Proceedings of tee Polytechnic Association. 849^ 



that it is free, without shake ; see if the two links are parallel at all 

 times, and the radius "arms at mid position ; if not, the arrangement 

 is not a parallel motion, and must be corrected. See that the ai*m 

 carrying the levers has no vertical shake ; see that the barrel mns 

 true, and adjust a pencil in place to bear lightly upon it. The scale 

 of the indicator should be tested by a mercury gauge, and the mark 

 on the spring corrected accordingly. This is important, for the repu- 

 ted scale is rarely correct, and during repairs it is often varied. The 

 instrument should never be connected to the cylinder ^ori!5/ nor in 

 any position where a current passes the connecting pipe. The con- 

 nection should be large, short, and direct. Be careful to give the 

 barrel the correct reduced motion of the engine piston. Other details 

 may be arranged as convenient. The instrument should be thoroughly- 

 heated before taking a diagram or marking the atmospheric line.. 

 The pencil should be made to bear as lightly as it will make a mark, 

 and it should be allowed to run over the paper several times. Both, 

 ends of the cylinder should be indicated. 



Before beginning an experiment, both engine and boiler should be 

 in average working condition. At the commencement the fire should 

 be clean, and its thickness noted. The contents of the ash-pit should 

 then be removed, and the coal be weighed, the same as in testing 

 boilers. Indicator diagrams should be taken once an hour, or every 

 half hour, or even less, if the load varies considerably. The pencil 

 should be allowed to remain on each diagram a considerable time, to 

 get a fair average. A register or counter should be attached to the 

 engine, the indications of which should be noted at the beginning and 

 end of the experiment, and every even hoiir intervening. If a regis- 

 ter cannot be obtained, the revolutions should be counted and recorded 

 every fifteen minutes. This should be continued not less than eight' 

 hours, and a longer time is preferable. At the end of the experiment 

 the fire should be clean, and of the same thickness as at the beginning, 

 the same as in testing boilers. A log should be kept during the pro- 

 gress of the experiment, showing the time, pressure of steam, revolu^- 

 tions of engine, weight of coal and ashes, and other matters of interest. 

 The calculations are simple, and need not be detailed. We will Lere 

 remark that the fault with most experiments is the short time for 

 which they are tested. To ascertain accurately the consumption of 

 fuel in a given case, requires, as has been said, at least eight hours 

 continuous action, and the mean power cannot be obtained, in many 

 instances, in much less time. A single diagram, taken occasionally^ 



[Inst.] 54 



