by the area of the piston in square inches, and by the number of strokes per minute; this product divided by 33,000 gives the horse power. To find the Load *. Rule 2.-Having by the preceding rule found the whole work done on one square inch in one stroke, divide it by the length of the stroke, and the quotient will be the load upon one square inch of the piston in one stroke. To find the Pressure at which the Steam is admitted. Rule 3.- Divide the whole length of the stroke by the number of feet described by the piston before the steam is cut off, find the hyperbolic logarithm of this quotient, add unity to this logarithm, and multiply this sum by the number of feet described by the piston before the steam is cut off for a divisor, and for a dividend multiply the load by the length of the stroke. The quotient is the pressure in pounds per square inch at which the steam is admitted. * The load is generally defined to be the mean pressure of the steam. |