1819.] lij Means of Windmill Sails. 359 



velocity, and being- further from the centre of motion, they would 

 have so much the more power. M. Parent, however, considers 

 that the figure of the sails of a windmill should be elliptical to 

 receive the greatest impulse from the wind ; and a windmill with 

 six elliptical sails, lie shows, would have more power than one 

 with four only, as the force of the six would be greater than that 

 of the four in the ratio of 245 to 231. 



The same author also considers which form among the rectan- 

 gular sailswiil be most advantageous, i. e. that which shall have 

 the product of the surface by the lever of the wind, the greatest. 

 The result of this inquiry is, that the width of the rectangular 

 sail should be nearly double ils length ; whereas usually the 

 length is made almost live times the width ! * The following- 

 peculiar construction of the sails -of a windmill used in the vici- 

 nity of Lisbon, merits notice from its being, according to the 

 opinion of Lord Somerville, superior to those used in Great 

 Britain. Their advantages are thus detailed by his lordship : 



1. The broad part of the sail is at the end of the lever, and thus 

 an equal resistance may be overcome with less length of arms. 



2. The sails, constructed upon this plan, maybe set to draw in a 

 manner similar to the stay-sails of a ship ; and as they are 

 swelled more than those of common mills, they render it unne- 

 cessary to bring the mill so frequently to the wind — a practice 

 attended with considerable 

 trouble. The annexed sketch 

 will, perhaps, convey a better 

 idea of its utility ; and at the 

 same time of its applicability 



o nautical purposes. 



Windmill sails may also be 

 made to act horizontally. A 

 scientific mill was, some time A 



since, constructed at Battersea, 

 on the principle, I conjecture, r 

 of the wind-towers of the 

 Asiatics, only that a number 

 of horizontal sails revolved 





■ ' 



/ 

 / 





* For ihe following calculation respecting elliptical sails, I am indebted to a 

 friend. " If the sails of a windmill form a complete ellipsis, whose transverse 

 diameter is HO feet, and coni.igale 01, and are so (lisposed that (he conjugate, or 

 father tenure unj*patt% forms the ien~lh of Ihe arms add by this disposition receives 

 (he whole force of lite wind, and !o>«s none, it is then lo he observed that the^e sails 

 embrace a surface, or rather present to the wind a surface of 4,021,248 square feet; 

 then, admitting Ihe v>ind to be h • i - k , or what nautical men term ' a snug breeze, ' 

 the uiir.l at that rate acts wiih a foree of about a pound on each square fool, or 

 4021 lbs. on the six sails combined ; or supposing, for argument, the dimensions or 

 ► urface pf the sails equal, 'i70.<i05 lbs. on each sail. Now, as the sails tire a lever 

 of ihe fir.,t order, of course tin - pou e'r of eu< h bail i> in propertion to the length of 

 ihe lever (or the circumference described by it), compared u ith ihe semi-axis (or 



