June 17, 1875] 



NATURE 



123 



water in motion. This Mr. CroU shows to depend entirely 

 and only on the amount of slope of the water-surface 

 from the equator to the pole, and not at all upon the 

 amount of fall from the surface in the polar regions, to 

 the lowest depth at which the water of maximum density 

 s found beneath the equator. Dr. Carpenter says that 

 this " would seem irreconcilable with the simplest prin- 

 ciples of physics," a statement easier to make than to 

 prove. For, as Mr. Croll shows plainly, the work done by 

 gravity in the descent, is done against gravity in the ascent 

 at the equator, and the two counteract each ether, except 

 only the extra amount of gravity which is called into action 

 by the shrinkage of the polar column from what would 

 have been its size under the average amount of solar heat, 

 and which alone can have any continuous effect. The solar 

 heat is a constantly supplied moving force which is used 

 indirectly in the ocean circulation, and any further amount 

 of gravity made use of in the circuit would involve the 

 idea of perpetual motion. Connected with this is Dr. 

 Carpenter's assertion that there is no difference of level 

 between the equatorial and polar seas. Since, how- 

 ever, this is the only proximate cause for the ocean circu- 

 lation, its denial would seem to cut the ground from 

 beneath his feet. It will be found, however, that though 

 he denies it in one place he asserts it in another, and his 

 theory essentially depends on it. It is true that water 

 tcfids to find its level when disturbed, as it is by the 

 action of polar cold, which tends to alter its level; but it is 

 just this tendency that causes the circulation. If one of the 

 forces were to be powerful enough to have its own way 

 entirely, no motion could occur ; i.e. if the water were too 

 viscous, a greater permanent change of level would arise ; 

 if it "wtre perfectly fluid, the equilibrium would be brought 

 about instantaneously and no visible motion would be 

 perceived. We must be content, then, with the fall of 

 level from equator to pole to produce the circulation ; is 

 it sufficient ? This depends entirely on the viscosity of 

 water. Mr. Croll bases his argument on the experiments 

 of Dubuat, who showed that water would not descend a 

 slope of I in 1,000,000, which is much greater than the 

 slope under discussion, and hence the fall of level is 

 too small to cause any circulation. He replies to Dr. 

 Carpenter's objection, that these experiments had refer- 

 ence to water running over solids and not over itself, by 

 saying that one layer of molecules alone would be in con- 

 tact with the solid and the rest with the water surface 

 only. The reply is plainly beside the mark, as Mr. Croll 

 should have seen by reading Dr, Carpenter's statement 

 following his objection, that the difference between a fluid 

 restoring its own equilibrium, and having a sensible 

 motion over solid surfaces, was well known in practice to 

 Mr. Hawksley and other hydraulic engineers. But in 

 reality no chamber experiments can determine such a 

 point satisfactorily ; and besides this, it seems to us that 

 an important point has been overlooked by Mr. Croll. 

 No doubt it would be hard for a single pound of water to 

 perform its whole circuit against all opposing frictions 

 under the impulse of the force due to so small an amount 

 of slope ; but if large masses of water move together, the 

 moving force would be proportional to the mass, but the 

 friction to be overcome would be simply that of the peri- 

 meter of the tube of flow, and it is an essential part of the 

 theory of ocean circulation that the moving water is of 



immense mass. This friction would not increase, like 

 statical friction, with the mass, since the pressure would 

 be the same at the same depths, and it is also more of 

 the nature of shearing force than friction, and therefore 

 nearly a constant quantity. 



It does not appear, then, that anything that has been 

 said by Mr. Croll disproves this theory of a general oceanic 

 circulation, though he may have successfully attacked it 

 in certain respects. Nevertheless we agree with him that 

 " if a vertical motion follows as a necessary consequence 

 from a transference of water from the equator to the poles 

 by gravity, it follows equally as a necessary consequence 

 from the same transference by the winds ; so that one is 

 not at liberty to advocate a vertical circulation in the one 

 case and to deny it in the other." 



This was the opinion also of Herschel in his letter to 

 Dr. Carpenter, that "henceforward the question of ocean 

 currents will have to be considered under a twofold 

 point of view." It would take too long to discuss the 

 other points [in which Mr. Croll enters into controversy 

 with respect to various currents, such as the Gibraltar or 

 the Baltic, and we must reserve for another notice the 

 interesting points connected with past time with which 

 the latter part of the book is occupied. J. F. B. 



{To be continued.) 



HILDEBRANDSSON ON UPPER ATMO- 

 SPHERIC CURRENTS 

 Essui sur les Courants sup'trieurs de VAtmosphhre, dans 

 leur Relation aux Lignes Isobarometriqties. Par H- 

 Hildebrand Hildebrandsson. (Upsal, 1875.) 



CLEMENT LEY, in "The Laws of the Winds pre- 

 vailing in Western Europe," expresses his opinion, 

 based on observations made near Hereford on the move- 

 ments of the cirrus cloud, that in general the upper 

 currents of the atmosphere flow away from the regions of 

 low atmospheric pressure, and converge upon regions ot 

 high pressure. This being a point of the utmost import- 

 ance with reference to the general circulation of the 

 atmosphere, M. Hildebrandsson, in December 1873, 

 organised a systematic observation of the cirrus cloud in 

 Sweden. Twenty of the Swedish meteorological observers 

 engaged in the work of observation, the network of sta- 

 tions extending over nearly 11° of latitude, from Tomarp 

 in the south to Qvickjock in the north. The above essay 

 discusses these observations with great ability. 



The question of the relation of the motions of the 

 cirrus cloud to areas of high and low pressure is cleverly 

 handled in the essay, and the method of discussion, illus- 

 trated by thirty-three charts, may be referred to as a 

 satisfactory and exhaustive treatment of the data of cirrus 

 observation, which are restricted only to one country. 

 Charts I. to VIII. represent well-selected instances of 

 storms advancing on Sweden from westward ; Charts IX. 

 to XVI. represent Sweden in the rear of storms ; and 

 Charts XVII. to XXIV. represent areas of high pressure 

 in various directions, S., E., &c., from Sweden. Com- 

 paring the direction of the upper currents with these 

 areas of high and low pressure, it is shown that quite 

 near the centre of the depression area of storms the upper 

 currents blow in directions nearly parallel to the isobars 

 and to the winds on the surface of the earth, but that in 



