Page:Popular Science Monthly Volume 90.djvu/782

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��Popular Science Monthly

��stream and found that in one minute the chip moved 24 ft. down the stream. At this point the stream had an average width of 12 ft., and an average depth of 3 ft. I then estimated that Q in cubic feet per second would be the width of the stream multipHed by the depth and by the speed of the chip in feet per second, which is 12X3X24/60 or 14.4 cubic feet per second. Because of rapids or other influences, such as great irregularities in the shape of the stream, it is not always possible to determine Q by this method. A satisfactory way is to consider the falls as a weir. To make our proposition general we will say


Chart showinpf'the monthly variations in cu- bic feet per second for a period of one year

that the width of the stream as it goes over the falls is W feet and that the depth of the water as it flows over the edge or crest of the falls is C inches. It may then be shown that the quantity of water flowing over the falls in one second is WC 5\/^/i5 cubic feet. Attention is called to the fact that the denominator 15 is the same as in the expression for horsepower developed. Care should be taken to get the total height C, as the depth of the water at the very edge of the falls is apt to be a little under the true depth.

The stream which we gaged by the chip method has in it a fall 12 ft. wide and the water has a depth of 7 in. as it flows over the edge. From the formula just given Q would be 12X7XV7/15 which is 14.8 cubic feet per- second, or approximately the same result which was obtained by the chip method.

The total height of the fall is 14 ft. so that the horsepower developed would be QH/15 or 14X8X 14/15 which is 1.4, or just slightly under one and one-half horsepower.

Remembering that one horsepower is 746 watts and that the efficiency of a

��generator of this small size would not be over 70 per cent, it will be seen that about 750 watts power would be available for use. This amount of power would light eighteen forty-watt tungsten lamps, which is all that would be required in an average house.

There is one other point to be considered and that is the constancy of flow. The flow of streams varies greatly over different periods of the year, as well as from year to year. An illustration of this is shown in the accompanying diagram which is a plot showing the average flow per month, in cubic feet per second, taken over a period of one year. Such a curve is called a hydro- graph. The hydrograph for this stream shows that the month having the greatest flow was March and that the month of minimum flow was January.

In an actual installation the question of pondage, or water-storage is an important one. By building a large pond, or by using natural basins it is possible to conserve the water so that it will be used only as required. This would mean a greater sup- ply and consequently more power when it was actually needed. Waterpower develop- ments include many questions of this sort but they are all outside of the scope of this article. The principal point to be remem- bered here is that horsepower is cubic feet per second multiplied by the head in feet divided by 15, P = QH/i5.

The installation shown in the illustrations is that of a power plant on a very small stream where it was not convenient to use a dam to store up the water for the power. The use of storage-batteries gave the same result. The water wheel is run all day long, generating all the power the stream affords and using that power to charge the storage-batteries, which were only used at night, except in the fruit storage bins where light was needed at any time.

The battery consists of 16 cells, 80 am- peres. The switchboard carries an ampere- hour meter that shows the condition of the battery; also an ammeter, which shows if more or less current is being used than is being generated; and an automatic switch which prevents the system from damage due to disturbances such as short circuits,- lightning, etc. The outside wiring is carried on poles from the generator-house to the switchboard and from there to the various buildings that are lighted by the plant — residence, fruit storage building, cooperage shed, barn, carriage house and garage — six in all.

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