330 Messrs. Guthrie and Boys on 



hundredweight of iron stands on tho wood to increase its 

 inertia and make the whole thing more steady. Near the top 

 of the shaft I a line screw is cut, which gears with the worm- 

 wheel m, of 100 teeth. This wheel has also cut on its edge a 

 screw of the same pitch as that on tho shaft I ; so that its 

 edge is divided into a series of square pyramids, and it gears 

 as a screw into the worm-wheel n of 100 teeth : n therefore 

 turns round once for 100 turns of m or for 10,000 turns of I. 

 At every turn m makes an electric contact and sounds a bell; 

 n has its face divided and numbered. Therefore by observing 

 occasionally the exact instant at which the bell sounds, and 

 then the reading on n, a continuous record of the speed can 

 be kept, at the same time that s is being observed with the 

 microscope. A pulley-wheel on I is connected by a catgut 

 band with a small steam-engine driven by steam from the 

 boiler which supplies the building. The steam-pressure 

 varies so much that the resulting changes in speed in the 

 engine have caused the greatest inconvenience, sometimes a 

 whole day being lost. Some form of absolute governor will 

 have to be devised in order to avoid great loss of time. 



On the lower side of the beam B is a small glass hook, from 

 which may be hung a disk of brass D, the use of which will 

 be explained below. The dimensions of most of the parts of 

 the apparatus may be found from the Plate, which is drawn to 

 the scale of almost exactly J. The torsion-thread is of hard 

 drawn steel wire about 13 feet long and 0'007 in. in diameter. 

 The space in which the cylindrical part of the screen is placed 

 between the poles of the magnet and the outside of the cell is 

 only about ^ in. wide, so as to get the greatest possible effect 

 from the magnets. When the machine was put together the 

 magnets were found to be slightly out of balance ; so a plate 

 of brass full of tapped holes was fixed to the light side, insuf- 

 ficient in weight to restore the balance, and then small screws 

 of brass were screwed into the holes until the balance was 

 perfect. By this means the magnets were made to run so 

 smoothly that at 3000 turns a minute scarcely any tremor 

 could be felt ; but always on passing a certain critical speed 

 (about 1300 turns a minute), at which the period of vibration 

 of the whole machine and mantlepiece was probably equal to 

 the time of rotation of the magnets, the machine set up a 

 vibration, gentle at first, but increasing with the speed, which 

 seemed at first as if high speeds could never be attained. 

 But it was found that if, as soon as this effect took place, one 

 of the screws of the upper bearing was loosened the oscilla- 

 tion ceased suddenly, and then the screw could be tightened 

 again, and the speed increased to any extent without the 



