1881.] Determination of the Ohm in Absolute Measure. 109 



term in C will represent resistances of the nature of Tiscositj, and that 

 in D a resistance such as is incurred in setting fluids in motion by a 

 fan or otherwise. By these resistances, if present, the speed of 

 working will be determined. 



In the water impulse engine, however, the work is not proportional 

 to the speed. At zero speed no work is done ; neither is any work 

 done at a speed such that the cups retreat with the full velocity of 

 the jet. The speed of maximum efficiency is the half of the last, and 

 the curve representing work as a function of speed is a parabola with 

 vertex directed upwards. If we draw upon the same diagram the 

 curve of work and the curve of resistances, the actual speed will 

 correspond to the point of intersection, and will be well or ill defined 

 according as the angle of intersection is great or small. At the higher 

 speeds of the coil (four to six revolutions per second) so much air is set 

 in motion that the resistance curve is highly convex downwards, and 

 no difficulty is experienced in obtaining a nearly uniform motion. 

 But when the speed of rotation is as slow as once a second, the 

 principal resistance is due to solid friction, and the requisite cur- 

 vature in the diagrams must be obtained in the curve of work. It 

 was necessary in order to obtain a satisfactory performance at low 

 speeds to introduce an additional reducing pulley, so that the engine 

 might run fast, although the coil was running slow. 



The revolving coil with its frame, and the apparatus for suspending 

 the magnet, were at first arranged as described by the Committee. 

 This description, with drawings, is to be found in the report, and it 

 is reproduced in " Gordon's Electricity and Magnetism," vol. i. The 

 water engine was ready about the middle of June, and towards the end 

 of the month the apparatus was mounted by Mr. Horace Darwin. 

 During July and August preliminary trials were made by Mr. Darwin, 

 Mrs. Sidgwick, and myself, and various troubles were encountered. 



The only point in which the arrangement adopted by the Com- 

 mittee was intentionally departed from was in the connexion of the 

 magnet and mirror. The magnet is necessarily placed at the centre of 

 the revolving coil, but in their arrangement the mirror is on the top 

 of the frame and is connected to the magnet by a brass wire. In 

 order to save weight, I preferred to have the magnet and mirror close 

 together, not anticipating any difficulty from the periodic and very 

 brief interruption caused by the passage of the coil across the line 

 of sight. A box was, therefore, prepared with a glass front, through 

 which the mirror could be observed, and was attached to the end of a 

 brass tube coming through the hollow axle of the coil. This tube 

 itself was supported on screws resting on the top of the frame. The 

 upper end of the suspension fibre was carried by a tall tripod resting 

 independently on the floor. 



The first matter for examination was the behaviour of the magnet 



