784 
DR. T. R. ROBINSON ON THE DETERMINATION OP 
that the free part of the thread is 23 feet long, and its torsion force is insensible in 
these experiments ; after being twisted during several series when left to itself it 
resumed its original position so slowly that ten minutes were required for about a 
third of a revolution. The lightness of the balloons and their large surface make 
them obedient to very slight currents, and extremely sensitive to any irregularity in 
its motion. The process of observing W is this : When the experiment has lasted for 
about a minute, the time b of two revolutions of one of the balloons was noted by 
a stop-watch (by Robert, of Paris) whose beats are 0'2 S . From the radius of the 
(1'94335) 
circle described by them, we find that W in miles per hour=L -1 X reduction 
to cross. This reduction is made on the assumption that the velocity in any part of 
the vortex is inversely as the square of the distance from the centre. This agrees 
as well with experiment as under the circumstances could be expected. Thus on one 
occasion, at the normal distance b= 83 s , when the distance was increased to 30 inches it 
became 104-6 3 . The inverse square gives 106‘2 S . Hence the factor for reduction 
= L -1 (0T0586). These balloons were very perishable; drops of oil were projected 
occasionally from the bearings at high speeds, which perforated them like shot, and by 
the commencement of June we had exhausted the stock in the neighbouring shops, 
and most of those in Dublin. They were then replaced (June 7th) by vanes of tissue 
paper 1 foot square, stiffened at their edges by rods of deal 0T25 inch thick, and 
suspended from the rod by slips of deal 0’5 inch broad, 0‘062 inch thick, and 4 feet 
long. These moved edgewise and were kept steady by lateral slight trusses of the 
same material and scantling. As the vortex reaches far above the rod these suspen- 
sions will not sensibly retard the vanes. Their centres are 20 ‘25 inches from the cross, 
but they give b 0 - 003 smaller than the balloons. Their reduction to the cross 
= L" 1 (0T48I0). 
(14.) This apparatus shows that the vortex current is very irregular : the balloons 
sometimes dart rapidly forwards, then move more slowly, and this not only when the 
anemometer is passing them ; sometimes they are violently projected upwards and out- 
wards, showing that in addition to the general translation of the air, it is affected by 
both horizontal and vertical eddies of considerable power and possibly magnitude. 
The vanes can only show irregularities in the circular motion, but fully confirm its 
lawlessness. Under such circumstances we can have no exact knowledge of the W 
which exists at the cups ; the balloons give its mean value through the circumference, 
but it is probably greatest immediately after the cups have passed, and decreases by 
the eddies and the friction of the walls till they again meet it. Any fixed meter 
would yield the same result, and the only proper plan would be one capable of giving 
the value of W at the cups for every instant. A pressure gauge carried with the 
cross and connected with a chronograph promises best ; but in air so agitated the 
relation between the velocity and pressure would be very doubtful. 
(15.) The horizontal arm has little effect in producing the vortex ; in the case already 
