ManrEN.—On Anemometry. 295 
6. Thus, practically, we may dismiss the estimated-relative-force system 
as a very rough-and-ready and a thoroughly unscientific and non-compar- 
able mode of anemometry. It remains to be seen whether the scientific 
systems be more accurate. 
7. It has been stated already that the most generally received method 
is by measuring velocity, and that the instrument used for that purpose is 
Dr. Robinson’s anemometer. To make comprehensible what is to follow, 
a brief description of this instrument is necessary. 
8. It consists of four hemispherical equi-distant cups, forming the ter- 
minations of as many arms, disposed horizontally, in the shape of a cross, 
and revolving on an axis, the cups being so placed that the concavity of 
one and the convexity of another are always exposed to the full force of the 
wind, simultaneously acting on their diametrical planes. The revolutions 
are numbered and recorded by a simple and ingenious mechanism, which it 
is unnecessary to explain. 
9. The circumference of the cirele described by the cups in each revolu- 
tion, and the number of revolutions made in a given time, being known, it 
is easy to calculate the distance travelled by the cups in such given time. 
Dr. Robinson calculated that from the resistance offered by the convex cup 
on the one side, the force of the wind on the concave one on the other side, 
would be only sufficient to propel it at one-third of the wind’s velocity, and 
that this rule was irrespective of the diameter of the cups, or the length 
of the arms. His calculations are given in Vol. XXII. of the “ Transac- 
tions of the Royal Irish Academy.” His mode of testing his original 
anemometer was by fixing it on a pole eleven feet in length, attached to a 
locomotive engine, which was driven a measured distance, in calm weather, 
on the Dublin and Kingston Railway, at various speeds, ranging up to 
nearly 70 miles per hour, the anemometer thus being pressed against the 
wind, instead of the wind pressing against the anemometer. The distance 
run by the engine being known, it was only necessary to compare it with 
the number of revolutions made by the cups, and the consequent com- 
parative distance they had travelled. The result was that they were found 
always to have travelled one-third of the distance run by the engine; and 
this “irrespective of their diameter or of the length of the arms." This 
went to confirm the theory Dr. Robinson had formed on the subject; but 
it is noteworthy that he apparently regarded that coincidence as somewhat 
fortuitous. Nevertheless, its correctness has been sustained by many sub- 
sequent experiments, including those made in Greenwich Park by Mr. 
Glaisher, who, testing the instruments at various velocities and under 
differing conditions, always found the result to corroborate the inventor's 
theory. ; 
