rOL. LI.1 PHILOSOPHICAL TRANSACTIOxVS. 353 



From these experiments it appears, that when the wheel made about 20 turns 

 in a minute, the effect was, nearly at the greatest. When it made 30 turns, 

 the effect was diminished about V-o- P^i*t ; but that when it made 40, it was di- 

 minished about i , when it made less than 1 8^, its motion was irregular ; and 

 when it was loaded so as not to admit its making 1 8 turns, the wheel was over- 

 powered by its load. 



It is an advantage in practice, that the velocity of the wheel should not be 

 diminished further than what will procure some solid advantage in point of 

 power ; because, caeteris paribus, as the motion is slower, the buckets must be 

 made larger ; and the wheel being more loaded with water, the stress upon 

 every part of the work will be increased in proportion ; " The best velocity for 

 practice therefore will be such, as when the wheel here used made about 30 

 turns in a minute ; that is, when the velocity of the circumference is a little 

 more than 3 feet in a second." 



Experience confirms, that this velocity of 3 feet in a second is applicable to 

 the highest overshot wheels, as well as the lowest ; and, all other parts of the 

 work being properly adapted, will produce very nearly the greatest effect pos- 

 sible : however this also is certain from experience, that high wheels may deviate 

 further from this rule, before they will lose their power, by a given aliquot part 

 of the whole, than low ones can be admitted to do ; for a wheel of 24 feet high 

 may move at the rate of 6 feet per second without losing any considerable part 

 of its power ;* and, on the other hand, I have seen a wheel of 33 feet high, 

 that has moved veiy steadily and well with a velocity but little exceeding 2 feet. 



IF. Concerning the Load for an Overshot Wheels in order that it may produce 



a Maximum. 



The maximum load for an overshot wheel, is that which reduces the circum- 

 ference of the wheel to its proper velocity ; and this will be known, by dividing 

 the effect it ought to produce in a given time by the space intended to be de- 

 scribed by the circumference of the wheel in the same time : the quotient will 

 be the resistance overcome at the circumference of the wheel ; and is equal to 

 the load required, the friction and resistance of the machinery included. 



V. On the Greatest Possible Velocity oj an Overshot Wheel. 



The greatest velocity that the circumference of an overshot wheel is capable 

 of, depends jointly on the diameter or height of the wheel, and the velocity of 

 falling bodies ; for it is plain, that the velocity of the circumference can never be 



* The 24-feet wheel going at 6 feet in a second seems owing to the small proportion that ihef 

 head (^requisite to give the water the proper velocity of the wheel) bears to the whole height. 

 VOL. XI. Zz 



