356 PHILOSOPHICAL TRANSACTIONS. [aNNO J 759. 



In plate 13, fig. 12, abc is a pyramidical frame for supporting the moving 

 parts. DE is an upright axis, on which is framed fg, an arm for carrying the 

 sails at a proper distance from the centre of the upright axis, h is a barrel on 

 the upright axis, on which is wound a cord; which, being drawn by the hand, 

 gives a circular motion to the axis, and to the arm fg ; and so carries the axis of 

 the sails in the circumference of a circle, whose radius is di, thus causing the 

 sails to strike the air, and turn round on their own axis. 



At L is fixed the end of a small line, which passing through the pulleys m, n, o, 

 terminates on a small cylinder or barrel on the axis of the sails, and, by winding 

 on it, raises p the scale, where the weights are placed for trying the power of the 

 sails. This scale, moving up and down in the direction of the upright axis, re- 

 ceives no disturbance from the circular motion. 



aR two parallel pillars standing on the arm fg, for supporting and keeping 

 steady the scale p ; which is kept from swinging by means of st two small chains, 

 which hang loosely round the two pillars, w is a weight, for bringing the centre 

 of gravity of the moveable part of the machine into the centre of motion of the 

 axis DE. 



vx is a pendulum, composed of 2 balls of lead, which are moveable on a 

 wooden rod, and thus can be so adjusted, as to vibrate in any time required. 

 This pendulum hangs on a cylindrical wire, on which it vibrates, as on a rolling 

 axis. Y is a perforated table for supporting the axis of the pendulum. 



The pendulum being so adjusted as to make 2 vibrations in the time that the 

 arm fg is intended to make one turn; the pendulum being set a vibrating, the 

 experimenter pulls by the cord z, with sufficient force to make each half revolu- 

 tion of the arm to correspond with each vibration, as equal as possible, during 

 the number of vibrations that the experiment is intended to be continued. A 

 little practice renders it easy to give this motion with all the regularity that is 



necessary. 



Specimen of a Set of Experiments. 



Radius of the sails 21 inches. 



Length of ditto in the cloth 18 



Breadth of ditto 5.6 



{Angle at the extremity 10 degrees. 

 Ditto at the greatest inclination 25 



instead of a weight, as in the former ; which was certainly best for the purposes of measuring the 

 impulse of the wind, or resistance of plains : but the latter is more applicable to experiments on 

 windmill-sails ; because every change of position of the same sails will occasion their meeting the 

 air with a different velocity, though urged by the same weight. — Orig. 



* In all the following experiments, the angle of the sails is counted from the plane of their mo- 

 tion ; that is, when they stand at right angles to the axis, their angle is denoted 0°, this notation 

 being agreeable to the language of practitioners, who call the angle so denoted, the weather of the 

 sail ; which they denominate greater or less, according to the quantity of this angle. — Orig. 



