VOL. LI.] PHILOSOPHICAL TRANSACTIONS. 3^5 



Table V. Containing the Ratio of the Felociti/ of the Extremities of Windmill 



sails to the F'elocity oj the Wind, 



It appears from the preceding collection of examples, that when the extremities 

 of the Dutch sails are parallel to the plane of motion, or at right angles to the 

 wind, and to the axis, as they are made according to the common practice in 

 England, that their velocity, unloaded, is above 4 times, and loaded to a maxi- 

 mum, above 3 times greater than that of the wind: but that when the Dutch 

 sails, or enlarged sails, are in their best positions, their velocity unloaded is 4 

 times, and loaded to a maximum, at a medium the Dutch sails are 2.7, and the 

 enlarged sails 2.6 times greater than the velocity of the wind. Hence we are fur- 

 nished with a method of knowing the velocity of the wind, from observing the 

 velocity of the windmill sails; for knowing the radius, and the number of turns 

 in a minute, we shall have the velocity of the extremities; which, divided by the 

 following divisors, give the velocity of the wind. 



Dutch sails in their common position unloaded 4.2, loaded 3.3. 



Dutch sails in their best position unloaded 4.0, loaded 2.7. 



Enlarged sails in their best position unloaded 4.0, loaded 2.6. 



From the above divisors there arise the following compendiums ; supposing 

 the radius to be 30 feet, which is the most usual length in this country, and the 

 mill to be loaded to a maximum, as is usually the case with corn-mills; for 

 every 3 turns in a minute, of the Dutch sails in their common position, the 

 wind will move at the rate of 2 miles an hour; for every 5 turns in a minute, of 



