Fig. 22. 



our science, or Hydrostatics proper, and most pass on to the ! 



:i, or that whirl. jiiidn in motion, and of 



the various modes of raising them, or deriving motion from 

 them. 



HYDRAULICS. 



n a liquid in contained iu any vessel it exert* a pressure 



against uul this pressure we found to vary with tho 



lielow tho surface. If now wo make an aperture in any 



..i th .-id.- ti liquid will rush out; and as the velocity 

 \virii which it Hows depend* on the pn--i*uro, the lower down 

 is situated the greater will be the Velocity with 

 it Hows. 



If we have a vessel of tho shape shown in I !<- 22, with 

 several jets inserted at diftV ts along the Mil.-, which 



can be opened or closed at pleasure, we can ascertain Inas- 

 much flows from each, and from this tho velocity with which it 

 issues. It is, however, necessary to maintain tho water at the 

 same levol during tho rx)i<-min-nt , and therefore a spout is made 

 at A, and tho vessel so placed that a stream 

 of water from a tap runs in rather more 

 rapidly than it issues from any of the jets. 

 The surplus water will escape by the spout, 

 and thus maintain a uniform level and pres- 

 sure. By a series of experiment* conducted 

 in this way Torricelli arrived at the conclu- 

 sion that if the distance of any jet, E, below 

 the surface is four times as great as that of 

 any other, B, the velocity with which the water 

 will issue from tho first is twice as great as 

 from the other; that is, that the velocity 

 varies as the square root of the height of the 

 water. Further experiments point out that 

 this velocity is just that which, under the 

 laws of gravity, tho liquid would acquire in 

 falling from the surface to the opening. Thus, if the jet be 

 one foot below the surface, the liquid will issue with a velocity 

 of eight feet per second, that being tho velocity a body acquires 

 i:. falling through a space of one foot. If, then, the aperture 

 have an area of 1 square inch, 96 cubic inches ought to 

 flow out in one second, but on trying the experiment wo find 

 that only about 60 cubic inches actually flow, or about 62 per 

 cent, of the calculated amount. This discrepancy seems at first 

 sight to show the inaccuracy of the law, but on further exami- 

 nation it only confirms it. 



If we make an opening in the bottom of a vessel (Fig. 23), 

 and carefully observe the water as it issues from it, we shall 

 notice that tho stream is not the same size throughout, but 

 narrows considerably just beyond the orifice, 

 so that the smallest area is a little way below 

 the opening. Thus, if A B be the aperture, 

 the part of the stream with the smallest sec- 

 tional area will be at a b. Tho particles of 

 water, in flowing along towards the opening, 

 acquire an onward motion, which they retain 

 as they flow out, and this narrows the stream. 

 Now the section at a & is found to be just T% 

 of that at A B, but the actual efflux we found 

 was actually this fraction of tho theoretical. 

 If, then, we take as tho area through which 

 the water flows, not that of the aperture, but 

 that of the section at a b, the actual flow will just correspond 

 with the calculated amount. 



The diminished stream at a 6 is called the vena contracta, or 

 contracted vein, and in all their calculations allowance is made 

 by hydraulic engineers for the existence of this. 



We have thus far supposed the water to flow from a hole 

 made in the side of the vessel ; tho actual amount that escapes 

 is, however, very much varied by inserting a jet or pipe from 

 which the water may issue. If a straight pipe, whose length is 

 about three times its diameter, be inserted in the opening, the 

 flow from it will be increased to about 82 per cent, of the theo- 

 retical amount ; if this pipe be slightly tapering outwards, the 

 issue will be still greater, while, if it taper inwards, the external 

 part being larger than the opening in the vessel, the flow will 

 slightly exceed the calculated amount. These differences are 

 partly accounted for by currents which are formed in the water, 

 and which by collision with the issuing stream destroy a portion 

 of its velocity. 



ANHWKkft TO EXAMPLES IH LEMOK V. (ps* TB). 

 I. ltapciBeffTritjri2M. 

 1 The ipocific fntritf ol the liquid U "*. or HIM. 



3. Tb* w>U>r iu tU Mk weifcs 1 trail*, while ttesMiw talk at 

 oil weigh, only 140 (niiw. IU sfMo gr- 

 0*848. 



would bo immersed either wjr , or U of iU <Uf*h. 

 10 Inch !<! U vertical It il.A. }{ x 10, or *} ioohM. U the offer 

 side, tbe irameraion Is T : \ toebM. 



ft. The rop* hu to nuUin flttf poooAi. or 1 too and 94f 



ELECTRICITY. 1 1 1 



FOBK8 Or BATTXKT (Cvntinutd) j UAXIKU/t, OOV'i, MTV. 



SIN'U, CALLAN'H CRUCIBLE UATTIHT MATMOOTM BAT- 



TIRT OBOVC'H OAU BATTBBT. HIJTI* AM TO WOEKIVO 



BATTJCB1M. 



VARIOUS other forma are sometime* given to Daniel!'* Battery 

 instead of that described in our last leMon. Tbo outer vesael, 

 v, may be made of glass or common earthenware, a* shown in 

 Fig. 14, and then a cylinder of copper, C, sometime* perforated to 

 allow the free passage of the solution, is placed in it ; the line, 

 too, z, may bo in tho form of a plate, or may bo bent round into 

 a cylinder instead of being a rod. If there bo any difficulty in 

 obtaining the porous cell, P, one may easily bo made by rolling 

 brown pujiur round a nil. -r, and tying a ping of wood in tbe 

 bottom, or it may be made of a piece of ox gnllct or any animal 

 membrane, tho object bc.ng to prevent the liquids mixing, but at 

 the same time to allow tho free passage of the electricity. Sail- 

 cloth even baa been tried and found to answer for this purpose. 



The theory of the action of this form of battery U somewhat 

 as follows : As the dilute acid acts upon tho zinc it dissolves a 

 portion of it, at the same time decomposing some of the water 

 and setting free its hydrogen. This gas, in the way already de- 

 scribed, passes through the diaphragm and the solutions by a 

 series of decompositions, till it reaches the copper plate, where 

 it is set free. It does not, however, escape here, nor remain 

 attached to tho plate, producing polarisation, as is the case ir 

 some of the batteries we have described, this effect being pre- 

 vented by the sulphate of copper solution which the hydrogen, 

 when in its nascent state, decomposes, precipitating the copper 

 on the plate of that metal, at the same time combining with it- 

 oxygen to form water, and setting free the acid. This acid in 

 part passes through the porous cell, and tends to replace that 

 neutralised there by tho zinc. We see thns that the battery 

 remains nearly constant for a time, provided crystals of sulphate 

 of copper are present in tho outer cell to replace that decomposed 

 by tho hydrogen. 



The copper cylinder need not be made very thick, as it is con- 

 stantly gaining by tho copper deposited upon it When a battery 

 of this kind is carefully sot to work, and the zinc well amalga- 

 mated, it will continue in action for six or eight weeks, or ere 

 longer, without further attention. But though it has this great 

 advantage, the quantity of electricity produced by it is not very 

 large, and hence, when great power is required as, for instance, 

 in working a large electro-magnet or for the electric light 

 Grove's Battery, or Bansen's (Fig. 15), is more commonly em- 

 ployed. 



In Grove's Battery zinc, z, i, as usual, the positive metal, 

 and is placed in a solution of diluted sulphuric acid, A, while a 

 sheet of platinum, P (Fig. 16), immersed in strong nitric acid, or 

 in a mixture of equal parts by measure of nitric and sulphuric 

 acids, is used as the negative one. Originally these cells werr 

 made of a circular form, as represented in Fig. 15, but now it is 

 found much more convenient to make them rectangular, as thu* 

 they pack much more closely and firmly when it i* required to 

 set up a number at onco. Tho illustration (Fig. 17) will show 

 the best form of construction for this battery. 



A represents the outer cell, which is usually made of white 

 earthenware ; ebonite is sometimes employed in place of this, a* 

 it i loss brittle; it is, however, much more expensive. A very 

 convenient size for these is about five or six inches high 1 

 inches wide. Inside this fits a piece of zinc, which is usually 

 bent in tho shape shown in Fig. 18. There are, however, wren 

 great objections to the use of pieces of rino shaped thun :- 

 surface of the bent part is much more easily acted upon by tl 

 acid, and therefore it soon breaks through there ; there is also 



