1068 
DR. T. R. ROBINSON ON THE DETERMINATION OF 
the assumption that y— 1 , and the possibility that the wind errors are not eliminated. 
As to the first, I have shown in paragraph (69) that it cannot be far astray ; even 
were the extreme diminution of it which is mentioned there to occur, the error for 
Y =100 would only be 6"1 ; but such is not likely to be the case, and y— 1 may be 
accepted as a major limit and one not far from the truth. As to the second it is certain 
that in a sufficient number of observations the + and — errors must balance each 
other ; but it may be a question whether the XXI. of Table XXI. were enough. Still, 
it is evident, from inspection of the column V —V' that there cannot be any large out¬ 
standing residue. I have pointed out the defects of the situation. Could I have had 
my wish I would have established the instruments on spars 20 ' high, erected on a level 
ground away from any influence of houses or trees, and used a better mode of applying 
friction to E x . I also regret that no strong gale occurred during these experiments to 
verify the formula for a very large v. Under favourable circumstances, I think this 
mode of determining x the best that is known. I have stated reasons for distrusting 
the results obtained by the whirling machine, and as yet no unexceptional mode of 
carrying an anemometer through the air in a right line has been devised. Even could 
we get a locomotive which could travel without disturbing the air through which it 
passes (and perhaps the new electric motors might effect this), and a line of rails 
certainly screened from wind, there would still remain the doubt whether the pressure 
is the same when a body is moved through a quiescent fluid or a current impinges on 
the body. 
(78.) The results obtained with other anemometers show that x is a function both 
of it, the length of the arm, and r, the radius of the cups. I subjoin its values. 
23'17 
r—0 
x= 1-5880 
24 
4-5 
1-5919 
12 
4"5 
1-7463 
8 
4-5 
2-1488 
26-75 
2 
1-8587 
10-67 
2 
2-5798 
If we take Nos. 2 , 3, and 4 in which the cups are equal, the dependence of x on R is 
manifest. In No. 3 it is yy larger than No. 2 , and in No. 4 - 3 . This is partly due to 
the air’s escape before the convexes being less easy as the circle described by them is 
less. Such a fact is strikingly shown by the whirling experiments (paragraph 69) 
which I made in search of a minor limit for y , in which I found the resistance = 30 - 61, 
and 7 9 for the three respectively. This was in quiescent air ; but a similar though 
much smaller effect must occur in the actual working of an anemometer. Its influence 
can be obtained only by experiments, such as the present.* 
* I thought to test this by removing two opposite cups in E g . As in this case there is only one cup in 
each semicircle at a time, the probability of their mutual disturbance was small. A set of ten gave the 
