254 



NATURE 



\yan. 1 8, 1877 



of Uiis superiority as " mainly due to superior cvaporalion in 

 the water hemisphere generally," it did not occur to me that I 

 could be misunderstood to mean that the excessive depression in 

 high south latitudes is due to excessive evaporation taking place 

 within those latitudes, an idea which, with Mr. Murphy, I regard 

 as absurd. 



The large areas of depression which the American meteoro- 

 logists have termed the " Polar Cyclones" appear, on exami- 

 nation, to be themselves aggregates of those local depressions, or 

 cyclones, which have penetrated either into the Arctic or Antarctic 

 regions, and have there, either partially or wholly, coalesced. 

 Local depressions starting from the edges of the great areas of 

 excessive evaporation seem to be governed in their course by the 

 distribution of relative humidity, and to be determined towards 

 those d'stricts in which precipitation is most in excess of evapo- 

 ration. Consequently their forward development is, as a general 

 rule— a rule to which there are in the northern hemisphere many 

 important exceptions — somewhat towards the poles. As they 

 advance between converging longitudes they commonly expand, 

 and therefore become united, while the influence of the earth's 

 rotation deflects their circulating currents further away from the 

 points of lowest pressure. Mr. Murphy's view that the imper- 

 fection of the Arctic as compared with the Antarctic depression, 

 is due to the amount of land in the northern hemisphere, and the 

 local air-currents produced thereby, is not in opposition to my 

 argument. Undoubtedly over Greenland the anticyclonic circu- 

 lations predominate (except in the summer quarter) over the 

 cyclonic. It is not improbable that somewhat analogous irregu- 

 larities of pressure dependent on the distribution of land and 

 sea may exist in high south latitudes. But I still think that it is 

 to the middle latitudes of the two hemispheres that we must 

 look in order to find the chief cause of the contrast between the 

 Antarctic and Arctic depressions, for it is in the middle latitudes 

 that the majority of the local depressions originate. In the 

 southern hemisphere those latitudes are almost entirely occupied 

 by surfaces in which evaporation is excessive. In the northern 

 they are represented to a large extent by areas of relatively 

 slight evaporation and predominant precipitation. 



The correlation of wind and pressure-distribution is of a kind 

 which can hardly be stated at the same time briefly and cor- 

 rectly. But if it is necessary to be concise, it seems more 

 natural in every case to regard the distribution of pressure as the 

 primary cause of the wind than to say " the cause of the depres- 

 sion round the Pole is the centrifugal force of the west winds." 



Lutterworth, January 9 W. Clement Ley 



Sense of Hearing, &c., in Birds and Insects 



Mr. Romanes (Nature, vol. xv., p. 177) is not quite correct 

 in supposing that the Death's Head is the only species of Lepi- 

 doptera known to "stridulate." Possibly the phenomenon is 

 far more general than is commonly believed, although only few 

 instances of its occurrence have been observed. In the current 

 number of the Entomologists Monthly Magazine, Mr. Swinton 

 details the method in which a sound is believed to be produced by 

 Vanessa io and V. urtica, viz. , by friction of a nervure of the 

 hind-wings against a " filed " nervure in the fore-wings. Chelo- 

 nia pudtca, one of the tiger-moths, has long been known to 

 produce a sound {cf. Solier, Annates Soc. Etitomol. de France, 

 1837). In 1864, Guenee {Ann. Soc. Fr., pp. 39Moi) notices 

 that the genus Setijta possesses a tympaniform organ on each side 

 of the breast, analogous to that found in the Chelouia, and in 

 the same volume he is followed by Laboulbene, who gives an 

 elaborate anatomical description of the organ in Chelonia, with 

 figures (pi. 10). Another tiger-moth {Euprepia matronula) is 

 said to stridulate {cf. Czerny, Verhand. zool.-bot. Vereins in 

 Wien, 1859) ; and the existence of the phenomenon is (at least) 

 suspected in members of other groups of Lepidoptera. 



Without being able to prove it, I suspect that birds obtain a 

 knowledge of the whereabouts of worms and subterranean larvae 

 by sight, and not by sound. In the case of the thrush, I think 

 the excrement rejected to the surface guides the bird to the right 

 spot. The starling during breeding-time feeds almost exclu- 

 sively on the larvae of Tipula. Here, again, I think it is sight, 

 and not sound that aids the bird. True, in this case there is 

 no rejected excrement on the surface, but there is something 

 that may be equally significant to the eye of the bird, viz., the 

 withered condition of the plants of grass, &c., telling a tale of 

 the mischief that is going on below. Furthermore, is it not 

 possible that if the movements of the larvae below the surface 

 cause sufficient sound to be heard above the surface, the move- 



ments of the bird should act as a vvarniiij, and cause the larva; 

 to cease feeding ? The withered plant tells its own tale ; if only 

 flagging, but yet with some amount of vitality in it, the chances 

 are that a larva is still at work at its roots ; if entirely dead, then 

 the larva has departed for another plant. 



Confessedly in the case of the curlew and allied birds, the 

 matter becomes very difficult of explanation, owing to the depth 

 below the surface at which the food is found. But do these 

 marine and other worms always keep at the depth to which the 

 bird is obliged to penetrate in order to obtain them ? Solen is 

 believed to remain near the surface until warned that an enemy 

 is near, when it descends with rapidity. The worm might also 

 be ordinarily near the surface, and the slight movement thereon 

 caused by its endeavours to bury itself deeper might result in 

 its destruction. I throw this out as a suggestion, because it is 

 hard to believe that sound produced by the movements of an 

 annelid could be transmitted through nearly a foot of sand. 

 There is yet another difftculty. In the case of the curlew the 

 sound w^ould have to travel nearly a foot aboz'e the surface before 

 it reached the auditory organs of the bird. 



Lewisham Robert M'Lachlan 



THE " CHALLENGER " COLLECTIONS 



THE following "Preamble" to a list of observing 

 stations, printed for the use of the naturalists en- 

 gaged in preparing the account of the voyage, contains 

 so much likely to interest naturalists generally that we 

 think it useful to publish it in Nature :— 



The special object of the Challenger Exploring Expe- 

 dition was to investigate the physical and biological con- 

 ditions of the great ocean basins ; and with this object 

 in view, during an absence from England of three years 

 and a half, and at intervals as nearly uniform^ as circum- 

 stances would permit, throughout a course of 68,890 miles, 

 362 observing stations were established. 



The following list of these stations has been compiled 

 for the use of those naturalists who have consented to 

 assist in the working out of the scientific results of the 

 expedition, with a view to their being published in an 

 official account of the voyage. Interesting observations 

 were made on land as opportunity occurred during the 

 short periods of the Challenoe?^s stay in port, and driring 

 her short visits to remote islands ; but these observations 

 were necessarily desultory and incomplete, and it has 

 been decided to omit their consideration from the present 

 work, and to publish such as may appear of sufficient 

 value' in the transactions of learned societies. The Official 

 Report will thus consist strictly of an account of the ad- 

 ditions which have been made to the knowledge of the 

 physical and biological conditions of the ocean by the 

 expedition. 



At each station the following observations were made, 

 so far as circumstances would permit. The position of 

 the station having been ascertained— 



1. The exact depth was determined. 



2. A sample of the bottom averaging from i oz. to i lb. 

 in weight was recovered by means of the sounding instru- 

 ment, which was provided with a tube and disengaging 

 weights. 



3. A sample of the bottom water was procured tor 

 physical and chemical examination. 



4. The bottom temperature was determined by a regis 

 tering thermometer. 



5. At most stations a fair sample of the bottom fau 

 was procured by means of the dredge or trawl. 



6. At most stations the fauna of the surface and 

 intermediate depths was examined by the use of the to 

 net variously adjusted. 



7. At most stations a series of temperature observj 

 tion's were made at different depths from the surface ' 

 the bottom. . 



8. At many stations samples of sea- water were obtaim 

 from different depths. , . 1 



9. In all cases atmospheric and other meteorological 

 conditions were carefully observed and noted. 



i 



