and Astronomical Notices. 355 



similar to that by which the heads of windmills are turned round to 

 the wind ; viz., a small wheel at the back of the mill, and with its 

 plane in the direction of the wind, so that in such a position it is not 

 acted on at all ; while if the wind changes its direction either way, 

 it then begins to turn round the small wheel, which, acting through a 

 train of wheel-work, is thereby enabled to turn the whole head slowly 

 round, and place itself again in the sheltered position. This con- 

 tfivance draws a very uniform line on the paper, in cases where the 

 mere vane might be vibrating through angles of 50° and 60°. 



The Rev. Dr Robinson, who pays almost as much attention to 

 meteorology as to astronomy in the observatory of Armagh, has in 

 use some arrangements of the anemometer, distinguished by the 

 peculiar elegance and efficiency of everything which he takes up. 

 For the strength of the wind, he uses Edgeworth's hemispheres, 

 which he was the first indeed to bring forward, and he has been 

 at much pains to ascertain the best size, and the exact value of the 

 revolutions in terms of the velocity of the wind : as, by carrying 

 it through still air at known rates ; by immersing it in water mov- 

 ing at a measured speed, when the friction of the moving parts 

 would be of far less moment than in the air ; and again, by alter- 

 ing the central distances of two opposite hemispheres, to ascertain 

 the proportions of radii that should give equality of pressure on the 

 concave and convex surfaces, whence the consequent velocity may 

 be easily deduced. 



A practical difficulty, however, was found to rise in the rapid 

 wearing of the lower pivot-hole of the vertical axis carrying the re- 

 volving-wheel of hemispheres ; the pivot was made of steel, and 

 worked in a conical hole in agate ; but such was the wearing power 

 of the ceaseless revolution of the spindle, day and night, that the 

 agate was bored through in a short space of time. A second was 

 made, but although well lubricated with oil, it soon shared a similar 

 fate to its predecessor. A large piece of sapphire being then obtained 

 by good fortune, was made into a bearing, but even that was actually 

 bored through before very long. 



The Doctor, therefore, then tried the plan of supporting the 

 weight of the revolver on balls rolling inside a box, and this seems 

 to answer perfectly, for while the friction is insensible, the wear or 

 other alteration is also as small. 



For the direction of the wind, Dr Robinson uses a simple vane, 

 but connects the lower end of the spindle, where it enters the room, 

 with a train of wheel- work, acting eventually on very large and 

 light fanners, formed merely of thin lath fans, with paper pasted 

 over them. The wheels are such, that for one revolution of the 

 wind vane, the fanners might revolve say fifty times ; and this velo- 

 city, combined with their large surface, causes them to offer an 

 almost infinitely great resistance to any sudden impetus, as the 

 impact of any little temporary gust. On the other hand, the small 

 weight of the fanners offers so little resistance of inertia to any 



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