Waves 5 
marine. During a heavy swell on the surface a submarine may 
experience no perceptible motion of the water. In certain 
scientific investigations-at sea, such as the determination of 
the force of gravity by means of a delicate pendulum experi- 
ment, it is important to prevent the apparatus from being 
jarred. A submarine then provides an excellent place for a 
laboratory. 
THE SHAPE OF A WAVE 
The shape of an ocean wave may be approximately de- 
scribed by the curve that the mathematicians call a trochoid 
TROCHOIDAL WAVE 
ee a Nr 
SOUND OR LIGHT WAVE 
( SINUSOIDAL WAVE ) 
(pronounced as in “TROlling for sharKs of cellulOID”). A 
trochoid is really quite a simple curve despite its peculiar 
sounding name. Suppose you put a spot upon a wheel and 
then roll the wheel along a plane in a straight line. The curve 
_that the spot describes in space is a trochoid. Now it has been 
observed that ocean waves very nearly approximate such a 
shape. | 
Extensive studies have been made of the shape of water 
waves by measuring photographs taken of them at different 
angles in much the same way that the contours of a country- 
