108 Lord Rayleigh and Dr. A. Schuster. [May 5, 



MK/G-H is equal to tan fi. where /a is the angle through which the 

 needle of a magnetometer is deflected when the suspended magnet (M) 

 is placed at a distance from it a/sin a to the east or west, with the 

 magnetic axis pointing east or west. In practice the difference of 

 readings when M is reversed is taken in order to double the effect, 

 and any convenient distance is used in lieu of a/sin <z, allowance being 

 easily made by the law of cubes. 



The correction for torsion is determined by giving the suspended 

 magnet one (or more) complete turns, and observing the displacement. 

 If this be S v reckoned in divisions of the scale, i.e., in millimetres, and 

 D be the distance from the mirror to the scale reckoned in millimetres, 



(8). 



The correction for scale reading, necessary in order to pass from 

 -J- tan 20 to tan 0, will be explained under the head of reductions. 



Corrections depending upon irregularity in the magnetic field, and in 

 the adjustment of the magnet to the centre of the coil are given in 

 the report. They are exceedingly small. The same may be said of 

 errors due to imperfect adjustment of the coil with respect to the axis 

 of rotation. 



In remounting the apparatus the first point for consideration was 

 the driving gear. The Committee used a Huyghens' gearing, driven 

 by hand, in conjunction with a governor. This, it appeared to me, 

 might advantageously be replaced by a water-motor; and Bailey's 

 " Thirlmere " engine, which acts by the impulse of a jet of water upon 

 revolving cups, was chosen as suitable for the purpose. As the pres- 

 sure in the public water pipes is not sufficiently uniform, it was at first 

 intended to introduce a reducing valve ; but on reflection it seemed 

 simpler to obtain a constant head of water by connecting the engine 

 with a small cistern at the top of the building. This cistern is just 

 big enough to hold the ball- tap by which it is supplied, and gives at 

 the engine a head of about 50 feet. 



The success of this arrangement depends upon attention to princi- 

 ples, as to which it may be well to say a few words. The work done 

 by many prime movers is within practical limits proportional to the 

 ^peed. If the work necessary to be done in order to overcome resist- 

 ances, as n overcoming solid friction, or in pulling up weights, be also 

 pioportional to the speed, there is nothing to determine the rate of the 

 engine, and in the absence of an effective governor the motion will be 

 extremely unsteady. In general the resistance function will be of the 

 form — 



in which the above-mentioned resistances are included under B. The 



