180 Lord Rayleigh on the Resistance experienced 



The plates uuder test can be cut from thin card and of 

 course must be accurately measured. In my experiments 

 the axis of rotation was a sewing-needle held in a U-shaped 

 strip of brass provided with conical indentations. The longi- 

 tudinal pressure upon the needle, dependent upon the spring 

 of the brass, should be no more than is necessary to obviate 

 shift. The arms connecting the plates with the needle are 

 as slender as possible consistent with the necessary rigidity, 

 not merely in order to save weight but to minimise their 

 resistance. They may be made of wood, provided it be 

 accurately shaped, or of wire, preferably of aluminium. 

 Regard must be paid to the proper balancing of the re- 

 sistances of these arms, and this may require otherwise 

 superfluous additions. It would seem that a practical solu- 

 tion may be attained, though it must remain deficient in 

 mathematical exactness. The junctions of the various pieces 

 can be effected quite satisfactorily with sealing-wax used 

 sparingly. The brass U itself is mounted at the end of a 

 rod held horizontally in front of the observer and parallel to 

 the direction of motion. I found it best to work indoors in 

 a long room or gallery. 



Although in use the needle is approximately vertical, it is 

 necessary to eliminate the possible effect of gravity more 

 completely than can thus be attained. When the apparatus 

 is otherwise complete, it is turned so as to make the needle 

 horizontal, and small balance weights (finally of waxj 

 adjusted behind the plates until equilibrium is neutral. In 

 this process a good opinion can be formed respecting the 

 freedom of movement. 



In an experiment, suggested by the case of the mirror 

 above referred to, the comparison was between a rectangular 

 plate 2 inches x 1J inches and an elongated strip '51 inch 

 broad, the length of the strip being parallel to the needle, 

 i. e. vertical in use. At first this length was a little in excess, 

 but was cut down until the resistance balance was attained. 

 For this purpose it seemed that equal areas were required to 

 an accuracy of about one per cent., nearly on the limit set 

 by the delicacy of the apparatus. 



According to the principle of similitude the influence of 

 linear scale (/) upon the mean pressure should enter only as a 

 function of vjVl, where v is the kinematic viscosity of air 

 and V the velocity of travel. In the present case z> = *1505, 

 V (4 miles per hour) =180, and Z, identified with the width 

 of the strip, =1'27, all in c.G.s. measure. Thus 



v/VZ = -00066. . 





