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is, on account of its uncertainty, and from the difficulty 

 occasionally of governing its impetuosity, fast sinking into 

 disuse. Being fully persuaded that this power deserves more 

 the attention of scientific men, after considerable thought 

 upon the subject, and at the risk of being thought presump- 

 tuous, I have ventured to question the truth of the conclusion 

 at which Sraeaton arrived after his numerous experiments, 

 and now submit, that the shape of sail which he recommends 

 is capable of improvement, and that with properly constructed 

 sails, the whole cylinder of wind in which they may be said 

 to revolve, may be advantageously intercepted, whereby the 

 same area of resisting surface may be presented to the wind, 

 with a reduction of 2-5ths of the diameter of the sail, or, 

 retaining the same diameter of the sails, the power of the 

 mill may be nearly doubled. 



I think it will be evident to all who pay attention to the 

 subject, that every portion of the sail should present a pro- 

 portionate resistance to the wind, and should recede from 

 its impulse with that velocity which would least interrupt the 

 wind's onward progress. If the object in view were to pro- 

 duce rectilineal motion, such, for instance, as the propelling 

 a carriage upon a railway in a right line oblique to the wind's 

 direction, the best form of sail would doubtless be a flat 

 surface, and the angle with the wind uniform throughout : 

 but the circumstances are much altered when a rotary motion 

 is required ; in that case the velocity of the sail varies con- 

 stantly from the extremity to the axle, whilst the wind 

 continues to act upon the sail with equal velocity throughout 

 the whole area of the revolving circle. In order, therefore, 

 that the sail should present a due resistance to the wind, and 

 on the other hand not disadvantageously intercept its pro- 

 gress, that form should be adopted which is the best calculated 

 for securing this object. 



