Vegetable Staticks. 115 



In very hot weather many air-bubbles would 

 rife, (0 as to make a froth an inch deep, on 

 the top of the fap in the tube. 



I fix'd a fmall air-pump to the top of a long 

 tube, which had 12 feet height of fap in it; 

 when I pumped, great plenty of bubbles arofe, 

 tho' the fap did not rife, but fall a little, after 

 I had done pumping. 



In Experiment 34. (where a tube was 

 fixed to a very fhort flump of a Vine, with- 

 out any lateral branches) we find the fap 

 rofe all day, and fafteft of all in the greateft 

 heat of the day : But by many obfervations 

 under the 37th and this 38th Experiments, 

 we find the fap in the tubes conftantly fub- 

 fided as the warmth came on towards the 

 middle of the day, and fafteft in the greateft 

 heat of the day. Whence we may reafon- 

 ably conclude, (conlidering the great perfpi- 

 rations of trees, fhewn in the firft chapter) 

 that the fall of the fap in thefe fap-gages, 

 in the middle of the day, efpecially in the 

 warmer days, is owing to the then greater 

 perfpiration of the branches, which perfpi- 

 ration decreafes, as the heat decreafes towards 

 evening, and probably wholly ceafcs when 

 the dews fall. 



But 



