554 



HYDRODYNAMICS. 



Oershot 

 Wheels. 



made 40 turns in a minute. The following Table, 

 for which we have computed the fourth column, con- 

 tains the results which he obtained. 



On the ef- 

 sect of over- 

 Jiot wheels. 



From this Table it appears, that the effect is a maxi- 

 mum when the number of turns is 8^, or when the 

 velocity of the circumference is 1 foot 4 inches per 

 second. The effect diminished by diminishing the ve- 

 locity, and the wheel was at last overpowered by its 

 load, as in Smeaton's experiments, which ought always 

 to happen when the resistance or load is equal to the 

 effect of all the buckets when acting upon a semicir- 

 cumferenceof the wheel with their respective quantities 

 of water. 



In comparing the relative effects of water wheels, 

 the Chevalier de Borda maintains, that an overshot 

 wheel will raise through the height of the fall a quan- 

 tity of water equal to that by which it is driven; 

 while Albert Euler affirms that the effect is greatly in- 

 ferior to this. The experiments of Mr Smeaton shew, 

 that when the heads and quantities of water are least, 

 the ratio between the power and the effect at the 

 maximum is nearly as 4 : 3 ; but when the heads and 

 quantities of water were greater, it is as 4:2; and by 

 a medium of the whole, it is as 3 : 2. When the powers 

 of the water, computed for the height of the wheel 

 only, are compared with the effects, they observe a 

 more constant ratio, the variation being only between 

 the ratio of 10 : 8.1 and 10 : 8.5. Hence the ratio of the 

 power, computed upon the height of the wheel only, is 

 to the effect, at a maximum, as 10 : 8, or as 5 : 4 near- 

 ly ; and the effects, as well as the powers, are as the 

 quantities of water and perpendicular heights multi- 

 plied together respectively. 



The form of the delivering sluice, and the method 

 of introducing the water into the buckets, will be 

 best explained in the description of different overshot 

 wheels. 



Smeaton's Overshot Wheel. 



The overshot wheel, as constructed by Mr Smeaton 

 for the upper paper mill of Thornton, is shewn in Plate 

 CCCXX. Fig. 1, where the diameter of the wheel is as 

 nearly as possible equal to the height of the fall; and 

 Fig. 1 & 2. another wheel, which he considered as of an improved 

 form, is represented in Fig. 2. where the diameter of 

 the wheel exceeds the height of the fall. In both these 

 Figures AB is the wheel, and MN the extremity of the 

 mill course, where the water is delivered into the buc- 

 kets. A vertical lever a b c turning round I as a cen- 



Smcaton'a 



cverthot 



wheel. 



PLATE 



CCCXX. 



tre, gives motion to the horizontal arm c d, and causes Overshot 

 one of the shuttles ef to advance or recede ; in conse- Wheels, 

 quence of which, the aperture on the right hand off ~~Y~~* 

 may be either increased or diminished, for the purpose 

 of regulating the supply of water which the wheel may 

 require. The iron bolt g goes through the bottom of 

 the trough between the two shuttles, and is intended 

 to prevent the bottom from sinking by the weight of 

 the water. From the form of the aperture at f, it will 

 be seen that the water will glide easily into the buc- 

 kets without any waste. In both these machines, the 

 water is turned back on the near half of the wheel ; 

 the consequence of which is, that the resistance of the 

 lower water is removed, as it runs off in the same di- 

 rection with the motion of the wheel. The wheel in 

 Fig. 1. is made to fit its sweep and the sides of the con- 

 duit as if it were a breast wheel, so that the water does 

 not get out of the buckets till it reaches the lowest point. 



Improved Overshot Wheel. 



An excellent overshot wheel, which we understand improved 

 is used in Yorkshire, is represented in Fig. 3. It dif- overshot 

 fers from the wheel in Fig. 2. in the construction of w ! leel - 

 the extremity of the mill course, and in the mode of s ' 

 delivering the water upon the wheel. A pinion rf, 

 turned with a handle, works in the teeth of a rack c a, 

 having a roller a, whose breadth is equal to that of the 

 mill course, fixed at its extremity. Upon this roller is 

 fixed a large piece of leather, which, after wrapping 

 round part of the cylindrical circumference, extends 

 downwards to b, where it is fixed, as seen in the Figure, 

 between two plates of iron or wood held together by 

 screws. This leather forms the shuttle in the follow- 

 ing manner. When the water stands so low in the 

 mill course MN, that none of it runs over the roller 

 so as to fall into the buckets, the pinion d is made to 

 move from right to left, so as to cause the rack c a and 

 roller a to descend. The leather shuttle is thus wound 

 up upon the roller, and the water is allowed to pass 

 over the surface a, and to fall into the buckets through 

 the apertures made of iron bars, as shewn in the Figure. 

 When the water, on the contrary, rises in the mill 

 course, so that too much of it flows over the roller, the 

 rack is made to move in the opposite direction, so as to 

 diminish the supply. In this construction of the regu- 

 lator, we see at once the advantages of having the dia- 

 meter of the wheel AB greater than the height of the 

 fall ; for we are at liberty to take advantage of the ad- 

 ditional head which is gained by any increase in the 

 quantity of water which is conveyed to the wheel. 



Description of Mr Burns' Overshot Wheel without a 

 Shaft. 



This very ingenious machine was invented and erect- 

 ed by the late Mr Burns, whose mechanical inge- 

 nuity we have already had occasion to admire. It is 

 represented in two different sections, in Figs. 4 and 5, 

 and forms a large hollow cylinder by its buckets and 

 sole, without having any shaft or axle tree. 



This wheel is 12| feet diameter, and 7 feet broad 

 over all, and has 28 buckets. The gudgeon is it inches 

 diameter, by 9 inches long. The flaunch is 1^ inch 

 thick at the extreme points. The arms are of red^ 

 wood fir, 6 inches square; one piece making two arms 

 in length, where they cross one another at the wheel's 

 centre, l| inch of the wood remaining in each, con- 

 necting the two opposite arms as one piece. The wheel 

 was made, by first fitting the gudgeon into a large 





Burns'over- 

 shot wheel 

 without a 

 shaft. 

 PLATE 

 CCCXX. 

 Fig. 4 & S. 



