170 W. P. Trowbridge on Deep Sea Soundings. 
3d. The uncertainty of knowing the exact instant of the weight 
striking the bottom, the effect of drift or currents, and the great 
loss of velocity in the descent, requiring so much time for a sin- 
gle cast, are therefore difficulties which cannot be overcome by 
this mode of sounding, and there is a limit at two or three thou- 
sand fathoms depth, beyond which, the method practically fails, 
and even at less depths large errors must enter into the results. 
The effect of increasing the specific gravity of the line as in 
making use of wire, is unfavorable, since at a certain depth the 
weight of the wire alone must overcome the resistance offered 
y the water, and it would run out of its own weight. The 
depth at which this would occur for a wire ;3, of an inch iD 
diameter is about 1700 fathoms; and for a wire ;4, of an inch m 
diameter, 400 fathoms. 
_ The same difficulty occurs when a large hempen line is used; 
a line two inches in diameter of this material would run out of 
its own weight with a velocity of 8 feet per second at the depth 
of 1900 fathoms. An instance of this kind is given in the ex- 
periment of Lieut. Joseph Dayman, R. N., in sounding across 
the Atlantic in 1857. 
Table No. IX gives the results of the experiment referred to; 
when the line was hauled in, 200 fathoms of it were coiled upon 
the sinker, the latter having struck bottom at 2200 fathoms. _ 
The next consideration is the mode of noting or measuring 
the depth within the limits which are accessible. For this pul 
pose several plans have been followed, none of which have been 
80 successful that the degree of accuracy attained, or the proba 
ble error of the determination, might be estimated. : 
___ The most natural method is to measure the length of the line 
“paid out. The errors arising from the drifting of the vessel, and 
rents acting upon a long length of line in the water, may 
amount to as much as the depth, and the method has 
epth from the time and rate of descent 
