NATURE 



\yune 17, 1875 



totally unreliable. We do not know the temperature 

 from which the sun raises the earth, except that it is 

 greater than that of space. The fourth argument, of 

 course, is nothing without the first three, and the frac- 

 tion \ we have seen may be much too large. We are not, 

 then, in a position to estimate accurately the thermal 

 effect of the Gulf Stream and other ocean currents ; but 

 we may consider it proved, as is indeed generally 

 acknowledged, that they have a very sensible influence? 

 and, as we shall see, bear a great part in the general cir- 

 culation of the ocean water. 



We must now examine how far Mr. Croll establishes 

 his position that a general oceanic circulation is im- 

 possible under the influence of temperature and gravita- 

 tion alone. Dr. Carpenter has already given (Proc. of 

 Roy. Geog. Soc, vol. xviii.) his reasons for his beli ef in 

 the adequacy of these influences, and his replies to Mr. 

 CrolFs objections, some of which are discussed in this 

 volume in no less than four chapters. Although it may 

 be familiar to most of our readers, it will be well to give 

 here an outline of Dr. Carpenter's " doctrine." 



The chilling of the salt water in both polar regions 

 renders it heavier and causes it to sink, its place being 

 supplied from the warmer water of lower latitu des, which 

 is itself supplied by the motion of the water from the two 

 poles towards the equator along the lower portion of the 

 ocean ; and these two masses meeting each other near 

 the equator, well up there, and bring the colder water 

 nearer the surface, while the heating of the surface water 

 in these regions keeps up the difference between the 

 specific gravities of the water supplied to and leaving the 

 polar regions, on which the whole depends. These 

 appear to be Dr. Carpenter's latest views (Proc. Roy- 

 Geog. Soc, vol. xviii., June 1874), though Mr. Croll's 

 objections seem, in some part at least, to be aimed at 

 details that do not affect the fundamental conception. 

 This is distinguished as a vertical circulation, because 

 the first origination of the motion is supposed to be in 

 the descent of the polar waters. Mr. Croll assents to the 

 facts, but ascribes the circulation to the initiation of the 

 winds, and denies that there is any circulation beyond 

 that produced by currents. We know that currents exist 

 on the surface, and it is generally agreed that they owe 

 their origin, in great part at least, to the system of prevailing 

 winds, and even on Dr. Carpenter's theory they must, so 

 far as they tend polewards, decrease by so much the 

 general circulation of the upper ocean ; but the known 

 or assumed under-currents are much more local, and the 

 depression of temperature at great depths is too general 

 to allow us to conceive that the return should be made by 

 circumscribed currents. 



In discussing the question whether the polar cold is 

 sufficient to cause circulation, Mr, Croll first objects that 

 the sea of the tropics is salter, and therefore denser, than 

 that of the poles, and that this would counteract the 

 effect of the cold. There is in reality but little force in 

 this objection as against Dr. Carpenter's theory. The 

 excess of temperature and of salinity counteract each 

 other in the surface layers of the tropics, and prevent 

 them sinking or rising j but as they have a nearly hori- 

 zontal motion, according to the theory, the objection is 

 nothing, the lower layers which alone have an upward 

 vertical motion deriving it from a vis-a-tergo ; and with 



regard to the polar area the lower layers cannot be more 

 salt than the upper, from whence they come, according 

 to the theory, and zxvy freezing on the surface must leave 

 the remaining water on the contrary salter. 



The next objection of Mr. Croll is far more formidable, 

 though it shows that some of the proofs adduced are 

 untenable, rather than the theory itself. The drifting 

 of icebergs from Newfoundland across the Gulf Stream, 

 and of the Atlantic c!ible buoy which travelled six hun- 

 dred nautical miles in seventy-six days, adduced by Dr. 

 Carpenter as proving the southward motion of the deeper 

 layers, proves too much according to Mr. Croll, as it 

 proves the existence of a sensible current, which Dr. 

 Carpenter admits cannot be formed by differences of 

 gravity. This may be true, and prove that other causes 

 operate in the motion of large masses of water ; but while 

 destroying one argument in favour of, it proves nothing 

 in opposition to, the doctrine of general oceanic circula- 

 tion. This class of objections, however, are far more 

 forcible than theoretical ones ; and the list of phenomena 

 that may be accounted for on either theory, and of those 

 that cannot well be accounted for on the gravitation 

 theory, e.g. the southward currents of Davis Straits and 

 the east coast of Greenland, shows that neither theory 

 alone will satisfy all the conditions to be fulfilled. Mr. 

 Croll, however, gives no satisfactory account of the 

 greater cold of the lower strata of the South Atlantic, 

 nor of the surging up of cold currents on eastern shores, 

 nor of the cold water coming nearest the surface under 

 the equator ; nor does his theory give that beautiful 

 account of the maintenance of life in the deep sea which 

 is so dependent on the change of the water. 



But Mr. Croll asserts that the gravitation theory is 

 physically faulty, and maintains the assertion in this 

 volume against Dr. Carpenter's last leply. In several of 

 his arguments it is impossible not to agree with him. In 

 examining them we will follow the order he takes. He 

 first shows that heat at the surface, as in the equatorial 

 regions, cannot produce circulation. But this, though 

 essential to Lieut. Maury's theoiy, has not been asserted 

 by Dr. Carpenter, who, on the contrary, states that any 

 effect due to^the heating at the equator may be practically 

 disregarded ; and why ? because the heat is here applied 

 at the top instead of at the bottom, as it should be to 

 produce convection ; but an application of cold to the top 

 would be equivalent to heat at the bottom, and this cold 

 is obtained in the polar area ; consequently Dr. Carpenter 

 regards polar cold as the primum mobile. Mr. Croll 

 objects to this that it is the difference of temperature only 

 we have to do with, and this may be said to depend on 

 either, and accuses Dr. Carpenter with confusion of ideas ; 

 but this is scarcely fair after arguing against the heat being 

 available to produce motion because apphed at the top, 

 showing that he perceived that not the difference only, 

 but where the lower temperature is found is of consequence. 

 Dr. Carpenter would say the temperature at the poles is 

 below the average, no matter how that average is obtained ; 

 which is a very different thing from saying the equatorial 

 temperature is above the average — since in the first case 

 the average might be obtained, as far as the theory is con- 

 cerned, by a nearly uniform temperature elsewhere. 



The next important question raised with respect to this 

 theory is the amount of force which is exerted to put the 



