*88 PLEASANT WA YS IN SCIENCE. 



length of the group, so that it must have come (arriving as it 

 did from the side towards which Hilo lies) from the north- 

 east It was, then, not the direct wave from Peru, but the 

 wave reflected from the shores of California, which produced 

 the most marked effects. We can understand well, this being 

 so, that the regurgitations of the sea were complex. Any 

 one who has watched the inflow of waves on a beach so 

 lying within an angle of the line that while one set of waves 

 comes straight in from the sea, another thwart set comes 

 from the shore forming the other side of the angle> will 

 understand how such waves differ from a set of ordinary 

 rollers. The crests of the two sets form a sort of network, 

 ever changing as each set rolls on ; and considering any one 

 of the four-cornered meshes of this wave-net, the observer 

 will notice that while the middle of the raised sides rises 

 little above the surrounding level, because here the crests of 

 one set cross the troughs of the other, the corners of each 

 quadrangle are higher than they would be in either set taken 

 separately, while the middle of the four-cornered space is 

 correspondingly depressed. The reason is that at the 

 corners of the wave-net crests join with crests to raise the 

 water surface, while in the middle of the net (not the middle 

 of the sides, but the middle of the space enclosed by the 

 four sides) trough joins with trough to depress the water 

 surface.* 



We must take into account the circumstance that the wave 

 which reached Hawaii in May, 1876, was probably reflected 

 trom the Calitbrnian coast, when we endeavour to determine 

 the rate at which the sea disturbance was propagated across 



* The phenomena here described are well worth observing on theii 

 own account, as affording a very instructive and at the same time very 

 beautiful illustration of wave motions. They can be well seen at many 

 of our watering-places. The same laws of wave motion can be readily 

 illustrated also by throwing two stones into a large smooth pool, at 

 points a few yards apart. The crossing of the two sets of circular waves 

 produces a wave-net, the meshes of which vary in shape according to- 

 their position. 



