DEEP-SEA INVESTIGATION. 63 



equal to that of lead, and the sinkers would float instead of sinking. 

 This might be the case were water as compressible as air, and if the 

 lead could escape compression ; but the amount of pressure that will 

 double the density of air will increase that of water by only one- 

 twenty-thousan.dth part, and it would require the pressure of more 

 than two hundred thousand atmospheres to squeeze water to the den- 

 sity of lead. The deepest water, five thousand fathoms, is not subject 

 to a pressure that can raise its density as much as one-twentieth part. 

 Moreover, the weight of lead is increased by pressure much faster 

 than that of water, so that, however dense the water may be, it would 

 have to encounter a still denser lead. This objection, however, falla- 

 cious as it has been shown to be, has been admitted by persons of high 

 authority, of course without sufiicient thought. 



A more real but exaggerated objection to the trustworthiness of 

 deep soundings is founded on the existence of currents likely to cause 

 deviations in the direction of the line, and to change the position of 

 the ship. There is no doubt concerning surface-currents ; they are 

 observed and measured every day, and form an important factor in 

 the navigator's daily reckoning. It has been inferred that they may 

 be complemented by return under-currents which will be harder to 

 deal with because they can not be so easily detected and nieasured. 

 Soundings taken in the presence of such currents are, it must be ad- 

 mitted, less to be relied upon than those taken in manifestly quiet 

 waters ; but the extent of under-currents has been very much exag- 

 gerated. By far the greater part of the ocean is, for sounding pur- 

 poses, practically still water. The surface-currents of any importance 

 are easily recognized, and so also are the under-currents. Just as a 

 physician can, by bringing his experience to bear on the sounds trans- 

 mitted to him by the stethoscope, divine what is taking place inside 

 the body of his patient, so the experienced seaman can, by observing 

 the behavior of his sounding-line, form a fair diagnosis, of what is 

 taking place in the depths of the sea. When the sinker passes into a 

 belt of under-current, the fact is very soon apparent ; but, even with 

 the greatest care, soundings taken under such circumstances are of 

 doubtful value, unless bottom is brought up. In the latter case, we 

 know the depth is not greater than the length of line used, and a cor- 

 rection, suggested by observation and experience, may be apj)lied, 

 which will bring our estimate of the depth very near the truth. It is 

 evident that this can not be satisfactorily done by the sounding-line 

 alone, and it early occurred to those who thought on the subject that 

 the method which promised most success was that which should give 

 the depth in terms of the height of the column of water ; in other 

 words, the barometrical measurement of altitudes was extended from 

 the land to the sea. * The instruments which have been suggested for 

 this purpose are constructed with a view to record the amount of 

 compression produced on a given mass of a certain elastic substance. 



