Feb. 20, 1890] 



NATURE 



363 



expression, and would be identical with it if P = j/o-ooi86 

 atmospheres. In that case the internal pressure due 

 to the salt in a solution containing 20 parts of salt to 

 100 of water would be about the same as the internal 

 pressure in pure water as given by Van der Waals. If, 

 however, we attempt to apply van 't HofTs theory of the 

 pressure due to dissolved substances, we find, as in the 

 examples quoted in the " Applications " {loc. cit.), that the 

 observed values of Vk' are many times greater than those 

 given by calculation. 



The second Report, by Mr. Buchan, on " Atmospheric 

 Circulation," of which we shall give some account in a 

 future number, is rather a treatise on meteorology than a 

 simple discussion of the Challenger observations. All the 

 data, other than those derived from the expedition (which 

 have been previously published), are set forth, and a vast 

 collection of meteorological facts from all parts of the 

 world is utilized. 



It would be impossible to attempt to discuss Mr. 

 Buchan's conclusions in detail, but one may be selected 

 as an example. Twenty-six thunderstorms occurred at 

 sea during the voyage, and of these only four took place 

 between 8 a.m. and 10 p.m. Nineteen occurred when the 

 ship was near the land, and these were pretty evenly dis- 

 tributed throughout the twenty-four hours. Over land 

 thunderstorms are most frequent during the day. At sea 

 thunderstorms are nocturnal, and occur chiefly during 

 the morning minimum of pressure. 



" Over the land the maximum of thunderstorms occurs 

 during the hours of the day when temperature is the highest, 

 but over the open sea during those hours when temperature 

 is lowest. The great majority of thunderstorms over the 

 land thus occur during the part of the day when the 

 ascensional movement of the air from the heated surface 

 of the ground takes place " (p. 32). 



These facts furnish Mr. Buchan with an interesting 

 suggestion as to the cause of these differences : — 



" As regards thunderstorms over the land surfaces of the 

 globe, the disturbance of atmospheric equilibrium, result- 

 ing in ascending and descending currents, is brought 

 about mainly by the superheating of the surface and 

 thence of the lowermost strata of the air. But as regards 

 the open sea, this mode of disturbing the atmospheric 

 equilibrium cannot take place, inasmuch as the influence 

 of solar radiation is only to raise the temperature of the 

 surface of the sea not more than a degree. Hence it is 

 probable that the disturbance of the equilibrium of the 

 atmosphere, in the case of thunderstorms over the open 

 sea, is brought about by the cooling of the higher strata 

 of the atmosphere by terrestrial radiation " (p. 34). 



There can be little doubt that Mr. Murray is right in 

 thinking that Mr. Buchan's Report will be a standard work 

 of refere. ce for many years to come. 



The third Report, by Commander Creak, is on the Mag- 

 netical Results of the voyage. As the author has himself 

 described the main results of his investigations in thepages 

 of Nature, it is unnecessary t j do more than refer to its 

 most salient features. We have two, and only two 

 criticisms to make. Commander Creak has employed 

 the British unit of force, and his paper will therefore be 

 used with less comfort and ease by most magneticians 

 than if he had employed the C.G.S. system. Perhaps, 

 however, as an Admiralty official he felt bound to adhere 

 to the traditions of his office. Again, we think that he 

 has been rather too modest in the amount of space he 



has claimed. Like Mr. Buchan, he has used information 

 from many sources which are not, or at all events are not 

 stated to be, generally accessible. These he has employed 

 in determining the rates of secular change during the last 

 40 years all over the globe. It would have been interest- 

 ing if means could have been devised for showing not 

 merely the results of this investigation but the data on 

 which they are based. Again, the map in which the 

 direction of motion — eastward or westward — of the north 

 pole of the needle is graphically shown for the period 

 considered would have been more valuable if the mag- 

 nitudes of the mean annual motion at different places had 

 been added. This has, in fact, been done in a recent 

 German work on the same subject. 



But if we are inclined to wish that Commander Creak 

 had claimed a larger share of space and given more 

 details, in what he has done he has gone beyond any 

 previous writer. His work is of the highest importance 

 as introducing a novel view of the causes of secular mag- 

 netic change, and in connecting it with certain definite 

 localities. 



Mr. Buchan has furnished us with new meteorological 

 maps. Commander Creak has prepared new magnetic 

 maps, which enable us to institute a comparison between 

 the magnetic state of the globe in 1880 and its condition 

 when Sabine portrayed it for an epoch some 40 years 

 earlier. The positions of the magnetic poles and foci 

 of maximum intensity do not appear to have altered. 

 The secular change is associated, not with these, but with 

 four points, towards two of which the north pole of the 

 needle is veering, and from two of which it is apparently 

 being repelled. The points of increasing attraction on 

 the north-seeking . pole are to the south of Cape Horn 

 and in the south of China ; the foci of diminishing at- 

 traction are in the Gulf of Guinea and near the north 

 magnetic pole in Canada. The existence of this last 

 focus is more or less hypothetical, but in the case of the 

 other three the various magnetic elements concur in in- 

 dicating the same neighbourhood as the centre of change. 

 Thus not only is the secular variation of the declination 

 of opposite signs to the east and west of these points, 

 but the increase of the downward attraction on the north 

 pole of the needle is a maximum near Cape Horn and 

 in China, and a minimum {i.e. a maximum decrease) in 

 the Bight of Benin. 



Again the annual change of horizontal force is very 

 small near Cape Horn, but it is decreasing in South 

 America, and the rate of decrease is a maximum at a 

 point between Valparaiso and Monte Video. These are 

 precisely the kind of results which would follow from the 

 gradual production of a subsidiary centre of relative 

 attraction on the north-seeking pole of the magnet near 

 Cape Horn. The real existence of the Gulf of Guinea 

 centre is similarly confirmed. Commander Creak 

 cautiously abstains from theorizing on these remarkable 

 facts, but there can be no doubt that he is right in thinking 

 that they must lead us to look for the chief causes of 

 secular variation within the globe rather than in solar or 

 extra-terrestrial influences. His paper will be a point of 

 new departure in the science of terrestrial magnetism. 



It will be seen from what has been said that the three 

 Reports which have been discussed are written with a 

 wider scope than the mere discussion of the observations 



