LESSONS IN GEOLOGY. 



ones have to nuntum the weight of those above, and will auuord- 



intfly be compressed tu a greater extent than thoM which aro 



liigher up in tlio pile, and therefore hare to sustain the weight 



of fewer. Junt in this way each layer of liquid has to laatain 



jht nf nil above it, and thm the lower layer* are more 



Mlly rom|u-i!saod. An illimtration of the groat preMore 



tlniM \.Ttnl is M.M-n in tho fact that if a tightly-corked bottle 



.k to a depth in the sea it will be broken, or else the oork 



will (. .Invon into it 



ire now to show that this pressure is quite independent 

 of tho shape of tho rossel. Instead of that shown in Fig. 4, let 

 ns have one umdo in the shape of a small tube fitted into the top 

 of a larger one, as shown in suction in Fig. 5. The pressure on 

 the port directly under u K will, as before, 

 . mi th height of tho column of 

 water abovo it. But every part of the 

 base M N must sustain tho same pressure, 

 for otherwise thero would not bo equili- 

 brium, but the liquid would move towards 

 that part where the pressure was least. 

 Every part of a horizontal layer sustains 

 then exactly the same pressure. Wo thus 

 arrive at the apparently strange result, 

 H that if the vessels represented in Figrfi. 4 

 and 5 be filled to tho same height, and the 

 areas of their bases be equal, tho pressure 

 .. on each base will be the same, although one 

 contains a much larger quantity of water 

 than the other. We must not, however. 



Fig. 5. 



suppose that, since the pressures are equal, the vessels, if placed 

 in opposite pans of a pair of scales, would balance each other. 



This paradox is easily explained. Suppose we have a box, 

 the lid of which fastens down by a catch, and we place a spiral 

 spring inside, so that when the lid is closed the spring is 

 powerfully compressed, the pressure on the bottom is manifestly 

 much greater when the box is closed than when it is open, and 

 yet it weighs no more. The fact is, the spring presses the top 

 of the box upwards with exactly the same force as it presses 

 the bottom downwards, and these two forces neutralise each 

 other. So in the vessel shown in Fig. 5, the pressure of the 

 liquid, being transmitted in all directions, presses up against 

 the surface p o P R, and balances a part of the pressure on 

 the base, and the pressure on the scale pan will be the difference 

 between these two, the upward pressure on p R being exactly 



equal to the weight of the ring 1 of water required to make up 

 the quantity there is in the other vessel. 



The following experiment affords a proof of this principle of 

 the pressure being dependent alone on the area of the sur- 

 face and the depth of liquid. Procure three vessels of the 

 shapes represented in Fig. C, and let their bases be made of 

 exactly the same size, and arranged so as to open like trap-doore 

 by means of hinges. 



To a similar part of the base of each attach a string, and let 

 these pass over pulleys and have equal weights affixed to their 

 ends, so as to keep the bottoms closed. 



If now water bo poured into each vessel it will be found that 

 the bottoms will open, not, as might bo supposed, when an 

 equal weight of water has been poured in, but when tho water 

 stands at the same level in each. 



We see thus, that when filled to the same height the bases 

 sustain exactly the same pressure, and this pressure is equal to 

 the weight of the fluid in tho middle vessel. 



Having thus seen that pressure is proportional to the depth, 

 we can examine the variations in it at the different ports of the 

 sides of any vessel or of an embankment. If we have a 

 column of water having a base 1 square inch in area, the 

 pressure on a layer of it at a depth of 1 inch will bo equal to tho 

 weight of a cubic inch of water, or 252'5 grains ; and at a depth 



of 2 hushes the prwsnre will b equal to the weight of 2 cubic 

 inches, and no on, varying in direct proportion to the depth. 



We see thus, that an embankment or tea-wall should also 

 increase in thickneM in the same proportion. The pressure 

 against inch an embankment u, it may be obsorvod, quite 

 independent of the extent of the body of water it sustains. 

 The same strength is required to resut the pressure on the aide 

 of a narrow mill-stream M in a sea-wall, provided the depth 

 be the same in each ease. 



If we divide the side of a rectangular Tessel into any number 

 of equal divisions, tho pressures at these divisions will be in 

 the proportion of the consecutive numbers 1, 2, 8, etc. 



Let these divisions be one foot apart. Then at the first, the 

 pressure on any portion will be equal to the weight of a column 

 of water one foot high. Tho pressure on a square foot at this 

 depth will therefore be equal to the weight of a cubic foot of 

 water. We must not, however, suppose that this will be the 

 pressure on a square foot of the side extending from the sur- 

 face to the first division, for at the surface the pressure U 

 nothing, and it gradually increases with the depth. The mean 

 pressure on the square foot is therefore equal to that at a depth 

 of 6 inches, and the total pressure is equal to the weight of a 

 column of water of this height. So if wo want to know the pres- 

 sure on the rectangular side of a vessel, we must ascertain iU 

 area, and multiply this by half the depth ; we shall thus find the 

 number of cubic feet of water to which the pressure is equal. 



An example will make this clear. Suppose we have a vessel 5 

 feet long and 4 broad, and it be filled with water to a depth of 

 4 feet, what is the pressure on the four sides, and what on the 

 bottom? We will take the sides first; each of these is 5 feet by 

 4, and has therefore an area of 20 square feet ; each of the ends 

 has also an area of 4 feet by 4, or 16 square feet. The total area 

 of the two sides and the two ends is therefore 40 + 32, or 1'i 

 square feet. Now the depth of the water being 4 feet, the 

 mean pressure is found at a depth of 2 feet, and thus the 

 total pressure on the sides is equal to a column of water 72 feet 

 in area and 2 feet in height ; that is, to the weight of 144 cubic 

 feet of water. 



In these calculations we must remember the following 

 weights: 



A cubic foot of water weighs about 1,000 ounces, or 62i pounds. 

 A cubic yard weights f of a ton. 

 A cubic fathom weighs 6 tons. 



The total pressure on the sides is therefore 144 x 62J = 9,000 

 pounds, or rather over 4 tons. The pressure on the bottom 

 is 5 x 4 x 4, or 80 cubic feet of water. This is equal to 

 80 x 62 or 5,000 pounds, which is nearly 2* tons. 



Sometimes the surface on which we want to ascertain the 

 pressure is not a rectangle, but we may always take the mean 

 depth as that of the centre of gravity of the surface, and, 

 multiplying this by the area, we obtain, as before, the pressure. 



We thus see that when water has to be confined by a wall or 

 embankment, the safest plan is to spread it out as widely as 

 possible so as to diminish the depth, and also to let the edges 

 gradually slope down to the middle. If the depth against the 

 embankment be great and a small leak occur as it may, fsom 

 the hole of a rat or some similar cause the water, when once 

 it has found a way, soon wears a larger hole, and the upward 

 pressure of the water is often so great as to blow up the bank. 



It is on account of tho great pressure thus produced by a 

 body of water that lock-gates havo to be made so strong ; and 

 to enable them to stand better, they are usually mode BO that 

 when closed they are in the form of an arch, the convex side 

 being turned in the direction in which the water is highest. 

 When the gates are large, a sliding panel, worked by a screw, 

 is introduced near the bottom, and through this opening the 

 water flows toll it stands at the same level on each side. With- 

 out this the pressure would be too great to allow of the gates 

 being opened. 



LESSONS IN GEOLOGY. XVI. 



DEVONIAN OR OLD BED SANDSTONE SYSTEM. 



OP all the geological systems, none sounds so familiarly to the 

 ear as " The Old Bed Sandstone," for this was the title of a 

 work by tho lamented Hugh Miller, which, at the time of its 

 publication, attracted great attention. Miller carefully studied 

 for years the rocks in which he quarried, and although they are 



