698 EEPORT— 1880. 



In the diagrams (Plate XIII., figs. 2 and 3) the ahscissse of the curves represent 

 the spaces passed over hy the piston, and the corresponding ordinates the times in 

 "which those spaces were described. 



The Differential Engine received its name because of its peculiar valve-gear — a 

 gear which aiitomatically effects the distribution of steam by the differential motion 

 produced in combining the motion of the engine-piston with that of a uniformly 

 moving subsidiary piston. 



In Plate XII., fig. 3, d is the differential lever, and c the connection to the en^ne 

 valves. The point r is attached to the subsidiary piston e. The motions of the 

 points E and c are opposite in direction, and when the motions are equal, there is no 

 motion of the point d. When the motion of e is quicker than that of c, the calves 

 are being opened — this occurs at the beginning of the sti-oke ; but as soon as the 

 engine motion c becomes quicker than that of the subsidiary piston e, the valves 

 are being closed. The initial velocity of the engine-piston varies according to the 

 resistance it has to overcome, and the resistance therefore determines the distribu- 

 tion of the steam. 



The Differential Engine is double-acting, and the weights w w, Plate XU. fig. 2, 

 support each other, and cannot move except when moved by the engine. Should 

 there be a vacant space in the pump at the commencement of the stroke, the engine 

 would have no resistance to encounter except its own inertia and fiiction, and would 

 move off veiy readily on the opening of the steam valve, so much so that its initial 

 motion would be greater than that of the subsidiary engine, and the steam vahe 

 would be closed again ; but immediately the plunger came in contact with the 

 water, the full 'load woidd be upon the engine, and a halt would be produced imtil 

 the subsidiary' engine had gained a lead sufiicient to have fully opened the steam 

 valve. Plate XIII., fig. 1, is a steam diagram taken under the conditions just 

 described. This element of safety is most important. Peferring to the Cornish 

 Engine, Plate XII,, fig. 1, the more the steam is expanded in the cylinder, the greater 

 the initial velocity of the plunger, and the more diificidt it is for the water to foUow 

 up the plunger closely, and the greater the xmsupported weight at the end of the 

 stroke. With an eightfold expansion, the plunger would have an initial velocitj' of 600 

 feet per minute, and at the end of the stroke the steam-pressure would only support 

 one-third of the weight w. Should there be a vacant space in the pump, the plunger 

 "would fall back with a great shock. This is one of the greatest sources of accident 

 in the Cornish Engine. All the early compound engines were single-acting, that is 

 to say, they were Compound Cornish Engines, and in addition to possessing defects 

 in principle and construction, were very cimibersome and costly for a given power. 



Plate XII., fig. 4, illustrates Sims's engine, which was a development of Trevet- 

 hick's Pole Engine* 



Plate Xm., figs. 4 and 5 give respectively the relatiAe strains and velocities in 

 the two engines. 



As a question of weight and first cost for a given power, the following is a 

 comparison of the three systems of engines described in the paper : — 



Power. Weiglit. Cost. 



The Compound Cornish Engine . 1 100 100 per cent. 



The Single Cylmder „ „ . 1 70 70 „ 



The Compound Differential „ , 1 45 50 „ 



5, Project for a Channel Railway.^ By Bkadfoed Leslie, M.Inst.C.E., 

 Agent and Chief Engineer, East Indian Railway . 



This was a pamphlet submitted by Mr. Ernest Benedict, M.Inst.C.E., M.I.E. & S. 

 Scotland, accompanied by two dra'R'ings, and describing a project for establishing 

 railway commimication between France and England in the neighboui'hood of Calais- 

 and Dover. The author proposes to lay a single line of rails -within a straightj 

 cylindrical steel tube, 16 feet in diameter and 2| inches thick, smooth outside ana 



' Pamphlet printed at the Stanhope Press, Calcutta."] 



