AND DIRECTION OF THE WIND AT SEA. 457 
with whom I have communicated, they seemed to look upon the effect of the 
motion of the ship as something insuperable, and in the face of which there was 
no need of aiming at any great accuracy. 
On beginning to consider the best species of anemometer for the purpose in 
question, it appeared to me that something on the principle of the log-line used 
for determining the ship’s speed through the water, would be appropriate ; for, 
notwithstanding the very scientific and accurate character of numerous instru- 
ments invented for the sarme purpose in later years, still they have one by one 
‘disappeared, or been forgotten ; and the old log-line has not only continued in ex- 
istence from the earliest times to the present, but ninety-nine out of every hundred 
ships that now go to sea are furnished with it, and with it alone. This peculiar vi- 
tality and power of withstanding the changes of fashions and times, may perhaps 
depend partly on this, that the quantity to be observed is measured on so very large 
a scale, that the clumsiest person can read it off to sufficient accuracy ; while, 
with the more modern methods, the accuracy of a person accustomed to delicate 
observation, is necessary for any trustworthy determination at all. ' 
The case in anemometry, perfectly parallel to the log-line, would be,—to have 
a float of some sort suspended in the air, and to note how many feet of line it ran 
out in a certain length of time, under the combined influence of the movements of 
the air and the ship. But though so perfect an imitation as this is prevented by 
the rarity of the atmosphere, yet the vane of a horizontal windmill is an approach 
to the same thing; where the float is supported in the air on a horizontal arm 
fixed to a vertical axis; and the distance run out, is measured by noting the nuin- 
ber of revolutions, and the magnitude of the circle described by the vane or float. 
The small motive power, however, of a horizontal windmill, only one-twelfth, 
according to SMEATON, of the vertical construction, together with the necessity of 
having a moveable screen to cover up one-half of the wheel from the action of the 
wind, has prevented the adoption of such a machine as completely for scientific 
as for industrial purposes. 
The vertical windmill, again, though it gains a far greater degree of mechani- 
cal power, is also inappropriate for our purpose, on account of the very different 
amounts of glancing off of the wind, at different velocities, from the inclined surface 
of the sail ; the unavoidable twisting of the necessarily light arms, which prevents 
the angle of the sail being perfectly constant; and the impossibility of fixing one 
uniform standard for the shape, size, and angle of the sail; as well as the neces- 
sity of having the plane of the sail-wheel always turned toward the direction of 
the wind. 
All these objections have, however, been very happily removed by a novel 
windmill, of the horizontal form, invented by Mr EpGrwortu, which requires no 
screen, but revolves by virtue of the shape of the float-boards ; which shape being 
a constant quantity in all strengths and directions of the wind, the revolution goes 
