40 Mr Forbes's description of a New Anemometer. 



A greater space of exposure than in this last case might, how- 

 ever, be desirable, that the wind might have an unrestrained 

 action through the machine. 



Suppose, in the first case we have calculated, that the den- 

 sity of the spherules was doubled, it might be easily shown 

 that 2 a would become 2 am, the density being altered as 1 :m; 

 and it is also demonstrable that the same change would be 

 produced by altering the diameter in the same ratio. The 

 following computation will serve for the case of density double 

 that of water, and that of a diameter double the last, or 0.4 

 inches, the velocity being the same. 



Supposing, therefore, that spherules of the diameter of 0.2 

 inches, having a specific gravity of 2, (which might easily be 

 made of some permanent composition approaching this den- 

 sity) be selected as the standard, (or, what is the same, having 

 half the diameter, or 0.1 inch, and a specific gravity of 4,) we 

 may compute for it the spaces corresponding to equal incre- 

 ments of velocity, and if we divide the ordinary velocities of 

 wind into three classes, under 20, under 40, and under 60 

 feet per second, corresponding to about 14, 27 and 41 miles 

 an hour, we shall have a sufficient illustration for our present 

 purpose. Such a computation once made for any instrument 

 will regulate the breadth of the spaces for the reception of 

 the balls, which, as we have remarked above, would not be in 

 the ratio of the velocities, were they equally divided. 



