Power is derived from a variety of sources, but what are called the prime movers are derived from heat, through the various fuels, from water, from the winds and from the tides and waves of the ocean. In the case of water the power depends on the head, or height, of the surface of the water above the discharging orifice.
A column of water an inch square and 28 inches high gives a pressure at the base of one pound; and the pressure at the lower end is equal in all directions. If a tank of water 28 inches high has a single orifice in its bottom 1" x 1" in size, the pressure of water through that opening will be only one pound, and it will be one pound through every other orifice in the bottom of the same size.
Power from fuels depends upon the expansion of the materials consumed, or upon the fact that heat expands some element, like water, which in turn produces the power. One cubic inch of water, when converted into steam, has a volume equal to one cubic foot, or about 1,700 times increase in bulk.
Advantage is taken of this in steam engine construction. If a cylinder has a piston in it with an area of 100 square inches, and a pipe one inch square supplies steam at 50 pounds pressure, the piston will have 50 pounds pressure on every square inch of its surface, equal to 5,000 pounds
In addition to that there will also be 50 pounds pressure on each square inch of the head, as well as on the sides of the cylinder.
Fig. 131 shows a cylinder (A), a piston (B) and a steam inlet port (C), in which is indicated how the steam pressure acts equally in all directions. As, however, the piston is the only movable part, the force of the steam is directed to that part, and the motion is then transmitted to the crank, and to the shaft of the engine.
|Fig. 131. Steam Pressure||Fig. 132. Water Pressure|
This same thing applies to water which, as stated, is dependent on its head. Fig. 132 represents a cylinder (D) with a vertically movable piston (E) and a standpipe (F). Assuming that the pipe (F) is of sufficient height to give a pressure of 50 pounds to the square inch, then the piston (E) and the sides and head of the cylinder (D) would have 50 pounds pressure on every square inch of surface.
In the use of fuels, such as the volatile hydrocarbons, the direct expansive power of the fuel gases developed, is used to move the piston back and forth. Engines so driven are called Internal Combustion Motors.
In this case power is derived from the force of the moving air and the calculation of energy developed is made by considering the pressure on each square foot of surface. The following table shows the force exerted at different speeds against a flat surface one foot square, held so that the wind strikes it squarely:
|SPEED OF WIND||PRESSURE||SPEED OF WIND||PRESSURE|
It is curious to notice how the increase in speed changes the pressure against the blade. Thus, a wind blowing 20 miles an hour shows 2 pounds pressure; whereas a wind twice that velocity, or 40 miles an hour, shows a pressure of 8 pounds, which is four times greater than at 20 miles.
It differs, therefore, from the law with respect to water pressure, which is constant in relation to the height or the head—that is, for every 28 inches height of water a pound pressure is added.
Many attempts have been made to harness the waves and the tide and some of them have been successful. This effort has been directed to the work of converting the oscillations of the waves into a rotary motion, and also to take advantage of the to-and-fro movement of the tidal flow. There is a great field in this direction for the ingenious boy.
In no direction of human enterprise is there such a wide and profitable field for work, as in the generation of power. It is constantly growing in prominence, and calls for the exercise of the skill of the engineer and the ingenuity of the mechanic. Efficiency and economy are the two great watchwords, and this is what the world is striving for. Success will come to him who can contribute to it in the smallest degree.
Capital is not looking for men who can cheapen the production of an article 50 per cent., but 1 per cent. The commercial world does not expect an article to be 100 per cent, better. Five per cent. would be an inducement for business