SCIENCE AND PRACTICE. 393 



of the soundings were made are given by the vertical dotted- 

 lines, and the corresponding depths in fathoms by the 

 figures at the surface. Sudden points may, and no doubt 

 do, occur which the soundings do not discover ; but when 

 the soundings are taken at moderate distances all the great 

 irregularities may be considered as being known. Soundings 

 for the English Government were successfully made by Rear- 

 Admiral Sir J. 0. Ross in his expedition to the southern 

 Antarctic Sea, in March, 1840, by a method which appears 

 to be the only trustworthy one yet devised that of sounding 

 by time. A lead is sent down upon a very light line, just 

 strong enough to withstand the friction of the water in 

 descending ; its purpose being only to give an indication, 

 when the lead reaches the bottom. The times which elapse 

 between the moment when the lead is eased down and each 

 successive mark passes out of the ship, as well as the moment 

 when the lead touches the bottom, are noted and compared 

 with data given by previous measurements or with the cal- 

 culated velocities. The velocity of a body falling free in 

 water is found to be, after the first few seconds, uniform. 

 When it hangs upon a line, however, the velocity of descent 

 decreases regularly by reason of the friction of the line 

 against the water. The results obtained in this way are 

 more to be depended upon than those obtained by direct 

 measurement of tlj.e line, which is liable to make curves 

 in the water by drifting, under- currents, &c., influences 

 which do not alter the times of descent when the resistance 

 of the line is very small. Both line and lead are, of 

 course, lost in this measurement. The French officials use, 

 on the contrary, a strong line, and a lead which they recover 

 always. Their results are therefore less reliable, unless the 

 soundings are taken from a small boat, which is not drifted by 

 the wind so much as the ship, and, even then, not where there 

 are any considerable under-currents. 



These diagrams of the bottom give a tolerable notion of 

 the amount of slack which should be expended at each point 

 of the line, and the engineer is able to r*egulate his break 

 accordingly. As a rule, the more cable thrown away in the 



