July i6, 1908] 



NA TV RE 



249 



Fig. 2. At surface, where the veins have been 

 affected by atmospheric agencies, there is a high pro- 

 portion of metallic silver. Much of the ore shipped 

 in 1904 was largely composed of native silver. The 

 discover!,' at Cobalt is an indication of Canada's bright 

 prospects of becoming a more important mineral- 

 producing countrv. In North America the territory 

 controlled bv Great Britain exceeds that of the United 

 States. But of this immense area of 3,600,000 square 



Fig. 2. — -Metallic <.iut'_ri>p or bilvt^r i 're, Lnhait, 

 Minerals of the British Empire." 



miles only a very narrow fringe has even been ex- 

 plored, and yet the rocks of which a very large pro- 

 portion of the unexplored area is in all probability 

 composed are those which in the United States carry 

 the most valuable mineral deposits. Here, and in 

 other parts of the Empire, the world awaits the estab- 

 lishment of new mining regions to compensate for the 

 steady impoverishment of the old. 



THE SECOND ARy OSCILLATIONS OF 

 OCEANIC TIDES.' 



TID.VL observers have long known that at certain 

 stations, mostly situated on bays or indentations 

 of the coast, the simple curve of the tide-gauge is 

 complicated by oscillations of level, often of consider- 

 able range and regularity of period. At first sight 

 nothing could be further removed from the study of 

 earthquakes than these irregularities of the tidal 

 oscillation, but a connection has been discovered by 

 the Japanese Earthquake Investigation Committee, 



1 "An Investigation on the Secondary Undulations of Oceanic Tides, 

 carried out by the Order of the Earthqualie Investigation Committee during 

 1903-6." By Drs. K. Honda, T. Terada, V. Yoshida and D. Isitani- 

 Preface by Prof. H. Nagaoka._ Pp. viii-i-ii3; 95 plates, 2 frontispieces. 

 Published as No. 26 of Publications of the Earthquake Investigation Com- 

 mittee in Foreign Languages, and as vol.x-viv. of the Journal of the College 

 of Science, Imperial University of Tokyo. (Tokyo, 190S.) 



whose diverse activity leads it to the study, not 

 only of everything directly or indirectly connected with 

 earthquakes, but also of everything which resembles 

 their effect. The discovery, by Prof. Omori, that the 

 periods of the earthquake-produced sea-waves were 

 not uniform at neighbouring stations, but in each case 

 agreed with those of the secondary oscillations of 

 the lidal curve, naturally led to an investigation of 

 this phenomenon, which has been noticed and made 

 the subject of speculation by various observers. The 

 investigation was carried out under the direction of 

 Prof. H. Nagaoka, and has been published in a bulky 

 and profusely illustrated quarto volume. 



.•\fter a description of the improved and portable 

 form of tide-gauge which was invented for the in- 

 vestigation, the ordinary limnograph being unsuitable 

 and the ordinary tide-gauge too cumbrous, we have 

 a detailed account, illustrated by reproductions of the 

 tide curves and charts, of the records from fifty-one 

 stations on the Japanese coasts. A general summary 

 of the results is given, from which it appears that on 

 the open Pacific coast, or in a bay of considerable 

 area communicating with the sea by a narrow outlet, 

 the tide curve is of a simple character, the secondary 

 Oscillations being small and irregular; but in more 

 open bays, the breadth of which is not too large in com- 

 parison with their length, secondary oscillations are 

 conspicuous, and often show great regularity of period. 

 .Simultaneous observations at different places along 

 the shore-line showed that the phase of oscillation 

 was usually the same throughout the bay, and that 

 the oscillations which were conspicuous within the 

 l.>ay could be delected, with the same phase, but much 

 ii'duced amplitude, at its mouth. From this it ap- 

 IH-ared.that the wholemass of water in the bay was 

 in simultaneous oscillation in a stationary wave, 

 .•inalogous to the sound-wave in an open organ pipe, 

 and that the bay selected from the multitudinous ocean 

 waves of various period the particular one to which 

 it was able to respond, and, like a resonator with 

 sound-waves, magnified and made it conspicuous. 



This conclusion was verified by experimental in- 

 vestigation. Models, to scale, of the bays were made 

 and sunk to the appropriate level in a large tank of 

 water; in this tank was immersed a leaden ball which, 

 being attached to a simple or a horizontal pendulum, 

 could be set in oscillation with any desired period, 

 and by its movement communicate to the water in the 

 tank a periodic oscillation, unaccompanied by anv 

 appreciable surface-wave; reflection from the walls of 

 the tank being checked by a thick layer of damping 

 material — wood-shavings, to wit. By e.xciting waves 

 with this arrangement the water in the model of the 

 bay was put into standing oscillation, the amplitude of 

 which was generally small ; as the period of the pen- 

 dulum approached the proper period of the bay, the 

 amplitude of oscillation gradually increased, and, 

 when the period exactly coincided with that of the 

 model, the amplitude reached its maximum, con- 

 tinuing, with a regular period, after the pendulum was 

 stopped. The phase of the water particles was the 

 same for all parts of the bay when the oscillation was 

 a fundamental one, but a binodal or trinodal oscil- 

 lation was easily produced in an elongated bay. 



An ingenious development of the experiment con- 

 sisted in sprinkling aluminium powder on the surface 

 of the water and photographing the model with 

 a camera suspended vertically above it ; by giving an 

 appropriate length of exposure, the movement of the 

 particles reproduced the course of the stream-lines on 

 the resulting photograph. In Fig. i we repro- 

 duce one of these photographs, showing the motion 

 where the induced wave is the fundamental oscilla- 

 tion, in which the mouth of the bay is a node for 



NO. 2020, VOL. 78] 



