1817.) Royal Society. 469 
tree be sawed in the direction from the bark to the pith, it speedily 
expands so much as to catch hold of the saw and prevent its action. 
If the parts be kept asunder by a wedge, and the sawing continued 
to the centre of the tree, the instant the wedge is removed the two 
sides of the cut close together with violence. If another slit be cut 
in the section from the bark to the centre at a short distance, so as 
to detach a slip of the tree altogether from the rest, this slip does 
not fall out, but is retained in its place by the expansion of the 
wood. [f the section be sawed in any other direction, so as to cut 
across the fibres of the silver grain, instead of merely separating 
them, no expansion takes place, and the saw continues to act freely. 
The pith in trees has a larger diameter when the tree is full of sap 
than when dry. He took branches, dried them well, and then 
forced in pieces of metal, so as to fill exactly the space occupied by 
the pith. The pieces of wood were then buried in moist earth, so 
as to absorb moisture. When in this state, the pieces of metal had 
become so loose, that they dropped out of themselves. 
On Thursday, May 15, part of a paper by Dr. John Davy, on 
the Temperature and Specific Gravity of the Sea, and on the Tem- 
perature of the Air over the Sea in Tropical Regions, was read. 
The observations contained in this paper were made by the author 
during his voyage from England to Ceylon. The temperature of 
the air was marked every two hours, both night and day. The sea 
water, when drawn up, was tried by means of a thermometer to 
ascertain its temperature, and then weighed in a weighing bottle 
capable of containing about 300 grains. During the latter part of 
the voyage the sea water was corked up in phials, and its specific 
gravity determined after he landed in Ceylon. It was taken at the 
temperature of 80°, which is nearly the mean heat of the tropical 
countries. The general result of these observations is, that the 
specific gravity of the sea is nearly the same every where. He does 
not agree with a modern traveller of high authority, who considers 
the specific gravity of sea water to differ in every zone. The small 
differences that exist are not easily accounted for. In one case he 
found the specific gravity diminished after very heavy rain. It was 
generally altered by squally weather. In general the temperature of 
the air was highest exactly at noon, and lowest just at sun-rise ; but 
in a perfect calm the temperature of the air was the same as on 
land; namely, its greatest height was some time after noon, The 
reason is, that heat accumulates both in the ship and in the sea. 
LINNEAN SOCIETY, 
On Tuesday, May 6, a paper by Andrew Knight, Esq, on the 
Species of the common Strawberry, was read. ‘The author is of 
opinion that no plants can be considered as constituting different 
species, excepting those incapable of propagating with each other. 
He therefore planted all the different varieties of strawberry known 
in this country in garden pots, and cultivated them in the proper 
situation to impregnate one another, and continued his experiments 
for several years, The result was, that there are only three distinct 
