173 



are full grown, and when the annual shoots have ceased to elongate, 

 than at any earlier period. This energy in the powers of vegetation 

 must certainly, Mr. Knight thinks, be employed in some very im- 

 portant operation. He has observed that the produce of his meadows 

 has been greatly increased when the herbage of the preceding year 

 had been left till the end of autumn, on ground that had been mowed 

 early in the summer ; from which he has been led to imagine, that 

 leaves are employed during the latter part of the summer in the pre- 

 paration of matter calculated to afford food to the buds and blossoms 

 of the succeeding spring. 



In order to determine whether the foregoing opinions were well 

 founded or not, Mr. Knight made the following experiments. 



Having made incisions in the trunks of sycamore and birch-trees 

 (some of these incisions being close to the ground, others at the ele- 

 vation of seven feet), he found that the sap obtained from the syca- 

 more close to the ground, was of the specific gravity of 1 '004, while 

 that obtained at the height of seven feet had a specific gravity of 1 '008. 

 The sap of the birch was somewhat lighter ; but the increase of spe- 

 cific gravity, at different elevations, was comparatively the same. The 

 sap of both these trees, when extracted near the ground, was almost 

 void of taste ; but when obtained at a greater height, it was sensibly 

 sweet. In one instance it was extracted from the sycamore-tree at 

 the height of twelve feet ; it was then very sweet, and its specific 

 gravity was 1*012. 



Mr. Knight then made an experiment to compare the sap obtained 

 from a recent incision with that obtained from an old one. He found 

 that the sap from an old incision was reduced in specific gravity to 

 1*002, while that from the recent incision continued at 1'004, as 

 before. These incisions were made in a sycamore-tree, and were 

 close to the ground. 



Some observations then follow on the variation in the specific gra- 

 vity of the alburnum at different seasons. After talcing every pre- 

 caution to avoid error, the author found the specific gravity of winter- 

 felled oak to be 0'679, and that of summer-felled oak to be 0*609, 

 after they had both been immersed five minutes in water. This dif- 

 ference appearing to Mr. Knight very considerable, he repeated the 

 experiment several times, but found no reason to suspect any error 

 in it ; and upon measuring pieces of both kinds of wood, which were 

 equal in weight, it appeared that the winter-felled pieces were much 

 less than the others. The more recently formed layers of winter- 

 felled wood had a specific gravity of 0*583 ; that of the summer-felled 

 wood was only 0*533. In another experiment the former was 0*588, 

 the latter 0*534. 



On pouring boiling water on equal quantities of summer- and of 

 winter-felled wood, it appeared that the latter communicated a much 

 deeper colour to the water than the former ; it also raised the spe- 

 cific gravity of the water to 1*002 ; the specific gravity of the other 

 infusion was 1*001. 



Mr. Knight thought he had reason to believe that the matter de- 



