\VATER-WIIKKI.S. 



WATEK-WHKKI.S. 



TM 



vein; and generally speaking this U effected by making the float* 

 about throe time* the depth of the water in the race, provided the 

 width doea not exceed 2 feet, or 2 feet 2 inches. The distance from one 

 float to another ahould be rather leas than the depth* of the float*, 

 thoir number being regulated principally by the purposes to which the 

 machinery U to be applied. The diameter of the wheel ia fixed in 

 moat mini by the number of revolutions it may be required to perform 

 in a given time, in order to drive the connected machinery at the 

 desired apeed, and with the intervention of the amalleat number of 

 parta ; and to a certain extent it U desirable to make the wheel act aa 

 a fly-wheel, to regulate tho movement of the machinery. In the beat 

 wheela. the velocity of the extremity of the floata U made r = 17 \ a, 

 in which r = the velocity, and u = the fall of water ; and the diameter 



is made D = V n in which N = the number of revolutions per 



minute ; and D = the diameter nought : it U rarely, however, that the 

 diameter of undershot-wheels varies beyond the respective limits of 13 

 and 20 feet. The numbers of floats usually adopted are, fur the dia- 

 meters Riven, aa follows : they are multiples of four, because millwrights 

 prefer distributing them equally upon Uic quadrants of the wheel ; but 

 it ia to be understood that the number may be increased without 

 inconvenience, if such course were desired. 



It is found theoretically that an undershot water-wheel, in which 

 the water thus acts by its shock upon floats working in a straight mill- 

 race, only yields an efficient power equal to half the dynamical effort 

 exerted upon it, even under the most favourable circumstances. But in 

 practice, Smeaton's experiments appear to show that the useful results 

 rarely attain even that value ; and that calling the weight of water r, 

 and the height of the fall 11. the real effect does not exceed O'iSi'ii. 

 The useful effect of these wheels is, therefore, very small ; but the 

 facility they present for variations in the diameters of the wheels, and in 

 their velocities, renders them at time* advantageous, when an unlimited 

 supply of water is at hand. 



In the previous remarks it was observed, that the useful effect of 

 the undershot-wheel was improved by making the race concentric to 

 the outer circumference of the wheel. The gain thus secured is propor- 

 tionate to the arc enclosed ; and the practice at the present day is, 

 therefore, to make that arc as large as the fall will allow (fy. 2), and 



fig. 1. 



usually it U made equal to | or J of the available fall. A space of 

 about half an inch is left between the circumference of the wheel and 

 the surface of the enclosing arc of the race ; tho width of the stream is 

 made such as to allow it to fall on the wheel with a thickness of about 

 8 inches, and the diameter and number of the floats arc calculated as 

 before. The useful effect of these wheels is said to range between 

 0-60 and 0-65r n. M. Poncelet has, however, increased still more the 

 useful effect of the undershot-wheels by making the floats curved, as 

 shown in fyi. 8 and 4, and by doubling their number. He confines the 



Fit. a. 



J and } of the vertical fall in the inverse proportion to the height of 

 the latter. But with all these improvement*, it aeeiua to be very 



Fif. 4. 



difficult to secure from the best undershot-wheeli a greater useful 

 effect than is represented by 0-60 p H. 



A species of undershot-wheel is sometimes used upon rivers, which 

 consists of a vertical float- wheel worked by the current of the river, or 

 even occasionally by the tide. The diameters of these wheels are rarely 

 of considerable dimensions, and the proportion of the useful el! 

 the power actually exerted is but small ; nevertheless, these wheels are 

 extremely useful in new countries, and they present one great advan- 

 tage from the fact of their not interfering in any way with the natural 

 drainage conditions of a locality, as the other classes of mills with 

 heads and dams almost inevitably do, from their upholding the water. 

 [TIDAL WHEEL.] 



The vertical vrmhot-Khetli with buckets, fa. 5, are those in which 



rig. i. 



the water is carried over the top of the wheel, and then made to strike 

 the buckets upon the side of the mill-tail, so that the descent of the 

 wheel is caused by the weight of the water on the unbalancul -i.i 

 shock of the falling water adding slightly to the motive power. As in 

 ordinary undershot-wheels, the overshot ones consist of the axle, arms, 

 rim, shrouds, sole, and buckets ; which may be of wood or of iron, 

 according to the resources of the locality where the mill is to be 

 erected ; the advantage in the use of iron consisting in the facility with 

 which the buckets can be worked to the best theoretical outline, and 

 in its durability; the advantage of wood consisting in its primary 

 economy, and its facility of repair. 



The number of buckeU on an overshot water-wheel is determined 

 upon the following scale, which is found practically to be the most 

 successful, namely, the diameters being given, the number becomes 



water within a close race, and takes more than ordinary precautions to 

 remove the tall- water; he makes the depth of tho floats vary between 



That is to say, assuming the depth of the shrouds to be, as it usually 

 is, 12 inches, and drawing a circle at one-third of that depth, measuring 

 from the inside, the buckets are spaced upon that circle at distances 

 apart of 1 3 inches, as nearly as may be. The length between th shrouds 

 will be determined by the quantity of water they have to cl. 

 and it is desirable not only to make the capacity of the bucket* suffi- 

 cient to receive all the water which may pass into them from Hi.- 

 hatch, but also to make their capacity equal to three times that 

 volume, or the water would have a tendency to splash over, and to 

 escape. Generally speaking, the water is carried over the top of the 

 wheel, at a slight distance from it, and is made to strike the oecou.l r 

 third bucket beyond the summit, at about 2 feet from the vertical 

 line passing through the centre ; because it is considered that the 

 shock thus obtained increases the dynamical force of the water. A 

 more important precaution still than cither of those named above, 

 consists in the provision of means of escape for the air contained 

 originally in the bucket, or carried down by the falling water ; and 



