April 26, 1877 



NATURE 



555 



He took the name of his hypothetical land from a 

 legend met with in the " Timseus " of Plato. In a con- 

 versation between a priest of Sa'i's and Solon, when in 

 E<?:ypt, mention is made of a great island of Atlantis, 

 situated beyond the pillars of Hercules, where lived a 

 powerful nation that ruled over Libya as far as Egypt, 

 and over Europe as far as Tyrrhenia. They tried to 

 subjugate the Hellenes, but that heroic people defeated 

 them. At a later period, during severe earthquakes 

 and great floods, the island of Atlantis sank into 

 the ocean. Such in brief outline is the legend. [The 

 lecturer, alluding to the translations of Jowett and 

 Whewell, referred to the puzzle this passage had been to 

 students to know where an Egyptian priest could have 

 known such a legend, or why (possibly) Plato had in- 

 vented it, and alluded to one explanation that it was 

 probably an exaggeration of some local phenomenon. 

 In the Journal of Botany for January, 1865, a list of 

 the literature of the subject is given.] This is as much 

 as time will allow to be said to indicate the nature of the 

 reasoning by which Unger, on the evidence of plant 

 remains in Europe and America, conjectured former in- 

 tervening land between the two, and why that hypo- 

 thetical land was called " the sunken " or " the lost 

 island of Atlantis." 



We now turn to the Challenger soundings, and with 

 these must be mentioned those of the United States ship 

 Dolphin, the German frigate Gazelle, and the British 

 ships Hydra and Porcupine. The generalisations of the 

 soundings taken by these vessels, with inferences drawn 

 from bottom temperatures, have been worked out by the 

 staff of the Challenger, and a contour map has been pre- 

 pared, of which the features which bear on our subject 

 are reproduced in the diagram. Some of the most im- 

 portant soundings were taken from the Challeti^^er htrs^M, 

 and as the working out of the whole results have been 

 performed by the staff of that ship it is not unfair, at any 

 rate in a short title for a popular lecture to mention only 

 " Challenger soundings." That there was no feeling of 

 international jealousy on the part of the Challenger staff 

 is fully evidenced by the fact that the northern portion of 

 the ridge has been named after the Dolphin, 



Before referring to the results it may be of interest, as 

 we have some of the Challcns;er apparatus here, to speak 

 of the method of deep-sea soundings. 



[The lecturer then briefly sketched the history of deep- 

 sea soundings, alluded to the impulse given by the laying 

 of cables, and mentioned how the improvement in me- 

 chanical appliances made possible now what was impos- 

 sible a few years ago.] 



The line used is about one inch in diameter ; on this 

 the twenty-five and seventy-five fathom distances are 

 marked with white thread, interwoven, the fifty by red, 

 and the 100 fathoms in blue. By this means the amount 

 of line paid out can be easily ascertained. The weights 

 to sink the line are so arranged, that when they touch 

 the bottom they release themselves. There are several 

 modifications of this apparatus, but the principle of those 

 used on board the Challenger is that round flat weights 

 with holes through them are placed one above another 

 with a rod or tube running through them, the number of 

 weights depending on the expected depth. To the 

 bottom of the lowermost weight a wire ring is fastened, 

 and a wire passes up and is fastened to a spring at 

 the top of the tube. The tube is then attached to 

 the line. So long as the strain of the weights is on this 

 spring it remains closed. Directly the weights rest on the 

 bottom the strain is removed, the spring opens, the wire 

 is released, and when the line is hauled in it brings up the 

 tube only, leaving the weights below. For taking tem- 

 peratures a cup lead is generally used to sink the line, 

 to which self-registering thermometers specially arranged 

 to withstand great pressures are attached at every 100, 

 and sometimes at every ten fathoms. It is not necessary 



here to speak of dredging, nor of the means for bringing 

 up water or samples of mud from the sea bottom. We 

 have now only to speak of soundings and taking tempe- 

 ratures. In both operations the line is passed through a 

 pulley-block, which is attached to a group of elastic 

 "accumulators," the object of this being to break the 

 shock of the roll of the ship. 



Dr. Spry, writing about the Challenger voyage, has 

 said : — 



" It has been found that in all deep soundings it is 

 necessary to use steam power. No trustworthy results 

 can be obtained from a ship under sail, as even in the 

 calmest weather the heave of the sea or the surface 

 current is sufficient to drift the ship in a very short time 

 a considerable distance from the place where the lead 

 was originally let go. . . . The first thing therefore to be 

 done is to shorten and furl all sail and bring the ship 

 head to wind, regulating the speed in such a manner 

 as to avoid forcing her through the water." 



The soundings and temperatures obtained by the Chal- 

 lenger have been from time to time issued in special 

 reports, of which there have been seven. In the seventh 

 is given a map on which the soundings have been marked, 

 together with those of the other ships already mentioned. 

 On this map the ridges and deep basins have been con- 

 toured, and where soundings have been wanting the 

 bottom temperatures have been taken as a guide of the 

 probable position of the separating ridges. For the 

 Challenger ridge there are plenty of soundings and nearly 

 as many for the Dolphin ridge. The connecting ridge is, 

 however, assumed from bottom temperatures. 



It is on this map our diagram is based. 



Having now obtained approximately the contour of this 

 ridge, which throughout the greater part of its range is 

 known as a fact from actual soundings, there are some 

 few speculations concerning it which naturally present 

 themselves for consideration. In the first place it will 

 be noticed that along the ridge itself there are four places 

 where it rises to dry land, at the Azores, at St. Paul's 

 rocks, at Ascension, and at Tristan d'Acunha. In the 

 deeper basins there is land rising above the sea-level at 

 Fernando de Noronha, at Trinidad, and at St. Helena. In 

 the deeper basins too there are five soundings which show 

 a depth of more than 3,000 fathoms. These are given on 

 our diagram. The greatest depth recorded is 3,450 fathoms. 

 A glance at the Challenger map on which the soundings 

 are marked in figures is sufficient to show that if the con- 

 tour lines were drawn at every 250 fathoms the Atlantic 

 would be found to be diversified by hills and valleys. 

 Geologists are familiarised with invoking former rises and 

 falls in land to account for some of the facts they study. 

 Indeed in some cases it seems almost as if it were believed 

 that the axis of the earth may be shifted and its ice-caps, 

 its soil-caps, and its continents moved about with im- 

 punity to suit any particular theory. At any rate it would 

 not be received as a startling idea that the whole area of • 

 what is now the Atlantic has been dry land. True we 

 know that deposits are now being formed on the floor of 

 the ocean, and at different rates, and consequently pro- 

 ducing different thicknesses ; rivers carry material which 

 is spread out according to conditions over large or 

 small areas, and so produce variations in the thickness 

 of their deposits ; and perhaps allowance must also be 

 made for currents. These circumstances may to some 

 extent modify the relative levels of parts of the 

 ocean bottom. But they could hardly account for 

 such extent of variations in the hills and valleys as 

 are met with. Some of the ridges may be the result 

 of submarine elevation analogous to that which has raised 

 high mountain ridges elsewhere, and in this case has never 

 brought the ridge above the sea except in a few peaks. It 

 must too be remarked that if we admit the ridge through 

 its whole length to have been dry land, it does not neces- 

 sarily follow it was so all at the same time. There is not. 



