VALVES AND SLIDES. 



26. "We are accustomed to consider the cylinder in a vertical 

 position, to call the covers of its ends the top and bottom, and to 

 speak of the up stroke and the down stroke of the piston. Such is 

 very often the position of the apparatus, but it is not necessarily nor 

 always so. The cylinder is often horizontal. It is almost always 

 so, for example, in locomotive engines, and often so in steamboat 

 engines. It is sometimes placed in an inclined position, and is some- 

 times moveable, changing its position with the motion of the piston. 



The motion of the piston from end to end of the cylinder is 

 called its STROKE, and the dimensions are usually expressed by 

 stating the diameter of the piston and the length of the stroke. 



27. The EFFECTIVE PRESSURE of steam per square inch on the 

 piston is found by deducting from the actual pressure the reaction 

 of the steam escaping, and the friction. This effective pressure 

 being multiplied by the number of square inches in the piston, 

 which is known by its diameter, gives the total effective force of 

 the piston, and this force, multiplied by the number of feet 

 through which the piston moves per minute, which is known by 

 the length of the stroke, and observing the number of strokes per 

 minute, will give the actual mechanical force produced per minute 

 by the steam acting on the piston. 



28. From what has been explained it will be apparent that 

 much of the efficiency of the machine must depend upon the pre- 

 cision and regularity with which the steam is alternately admitted 

 to and withdrawn from either end of the cylinder. If it be 

 admitted or withdrawn too soon or too late, it will either obstruct 

 the force of the piston, or delay its return to the other end of the 

 cylinder. For these reasons, and also because there is much 

 beauty and ingenuity in the contrivances 



by which the steam is admitted and with- 

 drawn, we shall here explain a few of the 

 expedients by which that object is attained. 



29. In the arrangement represented in 

 fig. 15, the object is attained by four conical 

 valves, two placed at each end of the cylin- 

 der. Let B and B' be two steam boxes, B the 

 upper, and B' the lower, communicating 

 respectively with the top and bottom of the 

 cylinder by proper passages D V. Let two 

 valves be placed in B, one, s, above the s / 

 passage D, and the other, c, below it ; and 

 in like manner two other valves in the 

 lower valve box B', one, s', above the passage 

 D, and the other c', below it. Above the 



valve s in the upper steam box is an opening at which the steam 



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