83Q 



APPLIED MECHANICS. [WATKR-POWBR OVERSHOT-WHKBL*. 



feet per second, or 135 cubic feet por minuto, weighing 



about 8,440 Ibs., mnd reckoning iU fall or effective movp- 



1 :> foot, the distance descended while it remains in 



the buckets, we should have found the power to be 



_ about 31 horse-power. But it must be 



33000 



remembered that all the backet* do not act with th-ir 

 full leverage to turn the wheel in descending through 15 

 feet, being nearer the centre of the wheel above and be- 

 low the middle level than when they pass that point. 

 The former estimate, therefore, of 3J horse-power, is to 

 be taken as the more correct one. 



In nnliT to ascertain what fraction it is of the power 

 actually developed by the descent of the water that is, 

 of the force necessary to raise the water up again to the 

 level whence it flowed we have to consider that 24 cubic 

 feet of water issue from the spout every second, and 

 descend 22 feet, or that a weight of 2J X 62$ = to about 

 140 Ibs., moves 22 feet per second, or 22 X GO = 1320 

 feet per minute ; or that 140 X 1320 = 184,800 Ibs. 

 moved through 1 foot per minuto. This is equivalent to 



moro than 5i horse-power. Of this the 



i.e. i::. 



184800 

 33000 

 mill has been found to render 3J horse-power, or about 



T cent available for driving machinery. 

 1 1 has been stated by some engineers, that as much as 

 70 to 80 per cent, of the power expended by the fall of water 

 has been made available by means of overshot- wheels ; 

 but we are inclined to think that, with the best known 

 construction and proportions, the useful effect does not 

 certainly exceed 70 per cent, of the water-power. 



Of late years, many of these wheels have been made of 

 iron : the partitions of the buckets 

 are constructed of iron plates, 

 bent to a curved form, and the 

 obliquity is made considerably 

 more than in the wooden shroud- 

 ing of former times (Fig. 122). 

 The diameter of such wheels is 

 made somewhat greater than the 

 height of fall, so that the water 

 enters the buckets some distance 

 below the summit, where the 

 inclination of the bucket is suited 

 for the reception of the stream. 

 Even if the buckets were filled at 

 the summit of a wheel, and did 

 not empty themselves till they 

 reached the lowest point, the 

 additional effect of their contents 

 would be of little advantage, as it 

 would act more to press the wheel 

 down on its bearings than to turn 

 it round. It will be found 

 advantageous in practice to rec- 

 kon the diameter of the wheel as 

 Jth more than the full. Thus, 



for a fall of 24 feet, we should make the wheel 24 and 

 J*h of 24, 3 ; altogether 27 feet in diameter. 



It has been r. commended that the velocity of the 

 wheel should be made dependent on the height of the 

 fall ; that is to say, that it should be Jth of the velocity 

 which the water would acquire in reaching the bottom 

 by free descent. We can see no reason why such a rule 

 should be observed ; for, as we have formerly stated, 

 the velocity of the circumference should bo so propor- 

 tioned to that of the water flowing from the spout, that 

 the buckets may be properly filled during their passage. 

 It is true that, by inclining the spout, we may increase 

 the speed of the stream flowing from it, and thus render 

 a greater velocity of wheel practicable ; but, being 

 limited to a certain fall, whatever inclination we give to 

 the spout, we take 10 much from the height after the 

 water is delivered on the wheel, and consequently 

 reduce the moving weight ou the descending side of the 

 wheel. Wo are, therefore, inclined to adhere to the 

 maxim formerly received among millwrights, that tho 



velocity of the wheel's circumference should not exceed 

 3 or 4 feet per second, and that, perhaps, it would most 

 advantageously be fixed at 2 to 3 feet per second. 



The number of buckets may be determined by making 

 it double the number of feet in the wheel's diameter : 

 thus, in a wheel 24 feet in diameter, the number of 

 buckets would be 48. According to this rule, the space 

 from lip to lip of buckets would always be about H feet 

 Where the stream in the spout is wide and shallow, it 

 may be made less ; and where tho stream is deep, it 

 should be greater. But, practically, its size within a 

 few inches is of no great importance ; and we should 

 recommend that a division of the circumference by 0, 8, 4, 

 or such numbers and their multiples, should be made, so 

 as to bring each division nearly to 18 inches. 



In order to provide for the escape of air from the 

 buckets, it is better to make their width exceed, by 

 several inches at each side, that of the stream, than to 

 provide air-slits in the sole ; for, by this arrangement, 

 each of the buckets may be made to hold a considerably 

 greater quantity of water than when the air-slits limit 

 its depth. 



Every precaution should be taken to secure a free 

 flow for the tail-water, as the resistance arising from 

 the immersion of the lower part of tho wheel in a 

 languid stream, takes considerably from its effective 

 force. It is better to sacrifice a few inches of head by 

 inclining the tail-course, so as to give the water some 

 velocity (at least that of the wheel) in its escape, than to 

 let it act as a drag ou tho wheel, by making the tail- 

 course too nearly leveL 



All descriptions of wheels where the water is received 

 on their circumferences, fall under the denomination of 

 overshot-wheels, even if the water be not shot over their 

 summits ; indeed, according to the systems now pur- 

 sued in rendering water-power available, there is no case 

 where a really orersAoi-wheol should be adopted. Instead 

 of making the diameter of the wheel loss than tho height 

 of fall, so that the spout could be carried over it, the 

 diameter should always bo greater, as we have described, 

 so that the water may be delivered at some point below 



Fig. 123. 



tho summit. Instead of an overshot-wheel, in somo 

 cases an endless chain of buckets (Fig. !-) has been 



