Mathematical and Physical Papers. 241 



shape or outline of the diagram depends principally upon the man- 

 ner in which the steam is admitted to and released from the engine 

 cylinder. 



Therefore, by careful investigation and measurement of these 

 outlines, and turning the varied information which they furnish to 

 practical advantage, the real value of the indicator is readily made 

 apparent. 



As a preliminary to the study of the diagram, suppose we knew 

 that, at a certain part of the stroke, the full boiler pressure should 

 be realized; now, if this does not appear to be the case on the dia- 

 gram, there is evidently imperfections existing, either from an in- 

 correct adjustment of the valves, 'or maybe due to inadequate 

 capacity of the steam-pipes and passages between the boiler and 

 the engine cylinder; and almost invariably happens also with en- 

 gines having insufficient or extremely light loads. 



Again, the diagram may show too great a pressure at other cer- 

 tain points, when we know there should be less, in order that 

 the demands for economy and efficiency in the engine be obtained. 

 This latter circumstance may also proceed partly from incorrect 

 valve adjustment, although it is principally caused by leakage 

 through the admission valves after cut-off; in combination with 

 the revaporation of steam previously condensed within the cylin- 

 der in the early part of the stroke. 



Any derangement of valve mechanism of the engine, such as 

 incorrect position of the eccentric on the shaft, or an uneven ad- 

 justment in the length of the valve-rods in connections, will in 

 consequence be revealed in the diagram by late admission or re- 

 lease, by low initial or high back pressure, also by absence of 

 compression ; either of which, in performing an equal amount of 

 work, will result in an increased consumption of steam. 



Consequently, where discrepancies of any kind occur, a thorough 

 investigation, study and reasoning of the diagram first become 

 necessary, in order intelligently to locate the cause of the defect, 

 and make changes and corrections accordingly, until the diagram 

 shows a proper distribution of steam pressure throughout the 

 stroke of the engine. 



In this paper we shall deal only with the interpretation of ir- 

 regularities in the lines of the card itself, and what these irregulari- 

 ties indicate. 



An indicator diagram is the result of two movements which are 

 at right angles to each other, one of which is the rotation of the 

 paper drum forward and back around its central stud, and is pro- 

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