in augmenting the Specie Gravity of different Woods. 369 
its specific gravity ; each specimen, on an average, having swell- 
ed 0.05 cubic inch in every solid inch of original dimensions, and 
gained 84 grains on every 100 grains of original weight ; that 
is, an increase of one-twentieth in size, and twenty-one twenty- 
fifths in weight. 
I have little doubt, but the degree of impregnation always 
increases with the increase of pressure ; but the air contained 
in the pores of the wood, which is never wholly disengaged, 
exerting an expansive force when the load of pressure is re- 
moved, forces part of the water out again. This was clearly 
discernible in some of the specimens used in the foregoing ex- 
periments, at the moment they were hauled up, their surfaces 
being covered with a thin pellicle of froth. Hence pieces of fir 
sometimes become buoyant, after being a few hours relieved 
from pressure, though kept constantly under water ; but all 
other kinds of wood yet tried, though they lose a little of their 
moisture, yet remain specifically heavier than water, as long as 
they are kept immersed. Blocks of wood, indeed, are now in 
my possession, that were soaked with sea-water in the year 
1817, and yet remain, at the bottom of a vessel of water, near- 
ly as heavy as when first drawn up out of the sea. 
The degree of pressure at the depth to which I sounded in 
my last experiment, is not a little astonishing, being, under a 
column of water, 6348 feet in length, at least S823 lb. or 25 cwt. 
23 lb. on one square inch of surface. Hence on the larger 
cubes of ash used in the experiment, though measuring only 
1.59 inches in diameter, the whole pressure must have exceeded 
nineteen tons ! 
XXIV . — Shetch of a Journey through Brazil in 1817 and 1818. 
By Mr Swains on of Liverpool. In a Letter to Professor 
Jameson. 
I DETERMINED on going to South America in the autumn 
of 1816. The enlightened policy which influenced several of 
the Continental Sovereigns in sending scientific men to explore 
those treasures which the country of Brazil offered to philosophi- 
cal investigation, the moment universal peace was restored, in- 
duced me to hope that our own Government would gladly have 
