THE CONSTANTS OF THE CUP ANEMOMETER. 
1059 
(G3.) It consists of an iron spindle 6" long, turning on a small toe below, and above 
in a brass collar carried by a transverse piece of wood supported on two uprights. It 
has a projecting piece to which the inner end of the spring is attached by a screw 
secured by a check nut; the outer end is fixed to one of the uprights. On the top of 
the spindle is screwed a disc of mahogany 13" diameter and 0"’5 thick, on the edge of 
which is turned a groove in which the thread that connects it with the brake arc is 
wound. On this disc is fitted one of the papers used with my original anemometer, 
which has its circumference graduated to half degrees, and is covered with circles 0 //- 05 
apart, every tenth one stronger than the rest. By pulling the cord the spindle is 
turned and the spring tended, the number of its revolutions is shown by a tell-tale 
fixed to one of the uprights, and the degrees by a pointer. 
(64.) It is thus used : tighten the clamp screws so that when the arm is held fast 
the anemometer shall turn without coming to a stop; pass the cord of the balance 
through a hole in the remote end of the arc, and tighten it till the increased tension 
keeps the arm nearly in the same position, then secure it to a pin provided for the 
purpose. In this state of things it is evident that the tension corresponding to 6, 
the angle through which the balance has been turned, is the moment of the friction at 
8", from which the moment at the cups is known, to which must be added the normal 
friction. The brake-ring weighs 14 oz., which would increase the friction a few grains, 
but this was obviated by hanging an equivalent weight on the relieving apparatus 
mentioned in paragraph (51). The ring was at first lined with cloth, but as it slightly 
abraded the bell-metal, I removed it and used the iron surface lubricated with lard. 
(65.) The relation between the tension of the spring and 6 was thus obtained: the 
balance being clamped to a table its cord was passed over a pulley; nineteen weights 
in regular succession from 2 oz. to 36 oz. were hung to it; and to eliminate the effect 
of friction the disc was turned a few degrees in advance and in rear of the positions 
of rest, when they were attained, the mean of the 6’ s was taken as that due to the 
tension. From ten to thirteen sets were taken for each weight. I had expected that 
the tension would be very nearly as 6, but with this spring such is not the case; at 
the beginning 1°=13 grains, at 4 rev. it =20, and the change is not uniform; so I 
formed from the series an interpolation table with 9 argument, from which T is easily 
computed by a formula analogous to that given by Stirling for equal intervals. 
(66.) In carrying out the work I was met by an unexpected difficulty : friction 
applied in this way is not constant; and I found that in strong breezes (when the 
wind is always fitful) the arm oscillated more than 90°, the utmost range which the 
opening of the iron box (paragraph 2) permitted unless the friction was reduced. These 
oscillations made it necessary to have a record of the tension, which was provided by 
clockwork moving a pencil from the centre to the circumference of the graduated 
paper at the rate of 0"‘5 per minute. This traces an irregular sector from which the 
mean 6 is easily obtained. But, besides this, it is necessary to reduce the oscillations 
below 90°, so that they all may be recorded. This was effected by connecting with 
6 u 2 
