356 Prof. C. Piazzi Smyth's Meteorological 



constantly acting force of even the smallest amount, that the mean 

 effect of the lightest airs will be well registered. 



Further, Dr Robinson has a plan of his own for overcoming the 

 difficulty which most, if not all other, self-registering anemometei-s 

 experience, from the wind sometimes going round the compass two 

 or three times successively in the same direction ; in which case the 

 pencil must be taken off any rectilineal scale on which it is expected 

 to register. Mr Osier meets this difficulty, in some measure, by 

 repeating his rectilineal scale three times over, so that if the pencil 

 is registering a south wind in the middle scale, and turns round once 

 either way, it will still be in a divided space, where it may go on 

 registering a south wind ; if, however, the wind turns round, when 

 no one is by, several times, this plan is manifestly incomplete ; 

 and the tripling of the points of the compass has the bad effect of 

 making the scale of each so very small, that all minute features 

 are barely discernible. Dr Kobinson, however, removes both objec- 

 tions by causing the wind, — not to move a pencil up and down in 

 straight lines on a rectangular piece of paper, but- — to turn a piece 

 of circular paper round, while a pencil is being moved across it in 

 a rectilinear direction by clock-work. The consequence is, that the 

 wind may go round the compass as often as it pleases, either always 

 the same way, or backwards and forwards — the pencil is still 

 on the paper, and the registers are made on a very large scale. 

 Small variations of the wind are exhibited, as well as changes of the 

 mean, but not so as to interfere with the latter. And the interest- 

 ing circumstance was thus ascertained recently by the Doctor, that 

 the noise accompanying wind is mainly due to rapid variations in 

 direction ; for one day lately, when there had been all the morning 

 a violent howling storm, there occurred on a sudden so dead a silence, 

 that the Doctor expected that the centre of a " cyclone " was pass- 

 ing over the observatory, and immediately ran up to the recording 

 apparatus to see the expected change of the wind through 180°; 

 but he found that the wind was blowing just as strong as before, and 

 in the same mean direction, but that the variations in that direc- 

 tion, which had been before exceedingly violent, had suddenly become 

 hardly sensible. 



The velocity of the wind is also marked on a circular paper, and 

 while this plan has the advantage of allowing the sheet to be 

 fastened on its metal disc with great ease, it appears to leave the 

 wind much freer from friction than when it has to give rectilineal 

 motion, instead of as here, a rotatory movement. As a specimen 

 of the extreme sensibility of the machine, it may be mentioned that 

 the revolving hemispheres are seldom or never at rest ; when not a 

 leaf is stirring, when 



*' There is not wind enough to twirl 

 The one red leaf, the last of its clan, 

 That dances as oft as dance it can, 

 Hanging so light and hanging so high 

 On the topmost twig that looks up at the sky," 



