TERRESTRIAL REFRACTIOX. ' 17 



This construction readily explains why it should 

 be easily affected by either moisture or dryness, but 

 does not evince that heat, or other changes in the 

 ambient air, might not likewise operate upon it in 

 the same direction. The following experiments satis- 

 fied me, however, that moisture alone unfolded the 

 stem. 



EXPERIMENT I. 



Having fixed a stem, about an inch long, with 

 six twists in it, on a piece of wax, a slit straw was 

 fastened at its upper end, by way of index : the 

 whole was then placed clear of any motion of the 

 air, in such a position as the nature of the experi- 

 ments required; and a graduated circle of paste- 

 board was so placed about it, as to indicate the num- 

 ber of revolutions and degrees which the index went 

 over. 



Then, on placing my hand within an inch of the 

 stem, the index generally moved from 5° to 10^ of the 

 circle, from left to rio/it; and as motion in this direc- 

 tion was invariably the consequence of moisture, it 

 was probable (but still it remained to be proved) that 

 the effluvia arising from perspiration principally af- 

 fected it in this case. 



EXPERIMENT II. 



By breathing violently on the stem, I generally 

 brought the index round from 70° to 90*^ in the same 

 direction; and it would go back to its original place 

 as soon as let free. 



This was a sufficient proof of its great sensibility ; 

 but to make sure whether moisture or heat caused it 

 to move in the two preceding cases, I had recourse 

 to the following : 



Vol. IX. c 



