1220

THEORY AND PRACTICE OF ENGINEERING.

Book II.

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which cross and continue up the back of each arm, to which they arefirmly screwed ; by these timber horns, as they are called, the wholeis very much strengthened.

The whips are generally placed at right angles to each other, andwhen 40 feet in length their scantling is about 10 inches by 8, or 10inches square : at j of the length of the arm from the centre of the wind-shaft is the first bar, which is inserted to make an angle of 60 degreeswith the axis of the wind-shaft : 20 other bars are then placed at regularand equal distances to the end, each making 1 degree of the angle less,so that the last bar at the end has only an angle of 20 degrees ; thebars are all of the same length, generally 5 feet.

The length of the sails of the Dutch mills is not more than 33 feet,and their width about 6, 5 feet being covered with cloth, and the other

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part boarded; where the board and cloth unite, a curve is formed,which gradually diminishes towards the end, and the angles formed bythe tangents of this curve and the plane of their motion is called theangle of weather, which at the foot bar is 30 degrees, gradually di-minishing to 12 degrees.

In 1772 Mr. Andrew Mickle con-trived a method by which the sails ofa windmill could adjust themselves;but that patented in 1804 by Mr. JohnBy water, of Nottingham, was the mostefficient; it consisted in rolling or fold-ing up the sails to the whip, by means —

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Fig. 1920. bywater’s self-adjusting windmill.

Fig. 1922.