^00 Qn Thrashing Machines. Bee. 



afterwards be completely rectified. Great mistakes iiave arisen 

 in erecting machines, from not knowing, or not attending tq 

 the velocity whicli the moving power ought to have, so as the 

 greatest effect might be produced. In an overshot wheel, 

 the greater the velocity of the wheel, the less will be the ef- 

 fect of the moving power, as water only acts with a relative 

 force ; for heavy bodies falling onV ^6 feet in the first second 

 of time, it is evident, that if the circumference of the wheel . 

 was m.oving at that rate, the water would have no effect upon 

 it. On the contrary, v»'hen tlie wheel is standing and the buck- 

 ets filled, the vrater then acts with its v/hole Vv'eight ; so of 

 course there must be a certain velocity of the wheel, where the 

 relative weight of the water will produce the greatest effect 

 on the niachine, or do the greatest quantity of work in a given 

 space of time. If your correspondent had pointed out what 

 this velocity ought to be for wheels of different diameters, he 

 "Wonld have performed a most essential service ; but by giving 

 dimensions of wheels and pinions, without mentioning the just 

 "vpelocity, or even any velocity of the moving power, no bene- 

 fit can he derived from his several calculations. 



What I have said respecting water machines is equally ap» 

 plicablc to those wrought by horses. There is a certain rate 

 per hour at which horses ought to move, beyond which they- 

 ought not to be driven ; and from this rate, all the other move- 

 ments fail to be regulated. Here I may notice what your cor- 

 respondent says concerning long levers. He says, that if the. 

 lever is increased one-third i?i lengthy one horse ivill perform 

 nearly the same ivor^ as two ; but I am afraid that if any of 

 your readers build their faith upon this statem.ent, they will 

 find themselves miserably deceived. In very small circles a 

 horse draws witliin the tangent to the circle, ard there is a 

 loss of power in the direction of the draught, and an increased 

 pressure upon the axis ; but in such circles as are commonly 

 used for tlirashing machines, the deviation of the draught line 

 from the tangent is so very small, that the loss of power is 

 frilling- This is not a matter of opinion, but admits of ma- 

 thematical demonstration; as also the consequences produced 

 by , increasing the length of the lever, it being a well knov/n 

 principle, that whatever is gained in power is lost in timc^ 

 From these things suffer me to I'cmark, that the observations 

 of your correspondent liaye not been duly considered by him ; 

 for instead of conveying instruction, they really tend to mis- 

 lead. 



I am al:^o inclined to thinii that tliere has been some mistake 

 In what he relates concerning the power of a fly on the cn.l of 

 ■•^ ■ ' the 



