202 ACCOUNT OF THE ARCTIC KEGIONS. 



feet. The degree of impregnation of the one-inch 

 cubes of ash, produced by immersion to the depth 

 of 2058 feet to 6348 feet, varies irregularly, but is 

 evidently as great at the depth of 2058 feet, as un- 

 der any superior pressure ; so that it is probable 

 that the greatest permanent impregnation by pres- 

 sure, of such open-grained woods as ash, elm, fir, &c. 

 is produced at the depth of 300 or 400 fathoms. 

 Hence it is clear that no use can be made of this 

 effect of pressure, for determining the depth, unless 

 it be within 2000 feet of the surface ; and even in 

 this limit, the results may be uncertain. 



From a comparison of column vii. with xiii., and 

 column iv. with xv., it appears, that an effect of the 

 impregnation of the wood with sea-water, was to in- 

 crease its dimensions, as well as its specific gravity ; 

 each specimen, on an average, having swelled 0.05 

 cubic inch in every solid inch of original dimen- 

 sions, and gained 84 grains on every 100 grains of ori- 

 ginal 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 impregna- 

 tion always iiicreiises with the increase of pressure ; 

 but the air contained in the pores of the wood, 

 which is never wholly disengaged, exerting an ex- 

 pansive force when the load of pressure is removed, 

 forces part of the water out again. This was clear- 

 ly discernible in some of the specimens used in the 

 foregoing experiments, at the moment they were 

 hauled up, their surfaces being covered with a thin 

 pellicle of froth. Hence pieces of fir sometimes be- 



