Now, suppose that after one second the eleventh
man is ordered to kneel and the first to stand ; after
two seconds the twelfth man to kneel and the
second to stand ; and so on. There will then con-
tinue to be a row of ten kneeling men, but during
each second it will be shifted one place along* the
line. The distant observer wUl therefore see a
depression steadily advancing along the line. The
state of things presented to his eye after twenty,
sixty and ninety seconds, respectively, is shown in
Fig. 2.
•1



10 TRANSFERENCE OF RELATIVE POSITION. [L § 6.
There is here no horizontal motion on the paxt
of the men composing the line, but their vertical
motions give rise, in the way explained, to the hori-
zontal transference of the depression along the line.
The reader should observe that for no two con-
secutive seconds does the kneeling row consist of
exactly the same men, while in such positions as
those shown in the figures, which are separated by
more than ten seconds of time, the men who form it
are totally different.
6. Let us now return to the sea-waves and
examine more closely the elements of which they
consist.
Fig. 3 represents a vertical section of one com-
plete wave.
I §6,] THREE ELEMENTS OF A WAVE. 11

Th( t in which the horizontal

plane i of the sea when at rest

cuts tl ire ; it is called the level-

line. ' n the two extreme points

of the ng this line, is called the

length the highest point of the

crest I point of the trough AED.
