NA TURE 



[May 21, 1874 



these makers deserve great credit for the help they have 

 thus given to the success of the expeditions. 



In all observations of the moon for determining the 

 longitude there are of course numerous corrections which 

 must be applied. Among these none is more important 

 than the correction for the paralla.x of the moon. 



Recapitulation. — In the case of every nation de- 

 pending upon De I'lsle's method and in the case of every 

 expedition when only one contact is observed, the longitude 

 must be determined with very great accuracy. This can 

 be done by any of the following methods : — 



1. By rockets, or flashing signals. 



2. By a trigonometrical survey. 



3. By the aid of chronometers, in which it would be un- 

 wise to neglect the method lately introduced of adding to 

 the chronometers one which is uncompensated. 



4. The telegraphic method, in which it is not desirable 

 to use relays, since verj' long lines with a Thomson's re- 

 flecting galvanometer will give good results, while the 

 employment of relays is objectionable. 



5. By observations of the moon's position which may 

 be made by either of the three following methods : — 



(a) By occultations of the moon. 

 (;3) By transit observations of the moon and moon- 

 culminating stars, 

 (y) By aid of an alt-azimuth. 



George Forbes 



{To be continued.) 



OCEAN CURRENTS 



T OBSERVE that in Nature, vol. ix. p. 423, Dr. 

 ■1 Carpenter re-states and maintains his opinion that 

 polar cold rather than equatorial heat is the prinuiin 

 mobile of his general oceanic circulation. In my papers 

 in the Philosophical Magazine for Oct. 1871 and Feb. 

 1874 I have proved, I trust, to the satisfaction of any 

 physicist who will be at the trouble to examine what I 

 have written on the subject, that this notion is based upon 

 a confusion of ideas in regard to the way in which differ- 

 ence of specific gravity produces motion. It is not my 

 object at present to enter into any further discussion of 

 this elementary matter ; but I wish briefly to refer to a 

 new and somewhat plausible-looking objection advanced 

 in Dr. Carpenter's article against the views I advocate in 

 reference to under-currents' The following is the para- 

 graph to which I refer : — 



" According to Mr. Croll's doctrine the whole of that 

 vast mass of water in the North Atlantic, averaging, say, 

 1,500 fathoms in thickness and 3,600 miles in breadth, the 

 temperature of which (from 40" downwards), as ascer- 

 tained by the Challenger soundings, clearly shows it to 

 be mainly derived from a polar source, is nothing else 

 than the reflux of the Gulf St/rain. Now, even if we 

 suppose that the whole of this stream, as it passes Sandy 

 Hook, were to go on into the closed Arctic basin, it would 

 only force out an equivalent body of water. And as, on 

 comparing the sectional areas of the two, I find that of 

 the Gulf Stream to be about 1-900 that of the North 

 Atlantic underflow ; and as it is admitted that a large 

 part of the Gulf Stream returns into the Mid-Atlantic cir- 

 culation, only a branch of it going on to the north-east ; 

 the extreme improbability (may 1 not say impossibility ?) 

 that so vast a mass of water can be put in motion by what 

 is by comparison a mere rivulet, the north-east motion of 

 which as a distinct current has not been traced eastward 

 of 30° W. long, seems still more obvious." 



The objection seems to me to be based upon a series of 

 misapprehensions: (i) that the mass of cold water 1,500 

 fathoms deep and 3,600 miles in breadth is in a state of 

 motion towards the equator ; (2) that it cannot be the 

 reflux of the Gulf Stream, because its sectional area is 

 900 times greater than that of the Gulf Stream ; (3) that 



the immense mass of water is, according to my views, set 

 in motion by the Gulf Stream. 



I shall consider these in their order : (i) That this im- 

 mense mass of cold water came originally from the polar 

 regions I of course admit, but that the whole is in a .state 

 of motion I certainly do not admit. There is no warrant 

 whatever for any such assumption. According to Dr. 

 Carpenter himself the heating power of the sun does not 

 extend to any great depth below the surface ; conse- 

 quently there is nothing whatever to heat this mass but 

 the heat coming through the earth's crust. But the 

 amount of heat derived from this source is so trifling 

 that an under-current from the Arctic regions far less in 

 volume than that of the Gulf Stream would be quite suffi- 

 cient to keep the mass at an ice-cold temperature. Taking 

 the area of the North Atlantic between the equator and 

 the tropic of Cancer, including also the Carribean Sea 

 and the Gulf of Mexico, to be 7,700,000 square miles, and 

 the rate at which internal heat passes through the earth's 

 surface to be that assigned by Sir William Thomson, we 

 find that the total quantity of heat derived from the earth's 

 crust by the above area is equal to about 88 X 10" foot 

 pounds per day. But this amount is equal to only i -894th of 

 that conveyed by the Gulf Stream, on the supposition that 

 each pound of water carries 19,300 foot pounds of heat. 

 Consequently an under-current from the polar regions of 

 not more than ^ the volume of the Gulf Stream would 

 suffice to keep the entire mass of water of that area within 

 1° of what it would be were there no heat derived from 

 the crust of the earth. That is to say, were the water con- 

 veyed by the under-current at 32°, internal heat would not 

 maintain the mass of the ocean in the above area at more 

 than 33°. The entire area of the North Atlantic from the 

 equator to the Arctic circle is somewhere about 16,000,000 

 square miles. An under-current of less than ^\ that of 

 the Gulf Stream coming from the Arctic regions would 

 therefore suffice to keep the entire North Atlantic basin 

 filled with ice-cold water. In short, whatever theory we 

 adopt regarding oceanic circulation, it follows equally as 

 a necessary consequence that the entire mass of the ocean 

 below the stratum heated by the sun's rays must consist 

 of cold water. For if cold water be continually coming 

 from the polar regions either in the form of under-cur- 

 rents or in the form of a general underflow, as Dr. Car- 

 penter supposes, the entire under portion of the ocean 

 must ultimately become occupied by cold water, for there 

 is no source from which this influx of cold water can 

 derive heat save from the earth's crust. But the amount 

 thus derived is so trifling as to produce no sensible effect. 

 For example, a polar under-current one-half the size of the 

 Gulf Stream would be sufficient to keep the entire water 

 I of the globe (below the stratum heated by the sun's rays) 

 I at an ice-cold temperature. Internal heat would not be 

 j sufficient, under such circumstances, to maintain the 

 • mass 1° F. above the temperature it possessed when it 



left the polar regions. 

 I (2) But suppose that this immense mass of cold water 

 1 occupying the great depths of the ocean were, as Dr. 

 Carpenter assumes it to be, in a state of constant motion 

 towards the equator, and that its sectional area were 

 900 times that of the Gulf Stream, it would not therefore 

 follow that the quantity of water passing through this 

 large sectional area must be greater than that flowing 

 through a sectional area of the Gulf Stream, for the 

 quantity of water flowing through this large sectional area 

 depends entirely on the rate of motion. 



(3) I am wholly unable to understand how it could be 

 supposed that this underflow, according to my view; is set 

 in motion by the Gulf Stream, seeing that I have shown 

 that the return under-current is as much due to the im- 

 pulse of the wind as the Gulf Stream itself 



I am also wholly unable to comprehend how Dr. Car- 

 penter should imagine that because the bottom tempe- 

 rature of the South Atlantic should happen to be lower, 



