no 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[April, 



operation with sufficient rapidity, if the ports be made of proper 

 dimensions. 



Th'' cam is made of steel, and the upper and under surfaces of the 

 square frame against which the cam presses are covered with plates 

 of steel, about three-quarters of an inch in thickness. The cam is 

 bolted upon a cast iron chuck, keyed upon the end of a horizontal 

 shaft, about 3j in. in diameter, which derives its motion from the main 

 shaft of the engine, at the same point where the governor derives its 

 motion. The square frame presses against this cast iron chuck, and 

 is steadied by it. 



The beam, parallel motion, connecting rod, crank, fly wheel, &c. are 

 of the usual description, and therefore require no particular notice. 

 The slide valves are, as we have already mentioned, of the description 

 known as three-ported. There is a loss of steam from the use of this 

 kind of valve. We consider a double two-ported valve to be in every 

 respect preferable. The governor is supported by a frame placed 

 immediately over the crank shaft, and the connection between it and 

 the throttle valve is maintained by a rod which runs beneath the floor 

 of the engine house. The diameter of the cold water pump is 10 in. ; 

 the length of its stroke 2 ft. 54 in. The diameter of the air pump is 

 20 in. ; the length of its stroke 3 ft. The delivery valve is situated 

 in the mouth of the air pump ; it is of the pot-lid construction, but 

 by a partial application of the principle of equilibrium, the noise and 

 force with which common pot-lid valves shut are greatly abated. A 

 valve so constructed as to be in perfect equilibrium would manifestly 

 not work in an air pump, as no upward pressure would open such a 

 valve ; but by the combination of the principle of the equilibrium 

 valve with that of the ordinary pot-lid or spindle valve, a point of 

 adjustment may be arrived at where the valve will open with suflS- 

 cient facility, without being closed with an injurious force. 



This will be better understood by the annexed section and plan of 

 the delivery valve. For this sketch, as well as for the indicator 

 diagrams which accompany this article, we are indebted to the Messrs. 

 Rennie. 



Fig. 1.— Section of Valve. 



be the force requisite to open it ; and if the faces were brought into 

 the same line, no force that the greatest pressure beneath the valve 

 could produce would ever open the valve. 



Eg. 2.— Plan of Valve. 



a a a a a a Fig. 1., is one half of the valve proper, or that part 

 which rises to permit the escape of the air and water lifted from the 

 condenser by the air pump ; 6 6 6 6 6 is one half of the valve seat, or 

 that part which remains stationary, and against which the valve 

 strikes. The valve seat is bolted by six bolts to the mouth of the 

 pump, by bolts which pass through the holes x x x x, Fig. 2. The 

 amount of force with which the valve is closed is that due to the 

 pressure upon the horizontal surface A, Fig. 1, on the ring A A A A, 

 Fig. 2, whilst the real area through which the air and water escape 

 is that represented by B, Fig. 1. Whatever pressure exists upon the 

 side of the valve C, is balanced by the equal and opposite pressure 

 upon the other side C. It is manifest that the more nearly the faces 

 f andy are brought into the same vertical line, the less force will the 

 pressure above the valve exercise in shutting it, and the greater will 



The species of valve applied to the air pump of this engine is cer- 

 tainly superior to that employed in engines in general ; but we believe 

 that the application of the slide valve to air pumps would be preferable 

 still. One slide valve would perform the united functions of the foot 

 valve, the bucket valve, and the delivery valve. The air pump piston 

 might then be made without any opening through it, the water being 

 expelled from the air pump by expression ; and if the piston were 

 made to travel a short distance beyond the port, no accumulation of 

 air could take place in the pump chamber. We cannot enlarge upon 

 this topic further than to say, that by using a slide valve worked \>j 

 the engine, a better vacuum might be attained in the condenser than 

 by any species of valve which is opened by the difference of pressure 

 upon its two sides. 



A certain quantity of the power which an engine exerts is exerted 

 in overcoming its own friction, lifting the water which has accom- 

 plished the condensation of the steam out of a vacuum, &c. The term. 

 horse power is used to denote the available quantity of power which 

 an engine is capable of furnishing for any useful purpose, aud is, there- 

 fore, the excess of the power produced over the power consumed by 

 the engine itself. Any estimate of the power of an engine, based 

 upon the assumption that the whole power exerted by the piston is 

 the true measure of the engine's beneficial exertion, is, therefore, 

 fallacious. An allowance of one-eighth of the power as being con- 

 sumed by the engine itself, is a usual and moderate allowance. 



The amount of economy to be obtained from steam working expan- 

 sively is precisely the same, whether the expansion takes place in one 

 or in two cylinders. The use of two cylinders serves to equalize the 

 action and to diminish the strain thrown upon the moving parts ; but 

 it is questionable whether the greater fluctuation of pressure when 

 only one cylinder is used might not be rendered equally instrumental 

 in the production of a regular motion, simply by using a larger fly- 

 wheel, or driving the fly-wheel at a greater velocity, and whether it 

 is not quite as simple to increase the strength of the moving parts a 

 little, as to add an additional cylinder and piston, to prevent them ftOM 

 being subjected to so great a strain. 



