AND DIRECTION OF THE WIND AT SEA. 459 
which any particular wind is made up; and although, in a mere mechanical poing 
of view, a wheel of great weight would tend to equalize and mean all the cur- 
rents of different intensity, still it can only do so with a certain amount of loss, 
and with the total omission of all very light impulses; and the only way accu- 
rately to sum up all those separate little quantities, is to employ an instrument 
which shall be as sensible as a feather, and take full and immediate account of 
the slightest motion of the atmosphere. 
After trial of floats 2, 3, 34, 4, and 6, inches in diameter, the 4-inch ones were 
considered as being the best; and the hemispherical shape was also preferred, 
as giving the greatest per centage of velocity with the least weight of material 
and smallest side resistance, as well as offering the shape, of all others, of the 
easiest and truest execution, and best understood everywhere. 
Various experiments were tried, of making the floats more or less conical, in 
order to diminish the pressure of the wind on their backs; but though that point 
was most eminently obtained, still the advantage was outweighed by the neces- 
sary increase of weight accompanying the increase of surface, the greater side 
resistance to the wind, and the diminished pressure on the concave side. 
In the month of January this year, I had the opportunity of trying the value 
of the revolutions of this anemometer, in company with Captain CockBurn. The 
instrument was mounted on the top of a cab, clear of the driver’s head, and 
driven at a pretty uniform speed of above seven miles an hour, first forwards 
and then back, on two miles of the London Road; the object being to measure 
the artificial wind produced by the motion of the vehicle, which would of course 
be equal to a natural wind blowing with the same velocity in the contrary 
direction. The first day there was a rather strong breeze, which would have 
completely vitiated the experiments, but that, as it was blowing almost exactly 
in the direction of that part of the road which was traversed, we expected to be 
able to eliminate its effects by taking a mean of the numbers given in going and 
returning. 
When going, having the wind with us, the instrument, which measured then 
only the difference between the velocities of the wind and the cab, made 209 
revolutions in one mile; but in returning, measuring the sum, it gave 921 revolu- 
tions in the same distance. The mean of these, or 565, when multiplied by 3:1415 
feet, or the space described by the centre of the float in one revolution, gives 
a velocity not exactly 4, but aa of that of the wind. 
The second day was all but perfectly calm; it was at the commencement of 
the long-continued frosty weather; and a proof of the general stillness of the air 
was offered in the dense, unnatural manner in which the smoke was accumulated 
and remained suspended over the city. In going out, 558 revolutions were made in 
one mile; and in coming back, 55). The mean of these, or 555, gives a velocity 
VOL. XVI. PART IV. 4 6B 
