may be conceived from what has already been stated concerning the amount of heat conveyed by that stream. The heat thus withdrawn from the North Atlantic would go to raise the temperature of the Southern Ocean and Antarctic regions. A similar result would take place in the Pacific Ocean. Were the equatorial current of that ocean removed greatly to the south of its present position, it would not then impinge and be deflected upon the Asiatic coast, but upon the continent of Australia; and the greater portion of its waters would then pass southward into the Southern Ocean, while that portion passing round the north of Australia (owing to the great strength of the N.E. trades) would rather flow into the Indian Ocean than turn round, as now, along the east coast of Asia by the Japan Islands. The stoppage of the Japan current, combined with the displacement of the equatorial current to the south of the equator, would greatly lower the temperature of the whole of the North Pacific and adjoining continents, and raise to a corresponding degree the temperature of the South Pacific and Southern Ocean. Again, the waters of the equatorial current of the Indian Ocean (owing to the opposing N.E. trades), would not, as at present, find their way round the Cape of Good Hope into the North Atlantic, but would be deflected southwards into the Antarctic sea.

We have in the present state of things a striking example of the extent to which the medial line between the two trades may be shifted, and the position of the great equatorial currents of the ocean may be affected by a slight difference in the relative strength of the two aërial currents. The S.E. trades are at present a little stronger than the N.E.; and the consequence is that they blow across the equator into the northern hemisphere to a distance sometimes of 10 or 15 degrees, so that the mean position of the medial line lies at least 6 or 7 degrees north of the equator.

And it is doubtless owing to the superior strength of the S.E. trades that so much warm water crosses the equator from the South to the North Atlantic, and that the main portion of the equatorial current flows into the Caribbean Sea rather than along the Brazilian coast. Were the two trades of equal strength, the transference of heat into the North Atlantic from the southern hemisphere by means of the Southern Atlantic and Equatorial currents would be much less than at present. The same would also hold true in regard to the Pacific.

Ocean-currents in relation to the Distribution of Plants and Animals. In the fifth and last editions of the 'Origin of Species,' Mr. Darwin has done me the honour to express his belief that the foregoing view regarding alternate cold and warm periods.

in north and south during the glacial epoch explains a great many facts in connexion with the distribution of plants and animals which have always been regarded as exceedingly puzzling.

There are certain species of plants which occur alike in the temperate regions of the southern and northern hemispheres. At the equator these same temperate forms are found on elevated mountains, but not on the lowlands. How, then, did these temperate forms manage to cross the equator from the northern temperate regions to the southern, and vice versá? Mr. Darwin's solution of the problem is (in his own words) as follows:

"As the cold became more and more intense, we know that Arctic forms invaded the temperate regions; and from the facts just given, there can hardly be a doubt that some of the more vigorous, dominant, and widest-spreading temperate forms invaded the equatorial lowlands. The inhabitants of these hot lowlands would at the same time have migrated to the tropical and subtropical regions of the south; for the southern hemisphere was at this period warmer. On the decline of the Glacial period, as both hemispheres gradually recovered their former temperatures, the northern temperate forms living on the lowlands under the equator would have been driven to their former homes or have been destroyed, being replaced by the equatorial forms returning from the south. Some, however, of the northern temperate forms would almost certainly have ascended any adjoining high land, where, if sufficiently lofty, they would have long survived like the Arctic forms on the mountains of Europe."

"In the regular course of events the southern hemisphere would in its turn be subjected to a severe glacial period, with the northern hemisphere rendered warmer; and then the southern temperate forms would invade the equatorial lowlands. The northern forms which had before been left on the mountains would now descend and mingle with the southern forms. These latter, when the warmth returned, would return to their former homes, leaving some few species on the mountains, and carrying southward with them some of the northern temperate forms which had descended from their mountain fastnesses. we should have some few species identically the same in the northern and southern temperate zones and on the mountains of the intermediate tropical regions." (P. 339, sixth edition.)

Thus

Additional light is cast on this subject by the results already stated in regard to the enormous extent to which the temperature of the equator is affected by ocean-currents. Were there no transference of heat from equatorial to temperate and polar regions, the temperature of the equator, as has been remarked, would probably be about 55 degrees warmer than at present. In such a case no plant existing on the face of the globe could live

at the equator unless on some elevated mountain-region. On the other hand, were the quantity of warm water which is being transferred from the equator to be very much increased, the temperature of intertropical latitudes might be so lowered as easily to admit of temperate species of plants growing at the equator. A lowering of the temperature at the equator some 20 or 30 degrees is all that would be required; and only a moderate increase in the volume of the currents proceeding from the equator, taken in connexion with the effects flowing from the following considerations, might suffice to produce that result. During the Glacial epoch, when the one hemisphere was under ice and the other enjoying a warm and equable climate, the medial line between the trades may have been shifted to almost the tropical line of the warm hemisphere. Under such a condition of things the warmest part would probably be somewhere about the tropic of the warm hemisphere, and not, as now, at the equator; for since all, or nearly all, the surface-water of the equator would then be impelled over to the warm hemisphere, the tropical regions of that hemisphere would be receiving nearly double their present amount of warm water.

Again, as the equatorial current at this time would be shifted towards the tropic of the warm hemisphere, the surface-water would not, as at present, be flowing in equatorial regions parallel to the equator, but obliquely across it from the cold to the warm hemisphere. This of itself would tend greatly to lower the temperature of the equator.

It follows, therefore, as a necessary consequence, that during the glacial epoch, when the one hemisphere was under snow and ice and the other enjoying a warm and equable climate, the temperature of the equator would be lower than at present. But when the glaciated hemisphere (which we may assume to be the northern) began to grow warmer and the climate of the southern or warm hemisphere to get colder, the medial line of the trades and the equatorial currents of the ocean also would begin to move back from the southern tropic towards the equator. This would cause the temperature of the equator to rise and to continue rising until the equatorial currents reached their normal position. When the snow began to accumulate on the southern hemisphere and to disappear on the northern, the medial line of the trades and the equatorial currents of the ocean would then begin to move towards the northern tropic as they had formerly towards the southern. The temperature of the equator would then again begin to sink, and continue to do so until the glaciation of the southern hemisphere reached its maximum. This oscillation of the line of maximum temperature to and fro across

the equator would continue so long as the alternate glaciation of the two hemispheres continued.

This lowering of the temperature of the equator during the severest part of the glacial epoch will help to explain the former existence of glaciers in intertropical regions at no very great elevation above the sea-level, evidence of which appears recently to have been found by Mr. Belt and others.

XXIV. On the Number and Weight of the Molecules of Æther contained in Electric Conductors. By HERMANN HERWIG*.

IN

N the following I will briefly indicate a way in which, certain hypotheses presupposed, very remarkable explanations of the relations of the æther might possibly be reached.

I start from the comparison of the expressions, on the one hand, for the thermal effect of a galvanic current, and, on the other hand, for the vis viva represented therein at any moment by the motion of the electric particles. I may mention beforehand that it is quite immaterial of what kind we suppose the motion of the electrical masses to be. Even a not uniform, somehow oscillating motion would only introduce into the calculation additional simple factors which would be quite insignificant for the final result. Such motions will therefore be left out of consideration.

This presupposed, and holding fast the notion of only one fluid, let e be, in electrostatic measure, the quantity of electricity in motion in 1 millim. length of the conductor, and v its velocity, the unit of time being the second. Then, in mechanical measure, the current-intensity is ev. If, further, also expressed in mechanical measure, R is the resistance of 1 millim. of the conductor, and L millim. the length of the latter, in it there is produced in 1 second, according to Joule's law, a quantity of heat which has in mechanical measure the expression

The motion of the electric masses in this current represents a permanently constant vis viva of the magnitude

Le v2

(II.)

where the masses are reckoned in the usual weights; therefore

n

signifies the quantity of electricity in milligrammes in the unit

of length.

Translated from a separate impression, communicated by the Author, from Poggendorff's Annalen, vol. cl. pp. 381–385.

Comparing expressions (I.) and (II.), it appears that they differ only by the factor 2enR. And if now we adopt the assumption, certainly favoured by many physicists, that e in the same portion of the conductor is for all currents constantly the same, and therefore the action of different currents in such portion differs only by the various values of v, it will be seen that, in the same conductor, expression (I.) is for all currents the same multiple of expression (II.).

There is, then, only necessary the knowledge of a number which (as one may perhaps dare to say) specifies how often in the unit of time the vis viva of the electric motion (constantly transformed into heat) is reproduced by the electromotive forces, in order from one value of the vis viva to deduce at once the total thermal effect of the current. Perhaps the following consideration may elucidate this number.

Let us imagine from any place in the conductor the resistance and consequently its thermal effect to cease; from that moment the space of the conductor must be much more filled only with electric fluid, and, indeed, at first of the same density as in the conductor. The electric motion would then be propagated in this fluid without enfeeblement. If we call the constant velocity of propagation of the motion in this case, expressed in millimetres, C, in 1 second a vis viva certainly not less than the quantity Ce v2 would arise*.

n 2

As starting-point of this entire motion a layer of the conductor 1 of the length is sufficient, if M denotes the number of moM lecules of the electric fluid contained in 1 cubic millim. of the conductor, understanding by "molecule" just so much fluid that a translatory motion of it of the sort supposed can be originated.

Ce v2

n 2

may

And now, according to the preceding, the value very well be supposed equal to the thermal effect actually accomplished in the same time, with the action of the resistance, by

where the factor 9811 is put because in (I.) the acceleration of

* It has been already remarked that no account need be taken of further factors in this expression, which at all events would be requisite if oscillations were thought of.