338 PHILOSOPHICAL THANSACTIONS. [aNNO I76&. 



XX n. A New Manner of Measuring the Velocity of fVind, and an Experi- 

 ment to ascertain to what Quantity of fVater a Fall of Snow is Equal. By 

 Jlex. Brice. Dated Kir knew ton, May \3, I766. p. 224. 



Mr. B. attempted to determine the velocity of the wind by letting light downy 

 feathers fly in it ; being the method used by Dr. Derham ; but in all the trials 

 he made, though he has let 50 of these f<jathers fly, one after the other, at a 

 time, he has never seen above 1 or 2 at most, on which he could have founded 

 a calculation. The velocity of the wind near the earth is very unequal, on ac- 

 count of the frequent interruptions it meets with from hills, trees, and houses ; 

 and even in open plains, the surface of the earth, though much smoother than it 

 commonly is, must reflect, and interrupt such a fluid as the air, and occasion 

 great irregularity in the velocity of its current : this is the reason, when a feather 

 is let fly with the wind, why it seldom, if ever, describes a straight line, but 

 moves sometimes in a kind of spiral, now high, and then low, sometimes to the 

 right, and then again to the left ; and why two feathers let fly at once, seldom, 

 if ever, keep together, or describe similar lines. 



But, at some considerable distance from the earth, the velocity of the wind 

 seems to be regular and steady : nothing can be more uniform than the velocity 

 of a cloud in the sky appears to be, even in the greatest storm : it is like a ship 

 carried away insensibly by a smooth and gentle current, passing over equal spaces 

 in equal times. This suggested the thought, that the motion of a cloud, or its 

 shadow, over the surface of the earth, would be a much more proper measure of 

 the velocity of the wind. In the end of March 17 63, Mr. B. had as favourable 

 an opportunity of putting this method into practice, as he could wish ; the storm 

 was exceedingly high, and moved with vast velocity ; the sun was bright, the sky 

 clear, except where it was spotted with light floating clouds ; he took his station in 

 the north window of a room, near the clock, from which he had a free prospect 

 of the fields ; the sun was in the meridian, the wind due west intersecting his 

 rays at right angles ; he waited till the fore-part of the shadow of a cloud, that 

 was distinct, and well defined, just touched a south and north line, which he had 

 marked on the ground ; at that instant he began his reckoning, and followed the 

 shadow with his eye in its progress, counting seconds all the while by the clock, 

 till he had reckoned up 1 5 seconds ; then he observed exactly where the foresaid 

 edge of the shadow was. This experiment he repeated 10 times in half an hour, 

 and seldom found the difference of a second, in the time which different clouds 

 took to move over the same space. On the 5th of May current, he repeated the 

 trial 4 different times, the sun being also near the meridian, the wind in the 

 west, with light clouds floating in a clear sky as formerly ; and found that the 

 shadows of different clouds took some of them 44, and others 45 seconds, to 



