468 



vations, and it appeared desirable to ascertain what influence the 

 long branch of the siphon-like tube had in the production of the cur- 

 rent. For this purpose the long vertical tube was made moveable, 

 so that the apparatus could be alternately converted into a siphon 

 with equal limbs 4 inches in length, or one with a short leg of 

 4 inches, and a long one of 96 inches. In twelve observations, when 

 the long leg was inserted, the mean depression of the hygrometer 

 was 2'5 Fahr. ; when the limbs were of equal length, 2 0< 25 Fahr. 



Considering it possible that the current of air existing in the tube 

 might have sufficient force to move a light body delicately suspended 

 in its track, an elbow was inserted into the upper orifice of the tube, 

 to which a piece of glass tube of the same diameter was adapted, 

 6 inches in length, and a disc of tissue-paper, weighing one grain, 

 which nearly occupied the area of the tube, was delicately suspended 

 by a hair, at right angles to the axis of the tube. A slide valve was 

 so adapted to the lower orifice, that this aperture could be opened or 

 closed without entering the room. The air of the room being qui- 

 escent, it was found that when the slide valve closed the lower orifice 

 of the tube, the disc of tissue paper remained perfectly quiescent ; 

 but that when the slide valve was withdrawn, leaving the lower 

 orifice open, oscillations of the paper occurred, and it was projected 

 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 an horizontal fly- disc, 

 suspended within the lower orifice of a tube, 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. 



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

 it would not have preserved the horizontal position. 



