112 OBSERVATIONS ON THE STATE OF THE 



ment of any very accurate results presented themselves. The wood of 

 different trees of the same species, and growing in the same soil, or that 

 taken from different parts of the same tree, possesses different degrees of 

 solidity ; and the weight of every part of the alburnum appears to increase 

 with its age, the external layers being the lightest. The solidity of wood 

 varies also with the greater or less rapidity of its growth. These sources 

 of error might apparently have been avoided by cutting off, at different 

 seasons, portions of the same trunk or branch : but the wound thus made 

 might, in some degree, have impeded the due progress of the sap in its 

 ascent, and the part below might have been made heavier by the stagna- 

 tion of the sap, and that above lighter by privation of its proper quantity 

 of nutriment. The most eligible method therefore which occurred to 

 me, was to select and mark in the winter some of the poles of an oak 

 coppice, where all are of equal age, and where many, of the same size and 

 growing with equal vigour, spring from the same stool. One half of the 

 poles which I marked and numbered were cut on the 31st of December 

 1803, and the remainder on the 15th of the following May, when the 

 leaves were nearly half grown. Proper marks were put to distinguish the 

 winter-felled from the summer-felled poles, the bark being left on all, and 

 all being placed in the situation to dry. 



In the beginning of August I cut off nearly equal portions from a 

 winter and summer-felled pole, which had both grown on the same stool ; 

 and both portions were then put in a situation, where, during the seven 

 succeeding weeks, they were kept very warm by a fire. The summer- 

 felled wood was, when put to dry, the most heavy ; but it evidently con- 

 tained much more water than the other, and, partly at least from this 

 cause, it contracted much more in drying. In the beginning of October 

 both kinds appeared to be perfectly dry, and I then, ascertained the 

 specific gravity of the winter-felled wood to be 0.679, and that of the 

 summer-felled wood to be 0.609 ; after each had been immersed five 

 minutes in water. 



This difference of ten per cent, was considerably more than I had anti- 

 cipated, and it was not till I had suspended and taken off from the balance 

 each portion, at least ten times, that I ceased to believe that some error 

 had occurred in the experiment : and indeed I was not at last satisfied 

 till I had ascertained by means of compasses adapted to the measurement 

 of solids, that the winter-felled pieces of wood were much less than the 

 others which they equalled in weight. 



The pieces of wood, which had been the subjects of these experiments, 

 were again put to dry, with other pieces of the same poles, and I yesterday 



