THE CONSTANTS OF THE CUP ANEMOMETER. 
781 
plane of tlieir rotation ; the arms (to the centres of the square mouths) 24 inches, and 
the weight =129 oz. 
The other parts (beside the anemometers) which move with the horizontal shaft 
weigh 178 ‘82 oz. 
(9.) The method followed in experimenting was this. The machine was put in 
motion with as small a D as would make the anemometer revolve ; when this had 
attained as uniform a motion as could be judged of by the eye, the electric circuit was 
completed by the commutator, and the action was continued generally for four minutes. 
During this time a person watched the chronograph to guard against its failing to 
record, which sometimes happened from oxidation of the contact maker, or from the 
points becoming blunt.'" I made the observations for vortex motion, to be soon 
described, and Mr. Vereker kept watch over the labourers. 
When this observation was finished, a larger D was applied to give increased 
velocity, and so up to 76 lb., which I only passed in one instance. Even this was 
very severe on the men ; the more so as the temperature of the place was often as 
high as 70°. 
(10.) When this series was completed a weight was hung on the brake, and a new 
one taken. On account of the increased friction the anemometer would not move 
with the first D of the preceding series, and I adopted the plan of keeping the Y’s 
nearly equal. I laid down on a card the average length of the mark made by the 
point which records N in each experiment of the first series ; and then set N alone to 
act ; I altered D till its trace was nearly as long as the corresponding one on the card. 
Then the A point was made to record ; and thus another series was completed. This 
was continued till the load on the brake was as large as could be used with safety to 
the levers in the case of No. 1 ; and with the others such as would admit of a sufficient 
value of v. The chronograph sheets, which are carefully preserved, were tabulated 
independently by two persons. 
(11.) Before proceeding to discuss these observations, it is desirable to consider the 
conditions which determine the amount of an anemometer’s motion. This obviously 
depends on the impelling force of the wind and the various resistances which oppose 
it. These resistances can only arise from the action of the anemometer itself on the 
air, and the friction of its parts ; and therefore V, the velocity of the wind, is a function 
of v and F. The nature of this function cannot be determined a priori in the present 
state of hydrodynamics; but a general conception of its form is easily obtained. 
Suppose the wind makes an angle 9 with the mouth of a cup (or the arm which carries 
it) which is revolving with such angular velocity that the centres of the cups move 
with the velocity v, I have shown in the paper already referred to that the velocity R 
with which the wind is incident on the cup = v /V : '-l-r ’ : f-2V-y sin 6, the negative sign 
belonging to the first semicircle. The pressure of this to turn the cup=R 2 X eq, cq 
* These points should have been diamonds, as in the chronograph at the Armagh Observatory, but they 
could not be immediately procured. 
