UPON THE WORKING OF WHEWELL's ANEMOMETER. 



37 



the hourly meteorological observations at the Dockyard, we are entitled to 

 say, so far as our experiments extend, that there is an annual movement of 

 the atmosphere in this latitude toward the north, under a mean pressure of 

 29*900 inches nearly, taken at the level of the sea, and a mean temperature of 

 52° Fahr. The complete and satisfactory working of the anemometer, now 

 that it has undergone certain amendments, found by experience desirable, leads 

 me to hope that its use will be persevered in by observers in meteorology, since 

 the principle on which it has been founded is undoubtedly very perfect and 

 satisfactory. I do not, after a very critical examination, and experience in the 

 use of the instrument, see any difficulty whatever in respect to its mechanism 

 which may not be easily conquered ; and it only now remains to find what 

 are the actual numerical values of its indications ; that is to say, having been 

 enabled to trace an annual movement of the air in the direction above stated, 

 we should at the same time be enabled to determine the rate of the motion. 

 This would seem at first sight a sufficiently difficult matter. We may hope, 

 however, to arrive at something like a fair approximation to such information, 

 by the following mode of experiment, now in progress. 



With a view of determining the amount of pressure as observable by ex- 

 posing surfaces varying in dimensions to the aerial current, the poi'table gauge 



I represented in the annexed figure has been suc- 

 cessfully applied. A brass quadrant d e, being 

 set in a frame of brass and divided in the usual 

 way, a pressure plate a is so applied on the top 

 of the frame as to act by a rod h I, and a silk 

 line over intervening pulleys on the spiral spring 

 6; the pulley c fixed in the centre of the quadrant 

 carries the index cf, which will rise on the gra- 

 duated arc in proportion to the pressure, the 

 amount of this pressure in terms of a standard 

 of weight being known by experiment, that is, 

 by placing different weights on the extremity of 

 the rod held in a vertical position, and observing 

 the corresponding degrees on the arc. If Lind's 

 gauge be employed as a standard, we may readily 

 examine the pressures corresponding to various pressure plates, and thus dis- 

 cover whether the same pressure on a unit of area is shown by different sized 

 plates. This being determined, we are in a state to employ plates of different 

 dimensions according to the violence of the wind, and hence readily compare 

 the pressure with the velocity more easily. 



To find the velocity by experiment, a cork stuck round with capacious 

 feathers is made to travel over a fine wire of a given length by the force of 

 the wind ; the cork is set on a common writing quill bushed with a small brass 

 plate at each end, and by which the whole is supported on the wire, fine holes 

 being drilled through tlie brass plates for receiving it. This contrivance 

 is extremely light, and will fly along the wire with the velocity of the wind 

 for a given distance, or very nearly so. It is in fact throwing, as it were, a 

 log-line upon the air. Observers may now compare the pressures, correspond- 

 ing to certain velocities, and to the descent of the pencil on the anemometer ; 

 and thus its indications are reducible by experiment to terms of absolute value, 

 when a sufficient number of observations have been made and tabulated. 



