Mr. H. Hamell on a New Anemometer. 101 



in an inverted siphon. Of this last kind, the" differential 

 barometer of the late Dr. WoUaston surpasses in sensibility 

 to an indefinite degree all others of the kind,— an invention 

 worthy of the accurate and comprehensive mind of that la- 

 mented philosopher; but it is to be apprehended that in many 

 cases it cannot be easily applied to practice*. It is the other 

 principle, that of a pendulous weight deflected, that I make 

 use of; and I shall now proceed to the description of the in- 

 strument I propose. 



Imagine two pieces, which I shall call bars, like the two 

 ruler pieces of the common parallel rule, and attached to each 

 other by two joints in a similar manner ; the joints being con- 

 structed so as to play with scarcely any friction. Let these bars 

 be placed, in contact, with the plane of the joints perpendicular 

 to the horizon ; it is clear that the weight of the lower bar will 

 cause it to separate from the upper, and the joints to arrange 

 themselves perpendicular to the horizon. To one end of the 

 lower bar (that end which is to be presented to the wind) let a 

 plane of given magnitude be affixed, perpendicular to that bar. 

 Now to use the instrument, let this plane be placed perpendi- 

 cular to the wind, the upper bar being in a fixed position ; 

 the two bars being separated to their greatest distance, and 

 the joints of course at right angles to the bars, and perpendi- 

 cular to the horizon. The impulse of the wind acts as a force 

 on the plane, and causes it to recede ; but in consequence of 

 the two connecting links and the two parallel bars, it must 

 still preserve its parallelism to its first position, while it will 

 continue to recede; the links turning round their centres until 

 they have reached such an angle from the perpendicular as 

 shall produce an equilibrium between the impulse of the wind 

 on the plane and the weight of the lower bar with its appen- 

 dages, now supported by the links in an oblique position; 

 the angle by which the links deviate from the perpendicular 

 to be measured on a circular arc attached to the fixed bar. 

 As the lower bar, plane and links ought to be made as light 

 as is consistent with the required strength, and further, per- 

 haps, their common centre of gravity to be brought by coun- 

 terpoise into the line joining the upper centres of the links, it 

 will be seen that the most moderate breeze will cause the two 

 bars to collapse, or nearly so. There must, therefore, be a 

 series of weights, one or more of which must be suspended to 

 the lower bar according to the strength of the wind, so as to 

 cause the equilibrium to take place with the links at an angle, 

 not greater than 45°, from their quiescent or perpendicular 

 position. 



• A notice of Dr. Wollaston's paper on his Differentia! Barometer will 

 be found in the Phil. Mag. and Annals, vol. v. p. 305. — Edit. 



Now 



