THE CONSTANTS OF THE CUP ANEMOMETER. 
7 79 
the cups of the instruments are 12 feet from the walls, 10 feet from the railing of the 
gallery, and, in the lowest case, 12 feet from the floor. The annexed sketch will show 
the disposition of the apparatus (except the driving machinery). A, A is the inner 
wall of the building ; D, D a portion of the dome ; G, G the gallery ; Y, Y the vanes 
of the apparatus for measuring the air vortex subsequently described in paragraph 13. 
(3.) The plan of having the plane of the anemometer’s rotation vertical instead of 
horizontal, is liable to three objections, which will be noticed hereafter, but I was 
induced to adopt it on two accounts. Had the brake apparatus been at the outer end 
of the horizontal arm, the centrifugal friction would have been twice as great, and the 
action of the brake less certain ; and secondly, the cylindric zone of disturbed air in 
which the cups move is little thicker than their diameter, while in the other case 
it would be nearly five feet across, and as the velocity of the air vortex decreases 
outwards, the uncertainty of its influence would be greatly augmented. 
(4.) From the above we find that calling N the number of revolutions of the vertical 
iN" 
axis in S seconds, the velocity of the anemometer’s centre Y = -xL _1 (1*58979) in 
miles per hour. The anemometers used had arms 2 feet and 1 foot long respectively. 
Calling A the turns in S, v=-xL -1 (0*93288) is the velocity of the centres of the 
cups in miles per hour ; and half this for the shorter arms. 
(5.) The revolutions of the vertical shaft and the anemometer are recorded on an 
electric chronograph a little modified from Krille’s construction ; which was placed 
in one of the rooms opening into the gallery. It has three tracing points, of which 
the left records N by transverse marks on the helical line which it traces on the barrel 
of the chronograph as it is moved uniformly by a screw. This line is drawn on peculiar 
paper enamelled with some white composition, which is coated over with black. The 
point cutting through the black exposes a white line beautifully distinct on the dark 
ground."" The right hand point records A similarly. The middle point was connected 
with Mr. H. Grubb’s regulator, and recorded S by interruptions of the helix, so that 
the three elements of the experiment were closely contiguous. The motion of the 
barrel was regulated by a governor clock, such as Mr. Grubb applies to his equa- 
torials, and its accuracy is very great, as is shown by the line of seconds ; so great, 
indeed, that before long I dispensed with the seconds record, each revolution of the 
barrel being exactly a minute. There was attached, of course, a commutator, which 
brings each point separately into action. 
(6.) The driving apparatus is carried by a vertical frame resting on the ground and 
attached at top to the gallery ; it is an application of Huyghen’s maintaining power, 
and shown in fig. 2. A B is one of the top bars of this frame ; it carries the driving 
pulley D and the free pulleys I, C, G, H, F, and K. The endless rope (J> inch 
diameter) passes from D round these. I and K are set obliquely to the plane of the 
This paper is supplied of excellent quality by Messrs. I)i: La Rub. 
