62 WAVES 



difficult to find an appropriate point from which to make the observa- 

 tions. 



The heights of waves have also been estimated by the alteration that 

 takes place in the readings of an aneroid barometer as the ship rises 

 from trough to crest and then sinks again from crest to trough. But 

 this method is liable to errors, the magnitude of which it is difficult to . 

 estimate; hence, it is not of much practical value under ordinary cir- 

 cumstances. 



Waves and breakers may be most easily measured from piers with 

 an ordinary sounding line. The lead is allowed to rest on the bottom 

 and the line held taut, when the extent of the rise and fall of the water 

 can then be measured on it. Or the difference in the elevation of the 

 crests and troughs, relative to a fixed point such as the railing of the 

 pier, may be measured by raising and lowering the lead with the sur- 

 face of the water, as the latter rises and falls. Another simple method 

 is to attach a scale of feet and inches to one of the piles of a pier, or to 

 a pound net stake, and to read the rise and fall of the surface of the 

 water from this, with the passage of successive crests and troughs, 

 using a field glass or telescope if it is necessary to take the readings 

 from a distance on the beach. Waves can also be measured from the 

 shore by means of an anchored float which bears a vertical mast with 

 cross arms at intervals of, say one foot, the float being observed with a 

 transit, as the rise and fall of the water surface causes the arms to pass 

 the cross hair in the instrument. And sighting devices of various other 

 kinds have been devised for the purpose. 



Recording meters have also been used in which a small float, rising 

 and falling with the waves along a vertical rod, operates a pen writing 

 on a rotating drum or on a moving tape, but these, and recording meters 

 working on other principles, have not as yet come into general use, 

 although they may be expected to prove useful. 



