WATER. 



line of the partition, and will therefore fplafli the water. 

 The fplaftiing may appear immaterial, but it is in reality 

 yery prejudicial, becaufe the broken water fills the mouth 

 of the bucket, and prevents the air from getting out rea- 

 dily, and it is for this reafon that it is very neceflary to 

 allow fo much of the fall above the height of the wheel, as 

 will make the water run into the buckets, with a little 

 greater velocity than the motion of the wheel. 



Dr. Robinfon, in the Encyclopaedia Britannica, defcribed 

 a plan for the buckets of an overfhot wheel, which was in- 

 vented by Mr. Robert Burns, millwright, and executed by 

 him at a cotton-mill in Scotland : it is (hewn in^. 5. Plate II. 

 IVatc-r-'wheeh. In this way, the wheel has two ranks of 

 buckets, one within the other. The buckets confill of a 

 partition A B, in the direftion of a radius of the wheel, 

 which is joined to another B C, inclined to that, and alfo 

 to a third C D, which is concentric with the rim of the 

 wheel. 



The bucket is divided into two, by a partition L M, alfo 

 concentric with the rim of the wheel, and fo placed as to 

 make the inner and outer portions of the bucket nearly 

 of equal capacity. It is evident, without any farther reafon- 

 ing, that this partition will enable the double bucket to re- 

 tain its water much longer than the fingle one could. When 

 they are filled only one-third, they retain the whole water 

 at eighteen degrees from the bottom of the wheel, and they 

 retain half of the water at eleven degrees. The only ob- 

 jeftion is, that they do not admit the water quite fo freely 

 as buckets of the common conftruftion. 



This arifes from the air, which mufl find its way out to 

 admit the water, but is obftrufted by the entering water, 

 and occafions a great fpluttcring at the entry. This may 

 be entirely prevented, by making the fpout confiderably nar- 

 rower than the wheel, and will leave room at the two ends 

 of the buckets for the efcape of the air. It was found in 

 praftice, that a flow moving wheel, allowed one half of the 

 water to get into the inner buckets, efpecially when the 

 partitions which form the inner buckets, did not altogether 

 reach the radius drawn through the lip D of the outer 

 bucket. The doftor confiders this as a very great improve- 

 ment of the bucket-wheel ; and when the wheel is made of 

 a liberal breadth, fo that the water may be very fhallow in 

 the buckets, it feems to carry the performance as far as it 

 can go. Mr. Burns made the firft trial on a wheel of 

 twenty-four feet diameter, and its performance is manifeftly 

 fuperior to that of the wheel which it replaced, and which 

 was a very good one. It has alfo another valuable properly. 

 When the liipply of water is very fcanty, a proper adjuft- 

 ment of the ftream of water iffuing from the fpout, will 

 direft almoft the whole of the water into the outer buckets ; 

 which, by placing it at a greater diftance from the axis, 

 makes fome addition to its mechanical energy. 



IV. Concerning the proper Velocity of the Circumference of 

 an overjliot Wheel, in order to produce the greatejl EffeB. — If a 

 body of water is let fall freely from the furface of the 

 water in the upper refervoir to the bottom of the defcent, 

 it will take a certain time in falling ; and in this cafe, the 

 whole aftion of gravity will be fpent in giving the water a 

 certain velocity. But if this water in falling is intended to 

 aft upon fome machine, fo as to produce a mechanical 

 cfFeft, the falling water muft be retarded, becaufe a part of 

 the aftion of gravity is then fpent in producing the effeft, 

 and the remainder only will give motion to the falling water, 

 which motion it will retain, after it has quitted the machine. 

 On this principle, the flower a body defcends the greater 

 portion of the aftion of its gravity can be applied to pro- 



2 



duce mechanical effeft, and in confequcnce the greater that 

 effeft will be. 



If a quantity of water falls from a ftream, into each 

 bucket of an overfhot-wheel, it is there retained until the 

 wheel, by moving round, discharges it. Now, the flower 

 the wheel moves, the more water each bucket will receive 

 becaufe it remains a longer time beneath the fpout, fo that 

 what is loft in the fpced with which the wheel moves, ie 

 gained by the prcffure of a greater quantity of water afting 

 in the buckets at once ; and if confidered only in this light, 

 the mechanical power of an overfhot-wheel to produce effefts 

 will be equal, whether it moves quick or flow. The popular 

 reafoning adduced to prove this has been of the following 

 kind. Suppofe that a wheel has thirty buckets, and that four 

 cubic feet of water are delivered in a fecond on the top of 

 the wheel, and difcharged, without any lofs by the way, at 

 a certain height from the bottom of the wheel. 



It IS clear that this ftream will fupply the fame quantity, 

 whatever is the rate of the wheel's motion ; and the buckets 

 muft be of a fufficient capacity to hold all the water which 

 falls into them when the wheel mores very flow. Suppofe 

 this wheel employed to raife a weight of any kind, for in- 

 ftance to draw a bafket of coals out of a deep pit or mine, 

 and that the rope winds upon a barrel of fuch fize that the 

 bafl<et will be drawn up with the fame velocity as the water 

 in the buckets defcends. Suppofe, further, that the wheel 

 will make four revolutions in a minute, or one turn in fifteen 

 feconds, when the load or weight in the bafket which forms 

 the refiftaiice to the motion of the machine is one-third of 

 the load of water contained in the buckets of the wheel. 



Now, during the time of one revolution, fixty cubic feet 

 of water will have flowed into the thirty buckets, and each 

 have received two cubic feet. In this cafe, the hafi<:et may 

 contain a weight equal to twenty cubic feet of water, which 

 weight will be drawn up a height equal to one circum- 

 ference of the wheel, during one turn of the wheel, or in 

 fifteen feconds of time. 



Now fuppofe the machine fo loaded, by making the 

 bafket more capacious, that the wheel can only make two 

 turns in a minute, or one turn in thirty feconds, then each 

 defcending bucket of the wheel will receive four cubic feet 

 of water. If the bafliet contained a double weight, tjz. 

 equal to forty cubic feet, the effeft produced by the ma- 

 chine would be the fame as before, becaufe the velocity is 

 only one half ; but we find in praftice, that it will raife 

 more than in this proportion when it moves flower, for if 

 we attend to what we have juft obferved of the falling body, 

 we find that fo much of the aftion of gravity as is employed 

 in giving motion and velocity to the whei 1 and water therein, 

 muft be fubtrafted from its preffure upon the buckets. 

 The produft made by multiplying the number of cubic 

 inches of water which aft on tlie wlieel at once by its velo- 

 city, will be the fame in all cafes ; yet, as each cubic inch, 

 when the velocity is greater, prelTes more lightly upon the 

 buckets than when the velocity is lefs, the power of the 

 water to produce effefts will be greater in the lefs velocity 

 than in the greater. This leads us to the general rule, that 

 the lefs the velocity of the wheel, the greater will be the 

 effeft produced by any given quantity, and fall of water. 



A confirmation of this doftriiie, together with the limits 

 it is fiibjeft to in praftice, is a matter of experiment and 

 obfervation which has been ably deciJi.d by Mr. Smeaton. 

 The velocity of the wheel fhould not be diminiftied, further 

 than what will produce fome folid advantage in point of 

 power ; becaufe, as the motion is flower, the buckets mufl 

 be made larger, that the increafe of their weight may com- 



pertfate 



