366 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1759. 



the Dutch sails in their best position, the wind moves 4 miles an hour , and for 

 every 6 turns in a minute, of the enlarged sails in their best position, the wind 

 will move 5 miles an hour. 



Table vi. Containing the Velocity and Force of 

 Wind, according to their common Appellations. 



The annexed table, which was 

 communicated by Mr. Rouse, 

 and which appears to have been 

 constructed with great care, from 

 a considerable number of facts 

 and experiments, and which hav- 

 ing relation to the subject of this 

 article, is here inserted as he sent 

 it; but at the same time it must 

 be observed, that the evidence 

 for those numbers where the ve- 

 locity of the wind exceeds 50 

 miles an hour, do not seem of 

 equal authority with those of 50 

 miles an hour and under. It is 

 also to be obsen^ed, that the 

 numbers in col. 3 are calculated 

 according to the square of the 

 velocity of the wind, which, in 

 moderate velocities, from what 

 has been before observed, will 

 hold very nearly. 



FIf. Concerning the jibsotute Effect, produced by a Given Velocity of the TVind, 

 on Sails of a Given Magnitude and Construction. 



It has been observed by practitioners, that in mills with Dutch sails in the 

 common position, that when they make about 13 turns in a minute, they then 

 work, at a mean rate, that is, by the compendiums in the last article, when the 

 velocity of the wind is 8^ miles an hour, or 12-§- feet in a second; which, in 

 common phrase, would be called a fresh gale. The experiments in tab. 4, N° 4, 

 were tried with a wind, whose velocity was 84 feet in a second; consequently had 

 those experiments been tried with a wind, whose velocity was 12^ feet in a se- 

 cond, the effect, by maxim 3d, would have been 3 times greater; because the 

 cube of 12^ is 3 times greater than that of 84. 



From tab. 4, N° 4, we find, that the sails, when the velocity of the wind was 

 84 feet in a second, made 130 revolutions in a minute, with a load of 17.25 lb. 

 From the measures of the machine, preceding the specimen of a set of experi- 



