178 OF THE CONSTRUCTION OF [SECT. v. 



of mercury, the atmospheric pressure being 30, the resistance is 120 x '4 = 48, 

 the inches equivalent to the friction ; and 30 + 48 = 78, the whole force on the 

 piston, consequently, 



1 



120 : 78 : : 1 : 0-65 = 



1-54 



the portion of the stroke to be made before the steam be cut off, when its force in 

 the boiler is 120 inches, or 90 inches in excess above the atmosphere. 



373. The excess of force in the boiler must be about four-tenths of the 

 pressure of the atmosphere, or twelve inches of mercury, to cause motion at the 

 proper velocity : but the absolute friction being only about half this force, the 

 engine may begin motion with about a force of six inches. 



374. The most common mode of cutting off the communication between the 

 cylinder and boiler, at the proper period of the stroke, is to give a slide two 

 motions ; the first shuts off the steam, and the second motion lets it on at the 

 opposite end, and opens the other to the atmosphere. Such a construction is 

 shown by Plate iv. Fig. 4. the position of the slide being shown when the steam 

 is cut off by its first motion. This construction represents the principle followed 

 by Messrs. Taylor and Martineau ; but they place the axis of the cylinder 

 horizontally, and construct the pistons of the cylinder and of the slides in a 

 rather different manner from those drawn. 



A horizontal piston rod never works well, and the expense of such a frame as 

 enables us to use it in a vertical position can rarely be more than equivalent to 

 this defect ; nevertheless, in mountainous districts, where mines are difficult of 

 access, a horizontal cylinder has the advantage of being very easily fixed. 1 



375. The power of an engine of this kind should be regulated by altering 

 the time of cutting off the steam ; its power may vary from full pressure through 

 the stroke to that obtained by cutting off at the point above determined : the 

 average state that will give the greatest advantage, being to cut off at a mean 

 between the point which gives the maximum of effect, and that which gives the 

 greatest power required for the work ; for a loss of power arises from cutting off 

 sooner than is indicated by the rule, when the right amount of friction is calcu- 

 lated upon. For modes of giving motion to the slide, see art. 478. 



" = . 4/ + 30 ' 



if we disregard the diminution of the pressure due to the fall of temperature in the process of 

 expansion. 



1 Belidor shows a method of constructing a piston for a horizontal cylinder by the addition of 

 friction rollers. Arch. Hydraul. torn. ii. p. 240. 



