26 LIFE OF 



board thus sawn is never seen to deviate from its horizontal 

 position when laid in a floor. If, on the contrary, a board be 

 sawn across the silver grain, it will during many years be inca- 

 pable of bearing changes of temperature and moisture without 

 being warped, nor will the strength of very strong nails be able 

 to prevent the inconvenience thence arising. On this account 

 quarter boards are always sold at a much higher price than 

 others, which are here called bastard boards. If a board of the 

 latter kind be laid in the floor with that surface uppermost 

 which grew nearest the centre of the tree, it will show a dispo- 

 sition to become convex ; if with the other surface uppermost, 

 concave. The latter being much more inconvenient, this cir- 

 cumstance ought to be attended to by workmen ; but it is, I 

 believe, wholly unknown to them. I do not suppose this pro- 

 perty in wood to have been attended to by the makers of 

 harpsichords or pianofortes ; if it has not, it is probably the 

 cause why some instruments keep in tune better than others. 



" You have, perhaps, remarked that when an oak has been 

 stript of its bark and exposed to the sun and air, its surface 

 becomes full of small clefts, which continue for a long time to 

 contract and expand with the changes in the weather you will 

 find that these are always formed by the laminae of the silver 

 grain having parted from each other. This restless temper in 

 it (of which I could point out other instances) has convinced 

 me that it was not made to be idle ; and as no other power 

 appears to me to have been discovered capable of propelling the 

 sap to the height described by Dr. Hales, I am much disposed 

 to believe that this is the office which nature has assigned it, 

 and that the following may possibly be the mode of acting. 

 All bodies being more or less expansible by heat, and the silver 

 grain appearing to be of a very irritable temper, I infer that it 

 will expand and press on the sap vessels, whenever the tempe- 

 rature of the surrounding air is increasing by the presence of 

 the sun or other causes ; and that it will contract again during 

 the cold of the night, or other adventitious decrease of heat. 

 These effects will first take place in the smaller branches later 



