568 



NATURE 



[Oct. ii, 1888 



immediate vicinity of the volcano, rapidly falling off in 

 size at a comparatively short distance from the Sunda 

 Straits. 



The longer waves, with the original period of two hours, 

 are traced by automatic and eye observations to have 

 proceeded mainly in a westerly direction from Krakatab, 

 being noticeable at Ceylon, all over the western part of 

 the Indian Ocean, the south coasts of Africa and South 

 America, the west coast of Australia, and possibly — though 

 the evidence is not free from doubt — as far as the west coast 

 of France and the entrance to the English Channel. In 

 other directions, such as the China Sea, the Pacific, and 

 the Gulf of Mexico, they do not seem to have been felt, 

 the supposed indications not being compatible in any way 

 with the times and distances. 



As a general result, it may be said that the mean depths 

 deduced by the formula V = v 'gh, from the best data for 

 the speed of the waves, corresponded fairly with that 

 given by the soundings, but in nearly every case the formula 

 gave a smaller depth than the soundings. This and other 

 circumstances lead us to conclude, not so much that the 

 formula is incorrect, but that, with so few, and in some cases 

 such badly placed, automatic gauges, and from such com- 

 plex oscillations as seem to have occurred in many of those 

 discussed in this section, it is scarcely possible to arrive 

 at anything but a very rough approximation to the mean 

 depths. The shelving of the bottom near land, which in 

 many cases is not well determined, and the possible exist- 

 ence of ridges in mid-ocean, constitute obstacles to a 

 determination of mean depth, which is all the passage of 

 such waves can indicate. In so far, however, as they yield 

 an approximate check of this kind on soundings, their 

 observation ought to be encouraged by the establishment 

 of more automatic gauges in suitable spots. 



One very peculiar feature of the Krakatab long waves 

 is that, while their original period when leaving Krakatab 

 was two hours, they became subdivided (possibly by an 

 interpolated series caused by reflection from the coast of 

 Java) into waves of half this period ; and, by the time they 

 reached the North Atlantic, into waves of about one-quarter 

 of this period. Their consecutive oscillations could thus 

 only be identified with those of the original oscillations 

 by doubling or quadrupling the observed periods. 



Although at great distances from Krakatab the height 

 of the largest long wave was, as might be expected, only a 

 few inches ; at such comparatively remote places through 

 the more open route to the west as Ceylon and Mauritius, 

 the higher and shorter waves made their presence felt 

 to heights of several feet, and created considerable 

 astonishment as well as damage in these localities. 



Like the air and sound waves, the occurrence of 

 seismic waves on such a scale and over such a wide area 

 appears to have been quite unprecedented ; and their dis- 

 cussion, like that of the former, will in the present case 

 probably yield results of considerable value to hydrography 

 as well as other branches of science. 



{To be continued.) 



FOUND A TIONS OF CORAL REEFS. 



THE following extract from a letter from Captain 

 Aldrich, R.N., H.M. surveying-ship Egeria, now 

 employed in the Pacific Ocean, is interesting from several 

 points of view. 



" . . . . The following morning at daylight (July 10) we 

 picked up 268 fathoms (volcanic rock) some considerable 

 distance southward of the Pelorus Reef. This, again, will 

 involve a further search. Twelves miles to the northward 

 the depth was 444, and two subsequent soundings at five- 

 mile intervals gave 713 (ooze) and 888 (ooze). From here 

 the soundings continued to grow shoaler, until in lat. 

 22° 51' S., long. 176 26' W., we sounded in 335 fathoms 

 (cinder), being close to the assigned position of the 



Pelorus Reef. The water deepened again to 719 (cinder), 

 when we hove to for the night. On July 11 we continued 

 about this position, the shoalest sounding being 246. On 

 the 12th we continued the search, and by following up at 

 quarter-mile intervals struck 95 fathoms late in the after- 

 noon. Prepared a beacon, and the following day (July 

 13), after excellent star observations, sounded and shoaled 

 as yesterday, and when the men were standing by to slip 

 the beacon, discoloured water was reported from the 

 mast-head ; it was almost immediately seen from the 

 deck, and by 9 a.m. the beacon was dropped in 24 fathoms, 

 with a stretch of light-greenish water extending in a 

 northerly and southerly direction for about half a mile. 

 The whalers were lowered, and remained all day in this 

 green water. 



" Meantime more discoloured water was reported from 

 aloft, and I sent Mr. Kiddle up with his glasses, and he 

 verified the report ; so, leaving the boats on the Pelorus, 

 I went with the ship, and, after going two miles, I made 

 out the small streak from the poop. It had remained 

 as steady as possible, and had every appearance of being 

 a very small shoal. The ship was taken to within 100 

 yards of it, and the dingy lowered to get a sounding on 

 it ; no bottom, however, could be got, so the ship was put 

 in the middle of it and a sounding of 150 (no bottom) 

 obtained. A bucket of this water was drawn and a 

 bottle of it preserved, but I do not see anything in 

 it to account for the light greenish colour, and it may 

 be that the colouring matter may not lie actually on the 

 surface ; the fact remains, that this small patch was 

 sighted at very nearly three miles distance from aloft, 

 and that even when within 100 yards of it I believed it to 

 be shoal- water, and that a sounding of 150 (no bottom) 

 was actually obtained in the middle of it. On our return 

 to the Pelorus, I was not, therefore, much astonished when 

 I found that no very shoal water had been got by the boats. 

 The ship was anchored in 14 fathoms, not far from the 

 beacon, and the wire machines put into the whalers, and 

 a search on bearings from the standard compass and 

 mast-head angles carried on during the afternoon and on 

 the next day, July 14. Nothing less than 14, however, 

 was got, and I am under the impression that nothing less 

 is to be met with, as the bottoms are loose ashes and 

 cinder ; so that, as in the case of the Graham Shoal, 

 there may have been a shoal quite recently which does \ 

 not exist now. I think that had there been anything 

 dangerous about it we should have seen it, as anchoring 

 in 14 fathoms mid- ocean caused many inquiring eyes tc 

 be cast around 



" Another curious thing about the greenish water is 

 that I went over it all in the ship ; and the line between 

 it and the dark water was most distinct. Moreover, th(j 

 shoalest sounding of 14 fathoms was not found in thci 

 light water, but in the dark water alongside it. Then 1 

 was no sign of coral among the bottoms brought up. . . 

 My attention was pretty well occupied at this time, and if. 

 did not occur to me to do more than have a bucket oj. 

 the water drawn from the green colour to preserve, whicl 

 has been done. Afterwards, I much regretted that I did noj 

 get specimens from different depths, as certainly this is ; 

 most curious instance of, in one case, picking up a shoa 

 from the existence of some colouring matter, not coral 

 and, in the other, of being almost positive that a shoa 

 existed where an actual sounding proved it not to do sc 

 I can quite excuse a man reporting a shoal under suet; 

 circumstances, and it may be that a good many of th 

 reported dangers have come on the charts in this way. . . 



The position of the Pelorus Reef referred to is in la 

 23° S., long. 176 25' W., about forty miles south c 

 Pylstaart Island, which is volcanic. The reef was origir 

 ally reported in 1861 by H.M.S. Pelorus, Commodor 

 Seymour (now Lord Alcester), the ship passing within on(. 

 third of a mile of it, when breakers were distinctly seen. 



It I 



