1150

THEORY AND PRACTICE OF ENGINEERING.

Book II.

Fig.1741.

Fig. 1740.

OVERSHOT WHEEL.

weight of water acting at the end of the lever n O, equal to the distance of its centre ofgravity, d, from a vertical line passing through the axis O. Thus the mechanical effect ofthe water in the bucket in-creases till it arrives at B, y

after which in its progresstowards C it diminishes. Thepower of an overshot wheeldepends much upon the formof the buckets, which shouldalways be the fullest whenat the point B, and shouldretain the water as long aspossible.

The form of bucket prefer-red(fig.l"41.)is composed ofthree partitions; AB and G I,called the start or shoulder,which lies in the direction ofthe radius of the wheel; B Cand I K the arm, inclined atan obtuse angle to the radius ;and CD, K L the wrest, in-clined to an angle less than180°, to the arm B C or IK; the depth,

A G, of each bucket is about 1^ of G II ;

A B is half A M, and the angle ABCis such that B C and G I prolonged wouldpass through the same point II ; it ends,however, in C, so that F C is five-sixthsof G II, and C D is so placed that H I) isnearly a fifth of II NI : hence, it followsthat the arc F A B C is nearly equal toD A B C, so that the quantity of waterF ABC will continue in the bucketwhen AD is in a horizontal position,which is the case when A B forms anangle of about 35° with the vertical line.

There are several methods for sup-plying overshot wheels with water; insome instances it is let on a little belowthe upper part, or is varied to differentheights, in which case the wheel must be adjusted to suitthe lowest head, which is a disadvantage.

The Pentrough is made of cast-iron, and the waterruns over the upper side of the roller R, through barredspaces into the buckets of the wheel.

Barker's Mill is supplied by water from a reservoir, andconducted by an upright pipe into the horizontal trunk(), which has arms of an equal length, and through theends of which the water runs out. The upright spindleis fixed in the bottom of the trunk, to which it is screwedby a nut, in such a manner that when the trunk or up-right pipe is turned round, the spindle moves with it;to this spindle the rynd of the upper millstone is at-tached : when water is admitted from the constant reser-voir into the cup at the top of the vertical pipe, andsuffered to run out by a continued stream from the endsof the trunk, the upper millstone, together with the trunktube and spindle, turn round.

The length of the axis in a mill of this kind mustalways exceed the height of the fall, and therefore thereis some difficulty in introducing Barker’s mill when thefall is very high. M. Mathon de la Cour proposed thatthe water should be introduced from the mill-stream intothe horizontal arms, which are fixed to the upright spin-dle without having any tube ; when the water would issue from the openings below in thesame manner as w'hen introduced at the top of the tube as high as the fall. There are

n r-i I | I

Fig.1742.

PENTROCGH,

Fig.1743.

barker’s mill.