BM 



APPLIED MECHANICS. [WATM-FOWER VELOCITY or STKKAHS. 



machinery, and a regularity of movement better suited 

 to all thoio mechanical operation! to which motive power 



U POWER OF STREAMS. The power which can be 

 derived from a given stream of water may be computed 

 with tolerable accuracy. When, by levelling the ground, 

 it is ascertained how much fall may be secured, making 

 ample allowance for the declivity of channel to and from 

 the intended wheel, the volume of water delivered in a 

 certain time is to be computed by measuring the area of 

 the existing channel, anil the velocity with which the 

 water flows through it The area of the channel may be 

 found by dropping a plumb-line, at numerous equal- 

 mea.Hun.xl intervals, across some part of the current 

 where the water moves with tolerably equable velocity, 

 iin.l tracing out the section according to the measure- 

 ments so token. The area can then be calculated by 

 the ordinary rule for mensuration of superficies : for 

 example, if the total width of the surface of the stream 

 bo 9 feet, and the soundings taken at every foot be those 

 marked in Fig. 127 (in feet and fractious of a foot), the 



Fig. 127. 



area is the sum of all those depths, viz. , 8 square feet. 

 The velocity of the current may then be ascertained by 

 throwing into it a floating body at some distance above a 

 marked length of channel, so that before it floats within 

 the range of the marked distance, it may have attained 

 the speed of the current. The time of its passage over 

 the marked distance may be then observed by a stop- 

 watch. We may assume, for instance, that the marked 

 distance is 20 feet, and that the floating body occupies 

 5 seconds in passing this ; we conclude that its velocity 



ia " = 4 feet per second. We must not, however, 

 5 



assume this to be the velocity of the whole stream, for 

 at the bottom, and particularly at the shallow sides of 

 the channel, the friction of the water on the rough 

 surface considerably retards it. The effect of this re- 

 tardation may be often observed upon a streak of foam 

 spreading across the channel, the middle part advancing 

 rapidly and breaking away from the side portions, 

 which sometimes are even caused to move backwards by 



Flc.ua. 



the eddying current*. In a shallow stream, like that in 

 the case wo have supposed, we should not, perhaps, be 

 safe in assuming more than half the middle surface speed 

 ai the average of the whole stream, say 2 feet per second. 

 Multiplying this by the area, we find that 8 X 2 16 

 cubic feet of water per second is delivered by the stream. 



Having levelled the ground in the neighbourhood of the 

 stream, we find that by building a dam 4 frut high at A 

 (Fig. 128), and excavating a channel with a gentle 

 declivity to B. a convenient site for a mill, and thence 

 into the bed of the stream, we can get a fall of 15 feet at 

 the mill-wheel, allowing for declivity of choiim-1 from 

 A to B, and a proper fall for the tail-water from B to 

 the stream at D. We should, therefore, have 16 cubic 

 feet of water per second falling down 15 feet of vertical 

 height as our moving power. To lift thin quantity up the 



Hi X I'M) X r,L>J x 15 

 height of its fall, we should require -CMKI ~ 



27 horse-power. By a proper arrangement of wheel, 

 we may expect to secure about jjrds of this for effective 

 working power, which wo should therefore estimate at 

 2" X J = 18 horse-power. A sluice would be fitted 

 across the lead at E, so that the water might be excluded 

 from it for purposes of cleaning or repairing the chann.-l ; 

 and the proper sluice and regulating valve would be 

 fitted at the mill with a sluice and waste water-course 

 C for emptying the lead when the entering sluice at E is 



TURBINES. When the volume of water is small, but 

 the fall considerable, an apparatus called a turbine is 

 frequently applied with great advantage. The principle 

 of its action is similar to that of the well-known tin-work 

 called the Catherine-wheel, or of the revolving jet some- 

 times applied to fountains. For a considerable period 

 it has been known as a philosophical toy, called Barker's 

 mill. This consists of a vertical tube, with two horizon- 

 tal branches closed at the end, mounted on a vertical 

 axis on wliich it can freely revolve (Fig. 129). Near the 

 Fig. 129. 



extremity of the horizontal arms, holes A A are made on 

 opposite sides ; and when water is poured into the upper 

 part of the tube, it flows through these holes, and makes 

 the arms revolve in the opposite direction. The cause 

 of their motion may be very simply explained. If we 

 suppose the apparatus at rest, and the holes closed, 

 when the tube and arms are filled with water, every 

 square inch of the inner surface of those arms is equally 

 pressed on by the column of water in the vertical tube ; 



