310 REPORT — 1841. 



that when the carriage sustains no traction, and the springs suffer therefore 

 no deflection, it rests over the centre of the revolving disc B D. 



In this position of the wheel F it would evidently receive no motion from 

 the revolution of the disc B D, however fast that disc might revolve, whilst 

 in any other position between F and D, the wheel F would receive a motion 

 from the revolution of the disc, which motion (if the revolution of the disc 

 were uniform) would be directly proportioned to the distance of the wheel F 

 from the centre of the disc, that is, directly proportioned to the separation of 

 the springs, or to the force of the traction. 



Since then, if the revolutions of the disc were uniform, that is, if the mo- 

 tion of the carriage wei'e uniform during any exceedingly small time, the mo- 

 tion of the wheel F would vary as the force of traction, and that evidently, 

 if the force of traction were constant, the motion of the wheel F would vary 

 directly as the motion of the carriage during the same time, it follows (by a 

 well-known principle of variation), that if both the force of traction and the 

 motion of the carriage were variable, the motion of the wheel F would vary 

 as their product ; and this being true of every exceedingly small period of 

 the motion, it follows that the whole motion of the wheel F during any finite 

 time, however long, is proportional to the sum of the products obtained by 

 multiplying each elementary space described by the can-iage, by the parti- 

 cular force of traction under which that elementary space is described, or, in 

 other words, that the whole space moved over by the circumference of the 

 wheel F is directly proportional to the whole work, or dynamical effect ex- 

 pended in moving the carriage, however varied the traction or the velocity 

 may have been. It was to an application to the steam indicator of that ad- 

 mirable and fruitful principle of M. Poncelet, which combines the motions of 

 the Avheel F and the disc B D, and which, in fact, performs the complex ope- 

 ration known in analysis as integration — integrating the traction, considered 

 as a function of the space — that the attention of the Committee was specially 

 directed by the Association, and that their labours have particularly been de- 

 voted. The indicator which they have now the honour to present to the 

 Association has nothing in common with the instrument which has just been 

 described, except the principle of M. Poncelet and the springs of M. Morin ; 

 these have been constructed according to the formulse given bj'^ that gentle- 

 man in his work already quoted (Appareil Dynamometrique, <&rc.), and have 

 been found admirably adapted to their use. The original design or project 

 of the machine was given by Professor Moseley, and he being the only 

 member of the Committee resident in London, the execution of it was placed 

 under his direction. 



Professor Moseley's Indicator. 



Theaccompanyingengraving(Plate VI.) represents the indicatorconstructed 

 by the Committee. C and D are cylinders, each four inches in length, commu- 

 nicating by the steam-pipes A and B with the top and bottom of the cylinder 

 of the engine to which the indicator is applied, and well clothed with felt to 

 prevent radiation. In these cylinders work two solid pistons, each four 

 square inches in area, fixed upon the extremities of the same piston-rod E F, 

 which piston-rod (when the steam passages A and B are open and the indi- 

 cator is in action) sustains in the direction of its length a pressure equal to the 

 difference between the pressures upon the two pistons fixed upon its extre- 

 mities, or (since these sustain the same pressure with equal portions of the 

 opposite sides of the piston of the engine) equal to the effective pressure of the 

 steam on four square inches of the piston of the engine. This pressure upon 

 the piston-rod is made to bear, by means of a shoulder Z, upon the steel 



