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 gi-eat accm-acy. 



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 same 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 fm-nished 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 modem 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 windmiU 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 num- 

 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 pm-poses. 



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 necessai'ily light arms, which prevents 

 the angle of the sail being perfectly constant ; and the impossibihty 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 Edgewokth, which requires no 

 screen, but revolves by virtue of the shape of the fioat-boards ; which shape being 

 a constant quantity in all strengths and directions of the wind, the revolution goes 



