[Illustration: PLATE IV.
"DIVERTING PLATE" OVERFLOW.
To face page 66.]
It is apparent, therefore, that a pumping machine is wanted which will work continuously without attention, and will not waste money when there is nothing to pump. There are two sources of power in nature which might be harnessed to give this result—water and wind. The use of water on such a small scale is rarely economically practicable, as even if the water is available in the vicinity of the pumping-station, considerable work has generally to be executed at the point of supply, not only to store the water in sufficient bulk at such a level that it can be usefully employed, but also to lead it to the power-house, and then to provide for its escape after it has done its work. The power-house, with its turbines and other machinery, involves a comparatively large outlay, but if the pump can be directly driven from the turbines, so that the cost of attendance is reduced to a minimum, the system should certainly receive consideration.
Although the wind is always available in every district, it is more frequent and powerful on the coast than inland. The velocity of the wind is ever varying within wide limits, and although the records usually give the average hourly velocity, it is not constant even for one minute. Windmills of the modern type, consisting of a wheel composed of a number of short sails fixed to a steel framework upon a braced steel tower, have been used for many years for driving machinery on farms, and less frequently for pumping water for domestic use. In a very few cases it has been utilised for pumping sewage, but there is no reason why, under proper conditions, it should not be employed to a greater extent. The reliability of the wind for pumping purposes may be gauged from the figures in the following table, No. 11, which were observed in Birmingham, and comprise a period of ten years; they are arranged in order corresponding with the magnitude of the annual rainfall:—
TABLE No. 11.
MEAN HOURLY VELOCITY OF WIND
Reference | Rainfall |Number of days in year during which the mean |
Number | for |hourly velocity of the wind was below |
| year | 6 m.p.h. | 10 m.p.h. | 15 m.p.h. | 20 m.p.h. |
—————+—————+—————+—————-+—————-+—————-+
1… 33·86 16 88 220 314
2… 29·12 15 120 260 334
3… 28·86 39 133 263 336
4… 26·56 36 126 247 323
5… 26·51 34 149 258 330
6… 26·02 34 132 262 333
7… 25·16 33 151 276 332
8… 22·67 46 155 272 329
9… 22·30 26 130 253 337
10… 21·94 37 133 276 330
—————+—————+—————+—————-+—————-+—————-+
Average 31·4 131·7 250·7 330·8
It may be of interest to examine the monthly figures for the two years included in the foregoing table, which had the least and the most wind respectively, such figures being set out in the following table:
TABLE No. 12