92 ACCOUNT OF SOME EXPERIMENTS ON THE 



If these are expansible under changes of temperature, or from any cause 

 arising from the powers of vegetable life, I conceive that they are as well 

 placed as is possible to propel the sap to the extremities of the branches ; 

 and their restless temper, after the tree has ceased to live, inclines me to 

 believe that they are not made to be idle whilst it continues alive. 



I shall at present confine my observations to the English oak, though 

 the same are applicable, in a greater or less degree, to every other kind 

 of wood. In sawing this tree into boards, it is usual to cut it, as much 

 as possible, into what are called quarter-boards ; which are so named 

 because the tree is first cut into quarters. In a perfect board of this kind, 

 the saw exactly follows the direction in which the tree most readily divides 

 when cloven : in this case, the laminae of the silver grain lie parallel with 

 the surface of the board ; and a board thus cut, when properly laid on 

 the floor, is rarely or never seen to deviate from its true horizontal 

 position. If, on the contrary, one be sawed across the silver grain, it 

 will, during many years, be incapable of bearing changes of temperature 

 and of moisture without being warped ; nor will the strength of numerous 

 nails be sufficient entirely to prevent the inconvenience thence arising. 

 That surface, of a board of this kind, which grew nearest the centre of 

 the tree, will always show a tendency to become convex, and the opposite 

 one concave, if placed in a situation where both sides are equally exposed 

 to heat and moisture. You may probably have observed, that when an 

 oak has been deprived of its bark, and exposed to the sun and air, its 

 surface has been everywhere covered with small clefts. These are always 

 formed by the laminae of the silver grain having parted from each other ; 

 and they will long continue to open and close again with the changes of 

 the weather. In the last summer, I very frequently placed pieces of oak, 

 recently deprived of its bark, in a situation where it was fully exposed to 

 the sun, but defended from rain. The surface of the tree, in a few hours, 

 presented a great number of small clefts, into which I put, in the middle 

 of the day, the points of small iron pins. Examining these late in the 

 evening, I found that the wood closed so much as to hold them firmly, 

 and, early in the next morning, they were not easily withdrawn ; but as 

 the influence of the sun increased, the clefts again gradually opened, as 

 in the preceding day, and the pins always dropped out. I could never 

 discover that any weight was gained by the wood during the night ; but 

 I was not provided with a balance of proper sensibility to ascertain this 

 point. This experiment was frequently repeated, and always with 

 precisely the same result. After long exposure to the air and light, the 

 wood loses this property. 



