April 20^ 1876] 



NATURE 



491 



from the reservoir to enter during only a part of the 

 course of the piston. But the admission of the air 

 ought to vary if it is desired to obtain the same final 

 effect, since the pressure in the reservoir diminishes 

 continuously ; and as the apparatus which regulates the 

 admission was arranged to correspond only to determined 

 fractions, but not to vary in a continuous manner, it fol- 

 lowed that there was a greater expenditure of air than 

 was necessary, and consequently a diminution in the 

 length of the course over which the locomotive could run. 



On the other hand it is necessary that the air should 

 arrive in the distributing apparatus with the least possible 

 pressure, for it is in this apparatus, in the slide-value, 

 that the greatest losses take place, and these losses in- 

 crease in proportion to the pressure. No means could, 

 however, be thought of for diminishing the pressure in 

 the reservoirs, which would have reduced considerably 

 the work which the machines were capable of doing, 

 unless by augmenting considerably the volume of the 

 reservoirs, the dimensions of which were already un- 

 usually large. 



At this stage M. Ribourc, the engineer of the tunnel, 

 devised an arrangement which allows the compressed gas 

 to flow at a fixed pressure whatever may be the 

 pressure in the reservoir. The gas in escaping from the 

 reservoir enters a cjlinder B (Fig. i), over a certain 

 extent of the walls of which are openings m m. that 

 communicate with another cylinder c, which surrounds it 

 to the same extent, and which is connected with the slide- 

 valve by which the air is distributed, or, more generally, 

 with the space in which this air is to be utilised. On one 

 side moves a piston E, which shuts the cylinder and 

 hinders the escape of the air. This piston carries ex- 

 ternally a shaft F, which supports externally a spiral 

 spring H, the force of which is regulated by means of a 

 screw. Internally it is connected by another shaft L with 

 a second piston X, which bears a cylinder M, movable in 

 the interior of the principal pump, and forming thus a 

 sort of internal sheath. This sheath presents openings 

 ««, which may coincide exactly with those already 

 referred to, and in that case the gas passes without diffi- 

 culty from the reservoir at the point where it is to be 

 employed. But if the sheath is displaced, the openings 

 no longer correspond, there is resistance 10 the passage, 

 and consequently diminution of the quantity of gas which 

 flows out, and hence lowering of pressure in the exterior 

 cylinder. By making the position of the sheath to vary 

 continuously we may make the pressure of exit constant, 

 notwithstanding the continuous variation at entry. But 

 the apparatus is automatic. In fact the part of the cylinder 

 B comprised between the bottom and the piston x c.)in- 

 mimicates by openings p (which are never covered with 

 the escape-tube of the gas), in such a manner that upon 

 its posterior face the piston N receives the pressure of the 

 gas at the moment when it flows, a pressure which it is 

 sought to render constant. The piston E receives on its 

 anterior face the action of the spring which can be regu- 

 lated at pleasure. As to the other faces of the two 

 pistons, they are subjected to equal actions proceeding 

 from the pressure of the gas at its entry, actions which 

 thus counteract each other ; so that the forces which de- 

 termine the position of the movable system are on the 

 one hand the tension of the spring, a constant and deter- 

 mined force, and on the other hand, the pressure of the 

 flowing gas ; and thus equilibrium oannot occur unless 

 the two forces are equal. If the gas should flow in too 

 great quantity, the pressure increases on the posterior face 

 of the piston n, the spring is overcome, and the movable 

 system advances a Uttle towards the left ; but then the 

 orifices are partly covered and the flow diminishes. If 

 the pressure then becomes too weak at the exit, the spring 

 in its turn prevails, pushes the sheath towards the right, 

 uncovers the orifices, and consequently a greater quantity 

 of air may enter. 



The machines which are now used at the St. Gothard 

 tunnel, genuine compressed air locomotives, are furnished 

 with M. Ribourt's apparatus. They consist of the fol- 

 lowing parts : — A sheet-iron reservoir to contain the 

 compressed air is mounted on a framework quite like 

 that of steam locomotives, and carr)'ing glasses, cylin- 

 ders, distributing apparatus, &c. The tube for receiving 

 the air possesses, within reach of the driver, the automatic 

 valve of M. Ribourt. The screw being easily regulated, 

 the air can with certainty be made to issue from the 

 apparatus at a determined pressure. This air then passes 

 into a small reservoir (about one-ihird metre cube) in- 

 tended to deaden the shocks, which are always produced 

 when the machine is set agoing or stopped. Lastly, this 

 small reservoir communicates with the cylinders, and the 

 air which reaches them acts in the same manner as steam 

 in ordinary locomotives. 



The pressure in the principal reservoir at the point of 

 exit depends on the power of the compressing apparatus ; 

 at St. Gothard it may attain 14 kilogrammes per square 

 centimetre, but is ordinarily about 7'35 kilogrammes. 

 The pressure in the small reservoir is arbitrary, depend- 

 ing on the regulation of the screw ; at St. Gothard it has 

 a mean of 4'2o kilogrammes. The entire machine weighs 

 about 7 tons. 



PHYSICAL SCIENCE EV SCHOOLS 



npHE passages from Mr. Wilson's essay of 1867 and his 

 -^ letter of 1876 appealed to me in contradiction on the 

 value of science in developing the power of reasoning and 

 of language, since in his letter Mr. Wilson says that 

 science should not be taught to boys till they have 

 attained a certain power of reasoning and language as 

 shown by their attainments in geometry and Latm ; and 

 in his Essay he speaks of science as supplying the want of 

 clearness and certitude deiUr than arithmetic or geo- 

 metry, and again, as of all processes of reasoning the 

 exhaustive illustration ; and I wished to know whether 

 Mr. Wilson had altered his opinion in the last ten years 

 on this point. 



The question at issue is as stated : " Given that boys 

 are going to remain under a system of liberal education 

 till eighteen or nineteen, at what stages is it shown by 

 experience that it is wise to introduce the different 

 sciences ? " Certainly my experience has not been so 

 extensive as Mr. Wilson's, but I possess the qualification 

 he demands for forming an opinion, that while (during 

 eight years) I have taught science I have also at various 

 times been occupied with mathematics and with language 



The extent to which science should be introduced into 

 the curriculum of a particular school, and the order in 

 which the various subjects should ba taken up, caimot, I 

 think, be practically determined without taking into ac- 

 count various points of mere expediency. If, for example, 

 expense were no consideration, I should prefer certain 

 branches of physics, for example, magnetism and elec- 

 tricity, as the subjects for i/ie Jirst practical work to be 

 undertaken rather than chemistry. But practically there 

 is this difference, that a class of twenty or thirty boys in 

 practical chemistry can be handled by one master with 

 fair success ; whereas the attempt to carry a class through 

 any such course of physics as that sketched in Weinhoid 

 (translated by Foster) could, I think, only end in ladure. 

 Two or three boys to whom one master could give his 

 whole attention might use the book with advantage, but a 

 class could not be so handled, except at the additional 

 cost of two or three assistants and considerable time for 

 preparation. 



Without, then, asserting that this is the plan theoreti- 

 cally best, we have been led by circumstances at Giggles- 

 wick into the following course : — 



The school is di\'ided into the upper school, and the 

 lower (or preparatory) school j the upper school consists 



