814 Account of some Meteorological Instruments. 



directing the variations to which they are perpetually liable, that we may be 

 enabled to compare these variations with other meteorological phenomena ; 

 and this desideratum seems to be supplied by Dr Traill's Anemoscope. 



This instrument, in its simplest and original form, is represented in Fig. 1, 

 where a represents a light but firm rod of brass, supported, in a vertical po- 

 sition, by circular apertures in brass cramps fixed in a wall, when the rod is 

 short; or moving between friction rollers, when it is of considerable length, in 

 the manner usually adopted in the vanes on public buildings. The lower 

 end of the rod is shod with a blunt cone of steel ; and, to unite cheapness of 

 construction, with freedom of motion, the socket consists of a flint, selected 

 for one of those smooth, natural round cavities, which are so often observed 

 in that mineral. 



This socket is firmly fixed in the centre of a polished circular plate of com- 

 mon writing-slate, on the exterior margin of which the rhombs of the com- 

 pass, and degrees of a circle, are engraved. The vane b is firmly attached to 

 the rod, and carries the rod along with it, when acted on by the wind. Four- 

 inches above the slate, the square brass-arm c is jointed to the rod, so as to 

 have a slight vertical motion, but to be carried around with the rod. On the 

 horizontal arm slides a spring tube d, intended to carry a piece of slate pen- 

 cil ; the point of which may trace the progress of the vane round the circle. 

 The spring-tube may be fixed at any distance from the centre by means of a 

 small nut-screw/; and the due pressure of the pencil on the slate is secured 

 by the ball e, which moves on the arm by means of a screw, formed on its 

 outer extremity. It is obvious, that in this manner the whole variations in 

 the direction of the wind, capable of moving the vane, may be read off even 

 to degrees of the circle, during a complete revolution of the instrument. As 

 it was found inconvenient, in some situations, to place the index-plate or slate 

 horizontally, Dr Traill afterwards altered the apparatus, as represented in 

 Fig. 2. 



Two equal bevelled wheels were added : one, e, was fixed horizontally on 

 the rod ; the other, c, was attached vertically to the axis of the index/. The 

 wheels have the same number of teeth ; and the motions of the index cor- 

 respond in extent to those of the vane ; for the only effect of the introduc- 

 tion of the wheels thus arranged, is to convert a horizontal into a vertical 

 motion. The whole of this subsidiary apparatus is included in a box of brass ; 

 and to prevent the entrance of dust, a small inverted cone of copper is cement- 

 ed at d, so as to cover the aperture for the rod, without impeding its motion. 

 This is represented detached at h. 



For the writing-slate Dr Traill substituted a surface of polished porcelain, 

 and fastened, by a small spring, a black-lead pencil in the index/. The 

 movements of this pencil on the porcelain-plate are remarkably smooth, and 

 the line is beautifully traced with very little friction. But more lately, the 

 inventor finding that the wearing of the pencil gave the necessity of frequently 

 replacing it, has rendered the instrument more easy of management, by sub- 

 stituting two light indices g g, which are fitted to the central hole in the index- 

 plate, with just sufficient friction to remain in the position to which they are 

 moved by a stud on the prime index/. The indices gg have a curve at half 



