586 Prof. Kia-Lok Yen on the Coefficients of 



attached to the suspension, upon a transparent scale situated 

 at a distance of two metres from the centre of the mirror. 

 This scale is divided into millimetres and is bent to form a 

 part of the circle the radius of which is two metres, in 

 order that the angle of deflexion may he accurately replaced 

 by the quantity d/2D, where D is the distance of the centre 

 of the mirror to the scale, and d the distance, or rather 

 the arc, through which the spot of light is deflected : all 

 measured in centimetres. This is the only alteration of 

 the arrangement as left by Mr. Harrington. 



As Mr. Harrington has given in his paper a much more 

 detailed description of the construction and the adjustment 

 of the apparatus, and also a full discussion of the deter- 

 mination of the various quantities that enter into the final 

 calculation of results, it only remains here to reiterate his 

 assertion that the probable error in the final result is not 

 more than one part in one thousand. 



The Observation. 



When the apparatus is filled with the gas the viscosity of 

 which is to be determined, the- temperature inside of the 

 chamber is recorded ; then the chronograph is set in motion 

 and this motion is transmitted to the outer cylinder through 

 the gears and shaft by means of a small clutch provided for 

 that purpose. The inner cylinder will at once begin to 

 swing in the direction of rotation of the outer, and if not 

 checked, will continue to swing beyond the proper angle 6 

 on account of its inertia, and will continue to oscillate for 

 some time. This oscillation is eliminated in the following 

 manner : — After the inner cylinder has turned through a 

 short distance the rotation of the outer cylinder is stopped 

 by stopping the motion of the driving apparatus, and the 

 inner cylinder is allowed to swing. When it is about at 

 the end of the swing, the driving mechanism is started 

 again. And if this happens to be anywhere near the 

 proper angle the cylinder will oscillate but little about 

 that point. If it is not anywhere near the proper angle 

 the spot of light will either continue to swing forward 

 or reverse and swing backwards, according to where the 

 proper deflexion ought to be. If it continues forward 

 the motion of the driving apparatus is retarded slightly 

 once a wmile until the spot remains stationary. On the 

 other hand, if it moves backwards the motion of the driving- 

 apparatus is slightly accelerated. By accelerating and 

 retarding the motion of the driving apparatus the spot 

 of light may be made to remain stationary for a long time. 



