230 



Royal Society : — 



at a small angle towards the upper orifice of the tube, demonstrating 

 the existence of a feeble current of air through the apparatus. 



The preceding experiment having proved the existence of a cur- 

 rent of air within the tube, of sufficient force to move a light body, 

 the author next proceeded to ascertain the velocity of the current 

 by means of an anemometer, in the form of a horizontal fly-disc, D, 

 suspended within the lower orifice of a tube (fig. 6), bent twice at right 

 angles below. The revolving disc was made of a circular piece of 

 stout writing-paper, cut into twenty-four equal segments, from the 

 circumference to near the centre, each of the segments being after- 

 wards inclined at an angle of twenty-five degrees*, like the vanes of a 

 windmill ; so that when properly suspended, a current of air entering 

 the lower orifice of the tube would cause the disc to revolve from 

 right to left. The disc was suspended in the same manner as the 

 needle of the mariner's compass, and by the same means. 



When the apparatus was arranged, the door of the room closed, 

 and the atmosphere in a quiescent state, it was found that a con- 

 stant regular rotation of the disc was established, and kept up by 

 the upward current of air through the apparatus, and continued so 

 long as the atmosphere of the room was quiet ; but that agitations 

 of the surrounding air either rendered the rotation uncertain, or 

 reversed it. 



Having thus ascertained that the current of air within the vertical 

 tube possessed sufficient force to cause the rotation of a lightly 

 suspended fly-disc, the question arose, what influence elongation or 



Fig. 9. 



Fig. 8. 



Fig. 7. 



shortening of the tube would exert on the velocity of the current. 

 For this purpose three tubes (figs. 7, 8, 9) of precisely similar con- 

 struction, but with long limbs of 12, 24, and 48 inches respectively, 

 were fitted as before with fly-discs, D, D, D, and placed near each 

 other in the centre of the room. 



In nineteen observations, the number of revolutions in the tube, 

 with a long limb of 12 inches, varied from 0'75 to 4'5 per minute; 



* A nearer approach to an angle of 45 would have crippled tiie paper, so that 

 it would not have preserved the horizontal position. 



