370 PHILOSOPHICAL TRANSACTIONS. [aNNO 175Q. 



than an oblique one of the same dimensions ; but that in a horizontal windmill 

 little more than one sail can be acting at once : whereas in the common wind- 

 mill, all the 4 act together : and therefore, supposing each vane of a horizontal 

 windmill, of the same dimensions as each vane of the vertical, it is manifest 

 that the power of a vertical mill with 4 sails, will be 4 times greater than the 

 power of the horizontal one, let its number of vanes be what it will : this dis- 

 advantage arises from the nature of the thing ; but if we consider the further 

 disadvantage, that arises from the difficulty of getting the sails back again against 

 the wind, &c. we need not wonder if this kind of mill is in reality found to 

 have not above ^ or -^i^ of the power of the common sort ; as has appeared in 

 some attempts of this kind. 



In like manner, as little improvement is to be expected from water mills with 

 oblique vanes : for the power of the same section of a stream of water, is not 

 greater when acting on an oblique vane, than when acting on a direct one : 

 and any advantage that can be made by intercepting a greater section, which 

 sometimes may be done in the case of an open river, will be counterbalanced by 

 the superior resistance that such vanes would meet with, by moving at right 

 angles to the current : whereas the common floats always move with the water 

 nearly in the same direction. 



'Here it may reasonably be asked, that since our geometrical demonstration is 

 general, and proves that one angle of obliquity is as good as another ; why in 

 our experiments it appears, that there is a certain angle which is to be preferred 

 to all the rest ? It is to be observed, that if the breadth of the sail in be given, 

 the greater the angle kin, the less will be the base ik : tliat is, the section of 

 wind intersected, will be less : on the other hand, the more acute the angle 

 kin, the less will be the perpendicular kn : that is, the impulse of the wind, 

 in the direction ik being less, and the velocity of the sail greater ; the resistance 

 of the medium will be greater also. Hence therefore, as there is a diminution 

 of the section of the wind intercepted on one hand, and an increase of re- 

 sistance on the other, there is some angle, where the disadvantage arising from 

 these causes upon the whole is the least of all : but as the disadvantage arising 

 from resistance is more of a physical than geometrical consideration, the true 

 angle will best be assigned by experiment. 



XIX. On the Remarkable Alteration of Colour in a Negro Woman. By Mr. 

 James Bate^ Surgeon in Maryland, p. 175. 



Frank, a cook-maid in Col. Barnes's family, a native of Virginia, about 40 

 years of age, remarkably healthy, of a strong and robust constitution, had her 

 skin originally as dark as that of the most swarthy African ; but about 1 5 years 

 before, tliat membrane in the parts next adjoining to the finger nails, became 



