256 THE SEAS 



read off a dial. When a very large amount of wire is off 

 the drum, its own weight comes dangerously near its 

 breaking strain and there is a risk that, when it and the 

 lead weight attached are hauled in, a break may occur. 

 This difficulty is overcome by employing a specially 

 designed weight which consists of a number of separate 

 sinkers, and on the lead striking bottom these are released 

 and the weight to be hauled up is thus considerably reduced. 

 In very recent years a much improved method of sounding 

 has come into practice. This method is an outstanding 

 example of the application of knowledge gained by research 

 in other branches of science, being a combination of the 

 use of sound and electricity. It is known as " echo sound- 

 ing " and the principle of it is that a sound is sent vertically 

 downwards to the bottom and a very delicate instrument 

 picks up the sound once more as it is reflected back from 

 the bottom, in other words it receives the echo. Now the 

 rate of travel of the waves of sound through water is 

 known (about 4,900 feet per second) and by noting the 

 interval of time between the first transmission of the 

 sound and the reception of its echo it is possible to calculate 

 the distance through which it has passed and hence the 

 depth of the bottom. In shallow water this interval of 

 time becomes extremely small, and it is only by the use of 

 elaborate electrical apparatus that the time can be measured. 

 With such an instrument a ship can travel swiftly through 

 the water sounding almost continuously as she goes, and the 

 time occupied in taking a reading is a striking contrast to 

 that required with weight and line. The original word to 

 " sound " (which is considered to be a form of the Old 

 English word " sund," meaning ' swimming ") would 

 nowadays appear to have been very happily chosen and 

 could not express better what is actually being done when 

 a depth measurement is being taken. 



