[ 1055 ] 
XXY. On the Determination oj the Constants of the Cup Anemometer by Experiments 
with a Whirling Machine —Part II. 
By T. R Robinson, D.D., F.B.S., dc. 
Received June 13—Read June 17, 1880. 
(55.)* In the preceding Part (Philosophical Transactions for 1878, p. 777) I gave the 
results obtained by anemometers attached to a whirling machine, which substitutes 
motion through the air for real wind. If the air were quiescent this method would be 
nearly unexceptionable ; but the whirling gives the air a vorticose motion for which it 
is impossible to make an exact allowance, and therefore some uncertainty affects these 
results. In the conclusion of that paper I expressed an opinion that greater certainty 
might be obtained by comparing two anemometers, similar and equal in every respect 
except friction; and stated that I would endeavour to carry this into effect. I 
propose now to give an account of my attempt to do so. 
(56.) The instruments used, and their arrangement, are described in paragraph (51). 
The situation in which they are placed would be a good one but for the dome of the 
west equatorial, which in some points of the wind may interfere with its full action on 
one or the other of the instruments. 
The diameter is 13'‘6 ; the height of its summit above the platform is 15 *75 ; that 
of the arms of the instruments being 16'. The horizontal distance of its centre from 
the Kew instrument (K) = 21' , 5, bearing from it S.S.E., 2° S. The distance from the 
experimental one (E) is 23', and its bearing S W. b. S. 
The distance between K and E=22'. Of course when the wind is S.S.E., K will be 
less acted on than E, and vice versa, but probably the difference will be much less than 
that caused by fluctuations of the wind itself. When the wind is E. or W. the 
eddies caused by the windward anemometers may perhaps reach the leeward, but not, 
I think, to any great extent. 
(57.) The chronograph record of each experiment was at first entered in groups 
during which v , the velocity of K, was nearly uniform; and A, the number of turns 
made by each instrument, was an integer. The length of the chronograph helix gives 
the time ; it is measured in eighths of an inch (as the Observatory possesses a scale of 
eightieths) and when divided by the length of a second on the same scale, we have the 
number of seconds. As the chronograph in its present situation is exposed to con¬ 
siderable variations of temperature, its rate is not as regular as it was at Ratlimines, 
but the second-space was determined each day of observations. The average in 
* For facility of reference the numeration of the paragraphs and tables is in sequence to that of Part 1. 
