Prof. Trowbridge on Deep-sea Soundinys. 101 



been produced by the descent of the lead. Here we have a reli- 

 able result to the depth of 1000 fathoms only. The diiference 

 between this result and the conclusions of Captain Deuhani is 

 simply the difference between one mile and nine miles. 



In measuring the distance to the sun, an error of eight miles 

 would hardly be worth noticing, perhaps ; but what conclusions 

 can be di-awn from a measurement in which the probable error 

 amounts to eight times the whole distance ? 



Popular ideas with regard to the sinking of bodies in the sea 

 have heretofore been vague, — for the reason, perhaps, that the 

 laws which govern this descent, and which are derived from the 

 well-known laws of fluids, have never been fully defined in 

 their application to the depths of the ocean. Some imagine that 

 ships which founder at sea, sink to a certain depth, and there 

 float about until broken to pieces, or thrown upon some bank 

 beneath the sea ; and, indeed, a recent writer in England has 

 published a book sustaining this absurd notion. Others, again, 

 believe that the buoyant force of the water at great depths is enor- 

 mous, and due to the whole pressure of the column of water 

 above, and that all bodies which are lighter than water at the 

 surface, will, if sunk to the bottom and detached from the sinker, 

 shoot upward with a great velocity ; or, in other words, that the 

 density of the water increases directly with the depth. These 

 views are erroneous. It is true the pressure increases with the 

 depth, to the amount of fifteen pounds upon every square inch 

 for every thirty-four feet in depth ; but the density is not thereby 

 sensibly increased, owing to the incompressibility of the water ; 

 so that neither the buoyant force, nor the resistance to the mo- 

 tion of any body, are sensibly increased from the surface to the 

 bottom. At the depth of 3000 fathoms, for instance, the pres- 

 sure upon a square inch is nearly 8000 pounds, but the column 

 of 18,000 feet of water is only shortened about 60 feet : the den- 

 sity is thus but slightly increased ; but the efl'eet of this enormous 

 pressure upon compressible bodies, as air, wood, &c., is to con- 

 dense them into a smaller bulk, by which they may be rendered 

 heaviei' than water, and will sink of their own weight. A piece 

 of wood cannot float at the bottom of the sea ; but a very slight 

 extraneous force will bring it to the surface. 



Now, how is it with the sounding-lead and line ? The lead, 

 if allowed to descend alone, will fall with a uniform and rapid 

 velocity to the bottom. This velocity will be attained within a 

 few feet of the surface, and will be due to the opposing forces of 

 gravity and the resistance of the water, whicli will be balanced 

 when the uniform velocity is reached. But if a line be attached 

 to the lead, a few hundred feet of the line will offer a resistance 

 to the motien nearly equal to the whole weight of the lead j and 



