48 
Arterial Drainage 
Co.'s patent windmills, with annular sails fixed for working 
a set of pumps, and with horse-gear attached. 
Wind-engines with four or five arms and cloth sails are very 
inexpensive, and much more simple in construction than those 
with annular sails, and will be found in frequent use in brick- 
yards for working a pump for emptying the brick-pit in winter 
time. If made with self-regulating wind-gear and patent wooden 
sails, they are more costly, but very effective little machines, 
applicable to a great variety of purposes. The next illustration 
(Fig. 10) shows one of these engines, as supplied by Messrs. 
Owen and Co., for working a pump from a deep well. 
Where the supply is not required at a great elevation, as, for 
example, for a small railway station or farm-yard, the tanks are 
frequently constructed of wrought iron, and placed on the top of 
the framing or tower carrying the sails. Such a tank could be 
made to hold about 1200 gallons. 
The pressure of the wind acting on any surface, expressed in 
pounds on one square foot, is equal to the square of the velocity 
of the wind in miles per hour, multiplied by •0049. 
The following table gives approximately the velocity and 
force of the wind, and the corresponding numbers of the Beaufort 
Scale used to denote its force by sailors and in the daily weather 
reports in the newspapers. 
Miles per 
Hour. 
Force on one 
Square Foot. 
Corresponding 
figure of 
Beaufort Sciile. 
1 
0 
Calm. 
5 
2 oz. 
1 
Hardly perceptible. 
10 
i)b. 
2 
Liglit breeze. 
20 
2 lbs. 
3 
Good steady breeze. 
30 
^ „ 
4 to 6 
High wind. 
40 
8 „ 
7 
Gale. 
50 
12 „ 
8 
Storm. 
60 
18 „ 
9 
Heavy storm. 
70 
24 „ 
10 
Hurricane. 
80 
32 „ 
11 
Hui-ricanc. 
100 
50 ., 
12 
Tearing up trees, &c. 
It has been found practically that a wind moving with a force 
of less than 10 miles an hour is not able to insure the working 
of a corn-mill ; when the velocity exceeds 20 miles an hour it 
is necessary to furl the sails.* 
To find the power given off by a wind-engine, the area of the 
sails in square feet must be multiplied by the cube of the velocity 
of the wind in feet per second, and the product divided by 
* Bumcll's, ' Hydraulic Engineering : " Pneumatics." ' Weale's Series. 
