TRANSACTIONS OF SECTION G. 



697 



3. On the Anthracite Goal and Coal-field of South Wales. 

 By C. H. Perkins. See Reports, p. 220. 



4. On the Expansion of Steam in Non-Rotative Pimiping Engines. 

 By Henry Davey, M.List.C.E., F.G.8. 

 (Plates XII., XIII.) 

 The Cornish Engine (Plate XII., fig. 1) has a piston, A, attached to one end of a 

 xuas^ve heam, the outer end of which is attached to the plunger of the pump. On the 

 ylunger is placed a heavy weight, w. When the piston is at the top of its stroke 

 steam is admitted on the upper surface of the piston of sufficient tension to impart 

 to it a high initial \elocity. The shaded diagram a' represents a steam diagram, 

 and the point b the point at which the pressure corresponds with the average pres- 

 sure. The area a, b, c, d, represents the work accumulated in the moving mass w, 

 whilst the piston is moving through the space a, b, and which is given out again 



whilst it is still moving through the distance b, c. 



the 



The energy of the mass at 



WV" 



time of its maximum velocity is expressed in foot-poimds by -^ — • Putting E for 



the resistance of the pump, or, as it is usually termed, the * water load,' and making 

 K = w, and F = the n umtjer o f foot-lbs. represented by the area a, b, c, d, we have 



F = --— and V = a/ . In the example — an eightfold expansion — v = 14-1 



feet per second, a velocity far too great for safe working. 



The Compomul Differential E7igine (Plate XII., fig. 2) is an engine in which 

 expansion can be carried to a greater extent with increased safety. The steam is 

 first expanded to a moderate extent in the cylinder a, and then further expanded 

 on the return stroke in the cylinder b. The engine is double-acting, and has double 

 the power of the Cornish engine. The weights w w bear the same relation to the 

 water columns as w does in the Cornish engine, but as there are in this case two 

 weights the mass is equal to twice the water-load. 



r' = 2wvlandv=.\/^ 



'64-4 



64-4 V 2w 



Assuming, as in the former examples, an eightfold expansion, produced by cutting 

 ofi" the steam at half-stroke in the first cylinder, and expanding that steam into the 

 second cylinder of four times the capacity of the first, the value of F = 58 and 



V = a/^=6-8. 



V 2av , 



The value of v would then be : — 



For the Cornish Engine v = 144 feet per second. 

 For the Compound Engine v = 6-8 „ „ 



In practice the comparison is still more favourable to the Compound Engine 

 Iwcause of the * gap ' in the diagrams. 



The author has constructed a velocity indicator, by means of which he has been 

 enabled to take diagrams showing at a glance the velocities in difierent parts of the 

 stroke. The diagrams (Plate XIII., fig. 2 and 3) were taken by this instrument. 

 From those, and from steam indicator diagrams taken in connection with them, he 

 has been enabled to form the comparison shown in the following table : — 



