358 Mr. Barthtt on propelling Vessels [Nov. 



sails, I apprehend, would derive an additional impetus from the 

 vessel's velocity. 



Whether or not this hypothesis be founded on a just datum, I 

 am not prepared to dispute, nor is it necessary to my purpose to 

 prove an acceleration of motion ; for, as the sails of a windmill 

 move with a velocity nearly equal to the squares of the velocity 

 of the wind, a maximum of effect may be produced equal to the 

 end proposed. 



To pursue the subject of the address ; and which I shall do 

 more by reference to a power familiar to all than by any mathe- 

 matical demonstration of mere speculative opinion, 1 assume 

 (and it is not too much) that one set of common windmill sails 

 is equal to a steam-engine of 20 horse power ; consequently is 

 capable of propelling a vessel of 120 tons (as that power has 

 been found to do) at the average rate of six or seven miles per 

 hour, against wind and tide. If, therefore, one set impart a 

 momentum equal to the propulsion of a vessel of moderate bur- 

 den, three sets, placed longitudinally, as the masts of a ship, and 

 acting independently of each other, with corresponding water- 

 wheels, must impart momenta equal to a steam-engine of 60 

 horse power, or the propulsion of a ship of very considerable 

 dimensions. Tins is but a common inference deduced from 

 known and acknowledged data, and which will, I conceive, 

 render the proposition self-evident. But, supposing the means 

 proposed to be inadequate to the end, an excess of power may be 

 obtained, as I will presently show, by the scientific application 

 •of those means, which would greatly exceed any sum of force 

 ■which might be required. The extremely rude construction of 

 the windmill sails in common use has often attracted the atten- 

 tion of men of science ; and the only way to account for that 

 species of mechanical power remaining unimproved for centuries 

 is, that a quantum of force was probably thereby acquired suffi- 

 cient for the purposes for which it was employed.* That this 

 power might be increased, I will cite the following, out of nume- 

 rous authorities, to prove ; at the same time I beg it to be under- 

 stood, that it is far from my intention (although the subject may 

 almost demand it) to swell this paper into an essay on windmill 

 sails. Ferguson i~ very ingeniously suggests, that as the end of 

 the sail nearest the axis cannot move with the same velocity 

 which the tips, or further ends, have, although the wind acts 

 equally strong, a better position, perhaps, than that of stretching 

 them along the arms directly from the centre of motion, might 

 be, to have them set perpendicularly across the further ends of 

 the arms, and there adjusted lengthwise to the proper angle ; for 

 in that case both ends of the sails would move with the same 



.' * According to Ferguson lliis is the case ; for lie shows that if 1 lie stones of a 

 mill revolve more than 70 times per minute, they pulverize the hiuu wilh the 

 flower. 



f Lect. on Mecli. p. 58. 



