188 BOAED OF AGRICULTURE. 



the weather, and happened twelve days before there were any 

 indications of growth to be detected in the buds. These 

 observations are qnite new and as yet wholly inexplicable, 

 but will receive further attention at a future time. 



The maximum pressure of the sap for the season was 

 observed at 10, a. m., April 11th, and was equal to sustain- 

 ing a column of water 31.73 feet high. This was an excel- 

 lent sap day, considering the lateness of the season. There 

 was noticed a general correspondence between the flow of sap 

 in other mnples and the pressure on the gauge. 



After April 29th, the mercury remained constantly below 

 zero, day and night. During the mouth of May, there was 

 a uniform suction equal to about eight feet of water, and the 

 unaccountable feature of this fact is that though apparently 

 produced by exhalation from the expanding leaves, it remained 

 the same, day and night, for several weeks. In June, the suc- 

 tion gradually lessened and finally disappeared, the mercury 

 standing steadily at zero. 



The fact that exhalation from the leaves of growing plants 

 would cause suction capable of holding up several feet of 

 water was discovered by Hales, but has no apparent con- 

 nection with these phenomena. 



On the 20th of April, two gauges were attached to a large, 

 black birch, one at the ground and the other thirty feet higher. 

 The next morning at 6 o'clock, the lower gauge indicated the 

 astonishing pressure of 56.65 feet of water, and the u[)per, 

 one of 26.74 feet. The difference between the indications of 

 the two o;auges was thus 29.92 feet, while the actual distance 

 between them was 30.20 feet, so that they corresponded 

 almost precisely as if connected by a tube. In order to learn 

 whether the same principle would prevail, if the upper gauge 

 was moved, it was raised twelve feet higher. The same cor- 

 respondence continued through nearly all the observations of 

 the season, notwithstanding the gauges were separated by 

 42.20 feet of close-grained birch- wood. 



At 12.30, p. M., April 21st, a hole was bored into the tree 

 on the side opposite to the loAvcr gauge, and at the same level. 

 Both gauges at once began to show dijninished pressure, while 

 sap issued freely from the orifice. In fifteen minutes, one 



