Nov. 15, 1883] 



NATURE 



S9 



ing ; and the result is that a current of cold and fresh air 

 is continually issuing from the machines at the face of 

 the heading, just where it is most wanted. In conse- 

 quence, in the St. Gothard, as just alluded to, the hottest 

 parts were always some little distance behind the face of 

 the heading. Although in this case as much as 120,000 

 cubic metres of air (taken at atmospheric pressure) were 

 daily poured into the heading, yet the ventilation was 

 very insufficient. Moreover, the high pressure which is 

 used for working the machines is not the best adapted 

 for ventilation ; and in the Arlberg tunnel separate venti- 

 lating pipes are employed, containing air compressed to 

 about one atmosphere, which is delivered in much larger 

 quantities, although not at so low a temperature. In con- 

 nection with this question of ventilation a long series of 

 observations have bsen taken at the St. Gothard, both 

 during and since the construction : these have revealed the 

 important physical fact (itself of high practical importance) 

 that the barometer never stands at the same level on the 

 two sides of a great mountain chain ; and so have made 

 valuable contributions to the science of meteorology. 



Another most important use of the same scientific 

 fact, namely, the properties of compressed air, is found 

 in the sinking of foundations below water. When 

 the piers of a bridge, or other structure, had to be 

 placed in a deep stream, the old method was to drive a 

 double row of piles round the place and fill them in with 

 clay, forming what is called a cofferdam. The water 

 was pumped out from the interior, and the foundation 

 laid in the open. This is always a very expensive pro- 

 cess, and in rapid streams is scarcely practicable. In 

 recent times large bottomless cases, called caissons, have 

 been used, with tubes attached to the roof by which air 

 can be forced into or out of the interior. These caissons 

 are brought to the site of the proposed pier, and are there 

 sunk. Where the bottom is loose sandy earth, the 

 Vacuum process, as it is termed, is often employed ; that 

 is, the air is pumped out from the interior, and the super- 

 incumbent pressure then causes the caisson to sink and 

 the earth to rise within it. But it is more usual to 

 employ what is called the Plenum process, in which air 

 under high pressure is pumped into the caisson and 

 expels the water, as in a diving bell. Work iien then 

 descend, entering through an air lock, and excavate the 

 ground at the bottom of the caisson, which sinks gra- 

 dually as the excavation continues. Under this system 

 a length of some two miles of quay wall is being con- 

 structed at .'\ntwerp, far out in the channel of the River 

 Scheldt. Here the caissons are laid end to end with each 

 other, along the whole curve of the wall, and the masonry 

 is built on the top of them within a floating cofferdam of 

 very ingenious construction. 



There are few mechanical principles more widely known 

 than that of so-called centrifugal force ; an action which, 

 though still a puzzle to students, has long been thoroughly 

 understood. It is, however, comparatively recently that 

 it has been applied in practice. One of the earliest 

 examples was, perhaps, the ordinary governor, due to the 

 genius of Watt. Every boy knows that if he takes a 

 weight hanging from a string and twirls it round, the 

 weight will rise higher and revolve in a larger circle as he 

 increases the speed. Watt saw that if he attached such 

 an apparatus to his steam engine, the balls or weights 

 would tend to rise higher whenever the engine began to 

 run faster, that this action might be made partly to draw 

 over the valve which admitted the steam, and that in this 

 way the supply of steanr would be lessened, and the speed 

 would fall. Few ideas in science have received so wide 

 and so successful an application as this. But of late y -ars 

 another property of centrifugal force has been brought 

 into play. The effect of this so-called force is that any 

 body revolving in a circle has a continual tendency to fly 

 off at a tangent ; the amount of this tendency depending 

 jointly on the mass of the body and on the velocity of the 



rotation. It is the former of these conditions which is now 

 taken advantage of. For if we have a number of particles 

 all revolving with the same velocity, but of different specific 

 gravities, and if we allow them to follow their tendency of 

 moving off at a tangent, it is evident that the heaviest 

 particles having the greatest mass will move with the 

 greatest energy. The result is that, if we take a mass of 

 such particles and confine them within a circular casing, 

 we shall find that, having rotated this casing with a high 

 velocity and for a sufficient time, the heaviest particles 

 will have settled at the outside and the lightest at the 

 inside, whilst between the two there will be a gradation 

 from the one to the other. Here, then, we have the means 

 of separating two substances, solid or liquid, which are 

 intimately mixed up together, but which are of different 

 specific gravities. This physical principle has been taken 

 advantage of in a so.newhat homely but very important 

 process, viz. the separation of cream from milk. In this 

 arrangement the milk is charged into a vessel something of 

 the shape and size of a Gloucester cheese, which stands on 

 a vertical spindle and is made to rotate with a velocity as 

 high as 7000 revolutions per minute. At this enormous 

 speed the milk, which is the heavier, flies to the outside, 

 while the cream remains behind and stands up as a thin 

 layer on the inside of the rotating cylinder of fluid. So 

 completely does this immense speed produce in the liquid 

 the characteristics of a solid, that if the rotating shell of 

 cream be touched by a knife it emits a harsh grating 

 sound, and gives the sensation experienced in attempting 

 to cut a stone. The separation is almost immediately 

 complete, but the difficult point was to draw off the two 

 liquids separately and continuously without stopping the 

 machine. This has been simply accomplished by taking 

 advantage of another principle of hydromechanics. A small 

 pipe opening just inside the shell of the cylmder is brought 

 back to near the centre, where it rises through a sort of 

 neck and opens into an exterior casing. The pressure 

 due to the velocity causes the skim-milk to rise in this pipe 

 and flow continuously out at the inner end. The cream 

 is at the same time drawn off by a similar orifice made in 

 the same neck and leading into a different chamber. 



Centrifugal action is not the only way in which 

 particles of different specific gravity can be sepa- 

 rated from each other by motion only. If a rapid 

 "jigging" or up-and-down m jtion be given to a mixture 

 of such particles, the tendency of the lighter to fly further 

 under the action of the impulse causes them gradually to 

 rise to the upper surface ; this surface being free in the 

 present case, and the result being therefore the reverse of 

 what happens in the rotating chamber. If such a mix- 

 ture be examined after this up-and-down motion has gone 

 on for a considerable period, it will be found that the 

 particles are arranged pretty accurately in layers, the 

 lightest being at the top and the heaviest at the bottom. 

 This principle has long been taken advantage of in such 

 cases as the separation of lead ores from the matrix in 

 which they are embedded. The rock in these cases is 

 crushed into small fragments, and placed on a frame 

 having a rapid uo-and-down motion, when the heavy lead 

 ore gradually collects at the bottom and the lighter stone 

 on the top. To separate the two the machine must be 

 stopped and cleared by hand. In the case of coal- 

 washing, where the object is to separate fine coal from 

 the particles of stone mixed with it, this process would be 

 very costly, and indeed impossible, because a current of 

 water is sweeping through the whole mass. In the case 

 of the Copp(^e coal- washer, the desired end is achieved in 

 a different and very simple manner. The well known 

 mineral felspar has a specific gravity intermediate be- 

 tween that of the coal and the shale, or stone, with which 

 it is found intermixed. If, then, a quantity of felspar in 

 small fragments is thrown into the mixture, and the whole 

 then submitted to the jigging process, the result will be 

 that the stone will collect on the top, and the coal at the 



