planes of a Vertical tVhidmill. 17J? 



The length, breadth, and angle of weather at the extremity 

 cf a vane being given ; to determine the angles of weather at 

 different diftances from the centre. 



Let ABj fig. 4, be the length of the vane ; BC its breadth; 

 and BCD the angle of weather at the extremity of the vane, 

 equal to 20 degrees. With the length of the vane AB^ and 

 breadth BC, conftruft the ifofceles triangle ABC : from the 

 point B draw BD perpendicular to CB, then BD is the proper 

 depth of the vane. 



Divide the line AB into any number of parts (five, for 

 itiflauce) J at thofe divifions draw the lines lE, 2F, 3G, and 

 4H, parallel to the line BC ; alfo, from the points of divi- 

 fion I, 3, 3, and 4, draw the lines il, 2K, 3L, and 4M, 

 perpendicular to lE, 2F, 3G, &c. all of them equal in 

 length to BD. Join EI, FK, GL, and HM : then the angles 

 lEI, 2FK, 3GL, and 4HM, are the angles of weather at 

 thofe divifions of the vane ; and if the triangles be conceived 

 to (land perpendicular with the plane of the paper, the 

 angles I, K, L, M, and D, becoming the vertical anoTes, 

 the hypothenufe of thefe triangles will give a perfeft idea of 

 the weathering of the vane as it recedes from the centre. 



On the fuppofition that fuch a vane were infinitely ex- 

 tended, let us confider which part of it is moft applicable to 

 produce power with fafety. T he part of the vane next the 

 centre, till it leflens the angle of weather to 80 or 70 degrees, 

 fuftains but little lateral prefliire; and when the weather 

 arrives at lefs than 20 or 10 degrees, it will fuftain no more 

 than at the centre, while, in the former cafe, the prelfure 

 tending to break the vanes is very confiderable. If we confult 

 a figure, conllrufted to fhew the lateral and direft prefTure 

 that would arife from fluids fl:riking furfaccs with different 

 degrees of obliquity, I think it may be. concluded, that the 

 part of the vane between the ivealher of 80 and 20 degrees, 

 will produce the greatejl effed with the Icajl danger. 



Vol. IV. N XI. On 



