242 



REPORT — 1844. 



with the time, or otherwise the elements from which this integral force may 

 be determined ; and it will be my endeavour in this report to show how far 

 these instruments are calculated for such a purpose. 



IVhewell's Anemometer. — It does not seem requisite to enter here upon 

 any lengthened description of this instrument, inasmuch as a very full ac- 

 count otit has already appeared in vol. ix.and x. of the Reports of the As- 

 sociation. It may however be desirable for the sake of perspicuity to revert 

 briefly to its general character as shown in the annexed cut. 



By means of a vane 

 V a windmill fly F is 

 constantly presented 

 to the wind, and it re- 

 volves more or less ra- 

 pidly according to the 

 velocity of the cur- 

 rent. Anintermediate 

 train of wheels, &c. 

 operated on by the fly 

 F, causes a pencil 1* 

 to descend vertically 

 over a fixed cylinder 

 C, leaving a trace 

 thereon of variable 

 length, according as 

 the wind is more or 

 less strong. The di- 

 rection is shown by 

 vertical lines on the 

 cylinder, correspond- 

 ing in position to six- 

 teen points of the 

 compass, and upon 

 some one of which the 

 pencil P is brought to 

 act by means of the 

 vane V. Supposing 

 the fly F to revolve in 

 the simple proportion 

 of the velocity of the 

 wind, we obtain in the 

 length of the trace or 



line described by the pencil, a space proportional to that which a parti- 

 cle of air would describe in a given direction in a given time, say one day, 

 taking into the account the strength of the wind and the time for which it blows. 

 Finally, by collecting these integral results and laying them off successively 

 by the graphical method of delineation, we obtain as in Plate XXXllI. figs 

 1, 2, 3, what may be considered the path of the wind for a given period of 

 time. The mean annual path thus obtained in any given place is called the 

 type of the wind for that place. As an abstract philosophical principle nothing 

 can be more perfect or better adapted to the end proposed to be attained, 

 and although the mechanical arrangements which have been resorted to in 

 order to carry it out effectually in practice may not be the best possible, 

 yet I shall endeavour to show that we have by means of this instrument 

 still arrived at extremely valuable examples of its general application. 



