358 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1759. 



Table hi. Containing Nineteen Sets of Experiments on fVindmill Sails of various 

 Structures, Positions, and Quantities of Surfaces. 



Observations and Deductions from the preceding Experiments. 

 I. Concerning the best Form and Positron of Windmill Sails. — In table 3, 

 N° I, is contained the result of a set of experiments on sails set at the angle 

 which the celebrated Mons. Parent, and succeeding geometricians for many 

 years, held to be the best, viz. those whose planes make an angle of 55° nearly 

 with the axis; the complement of which, or angle that the plane of the sail 

 makes with the plane of their motion, will therefore be 35°, as set down in col. 

 ' 2 and 3. Now if we multiply their number of turns by the weight they lifted 

 when working to the greatest advantage, as set down in col. 5 and 6, and com- 

 pare this product (col. 8) with the other products contained in the same column, 

 instead of being the greatest, it turns out the least of all the rest. But if we 

 set the angle of the same planes at somewhiit less than half the former, or at any 

 angle from 15° to 18°, as in N° 3 and 4, that is, from 72° to 75° with the axis, 

 the product will be increased in the ratio of 31 -.45; and this is the angle most 

 commonly made use of by practitioners, when the surfaces of the sails are planes. 



