ART. XXXII.-Review of Croll's Climate and Time with especial reference to the Physical Theories of Climate maintained therein;* by SIMON NEWCOMB.

THE present notice of Mr. Croll's work is confined to an examination of his physical theories of climate, avoiding all those portions which have a geological bearing. The physical theories propounded have two distinct applications; the one to the present climate of the earth; the other to the changes of that climate during past geological ages. In the latter department of the work the principal object is to account for the glacial epoch or epochs, the author conceiving that there may have been several such epochs. The data from which his conclusions. respecting the past are derived are necessarily founded on his theories of the causes of present climate, since it is only by a thorough discussion of the way in which all climatic causes operate, and by tests of all the conclusions by a comparison with the present climate of the globe, that any safe rules can be formed for judging of the climate of the past.

We are forced to say at the outset that the physical data for forming a reliable estimate of the separate effects of various causes on climate are almost entirely wanting. The physical theory of cosmical heat is, at the present time, in a state nearly approaching the chaotic, a circumstance all the more surprising when we consider the advanced state of other departments of the theory of heat. Cournot and his successors have devoted to the mechanical theory of heat an amount of profound research which has made it a branch of the most exact of the sciences. On the other side, Melloni and his successors have done a great deal for what we may call the chemical theory of heat. Between these two lie the physical theory, as affecting climate and cosmical temperature, which has, comparatively speaking, been neglected entirely. To illustrate what we mean let us consider the temperature of the earth from the widest point of view. Practically, there is but one source from which the surface of the earth receives heat, the sun, since the quantity received from all other sources is quite insignificant in comparison. There is but one way of losing heat, by radiation into space. The temperature of the surface being in a state of permanent equilibrium, the quantity of heat radiated and reflected must be equal to the total quantity received from the sun. It is this equality which determines the mean temperature of the surface of the globe.

If the earth were not surrounded by an atmosphere, if, con. sequently, the amount of heat radiated from each square foot of the land, as well as from the whole surface, were equal to that

*Climate and Time in their geological relations: a theory of secular changes of the Earth's Climate. By James Croll. New York, D. Appleton & Co., 1875.

received from the sun, the problem of climate would be a quite simple one. But the atmosphere, and especially the vapors suspended in the atmosphere, exert a powerful influence in various ways. Perhaps the most general and wide-spread source of this influence may be found in the probable unequal diathermacy of the atmosphere to solar and terrestrial heat which may result in the mean temperature being higher than it would be if there were no atmosphere. To investigate this influence the first datum necessary is the mean temperature, first of the whole earth and then of its various zones, which would be maintained if there were no atmosphere. In other words, we wish to know what would be the temperature of a small solid body revolving round the sun at the mean distance of the earth, and presenting all its sides equally to the sun in rapid succession. This temperature may be called the normal temperature of the region in which the earth is moving.

We repeat that the foundation stone of any reliable investigation of terrestrial climate, with respect to its causes, must be a knowledge of this normal temperature. Without it we may have any quantity of material for discussion but nothing on which we can base a theory worthy of the slightest confidence. There are of course many other questions to follow it, but this is the one which the investigator of this subject meets at the very threshold of his investigation, just as the surveyor who attempts to make a geodetic measurement first meets with the question of the length of his measuring rod. Now, no stronger example of the chaotic state of the theory of cosmical heat can be given than the simple fact that not only is this normal temperature entirely unknown, but, so far as we are aware, no attempt has ever been made to determine it. What adds to our surprise is that while no one has attempted to determine what temperature a body like the earth would acquire in free space exposed to the solar rays, there have been a number of attempts to answer the experimentally impossible question what temperature such a body would acquire if the solar heat were cut off, so that the body should be exposed to stellar radiation alone, a temperature known in our books as that of space.

In justice to physicists it must be said that one step toward determining this fundamental temperature was taken many years ago. Pouillet and Herschel determined the actual quantity of heat radiated by the sun, and their results have been of the greatest value in investigating the thermal relations of the solar system. The remaining part of the problem is more laborious, but not, we conceive, more difficult.

Since Mr. Croll had not at hand the means of commencing a complete investigation of the causes on which terrestrial climate depends, his theory must, of necessity, fail to be entirely conclusive. Still it is worked out in a manner so laborious as to

render it worthy of very careful consideration, although, owing to the diffuse mode of treatment adopted, the complete mastery of his views is a very difficult task. For this reason it is not easy for the reviewer to feel sure that he is giving such a statement of the author's views as the author himself would regard as entirely satisfactory. We may say, however, in brief, that one great object of the author is to insist upon the important agencies played by ocean currents in influencing climate. Indeed, beyond the regular astronomical variation of climate with the latitude, this seems to be the only influence which he will allow to be important. The influence of the Gulf Stream in modifying the climate of Northeastern Europe receives especial attention, and his views of this influence seem quite well grounded. We had supposed the view that the warm and equable climate of that region was due to the Gulf Stream to be one universally held, although no one had attempted to render it plausible by an actual calculation of the amount of heat conveyed by that stream. This calculation Mr. Croll has made, and having reduced his own estimate of the volume of the stream to one-half, in deference to the views of some of his opponents, he shows that the amount of heat annually conveyed away by the stream is equal to the whole amount which a belt of the earth sixty-four miles broad, extending all round the equator, receives from the sun. We make the quantity a little less, but yet equivalent to more than the total amount of heat which falls on a million of square miles at the equator. Making all allowance for the uncertainties of these data, and for the fact that only one of the two branches of the Gulf Stream passes over to Northeastern Europe, it must yet be admitted that the quantity of heat which that region receives from the Gulf Stream is not an inconsiderable fraction of that which it receives from the sun.

An essential part of Mr. Croll's system is the wind theory of oceanic circulation, essential, however, to his views of the climate of the glacial period rather than to the climate of the present. This is a point on which there is some difference between Mr. Croll and his numerous opponents, especially Dr. Carpenter. Having made no examination of the views of Dr. Carpenter, we cannot pronounce them wrong, but the view maintained by Mr. Croll, that the winds are the principal causes of ocean currents seems well sustained. The direction of these currents may be materially modified by the earth's rotation, a cause which can be investigated only by mathematical methods, and until the mode of operation of this cause is fully understood, we cannot feel sure that the theory is complete. So far as we are aware Mr. Ferrel is the only mathematician who has entered upon this investigation, but Mr. Croll does not seem to have made much use of his results. The principal

support of the wind theory is found in the very obvious general correspondence between the winds and currents of the ocean, a correspondence so striking that it is difficult to see how the strongest presumption of a causal connection can be avoided. That the winds are, in a general way, amply sufficient to produce regular currents in the ocean seems to be shown by a familiar phenomenon on our Eastern coast. It is well known that the tides are there materially modified by the winds, so that the time of high water may be delayed or accelerated by an entire hour or more, and the height changed by one or more feet in consequence of a heavy wind. The effect of a wind thus determined must be the same as that of a difference of level equal to that which the wind is found to produce, and this again must be sufficient to produce a very strong surface current. Moreover, a continuous surface current must, in time, extend itself to a great depth through friction.

In thus sustaining the wind theory, we must not be understood to deny the existence of a general law of oceanic circulation which we understand to be due to Dr. Carpenter, and by which an undercurrent of cold water runs from each pole to the equator, to return as a surface current of warm water. That the mass of ice-cold water which forms the depths of the ocean came from the poles, and that to keep it cold, the supply must be constantly though slowly renewed will, we conceive, be disputed by no one. And the renewal of the water necessarily implies a surface set from the equator toward the poles. But, when we inquire whether the quantity of water thus interchanged can be so great as to give rise to the observed ocean currents, the answer is not quite clear, and the probabilities seem to incline to the negative. At the same time, we may have here an important feature among the causes which produce ocean currents, and the scientific method of investigating the subject is not by mere arguments, but by actual calculating the effect of each cause with judicial impartiality. Perhaps it would be unfair to say that Mr. Croll does not attempt to do this, but the impression left on the mind of the reader is that the "gravitation theory" of oceanic circulation is examined rather to refute it than to determine with mathematical precision what part differences of gravity between the polar and equatorial waters do really play in the phenomena in question.

While we agree with Mr. Croll in the important part he assigns to oceanic currents in modifying climate, we cannot accept the reasoning by which he attempts to prove that the corresponding influence of ærial currents is entirely insignificant. Speaking of the possible amount of heat conveyed by the upper currents, or anti-trades, from the equatorial to the polar regions, he says:

"The heated air rising off the hot burning ground of the equator, after ascending a few miles, becomes exposed to the intense cold of the upper regions of the atmosphere; it then very soon loses all its heat and returns from the equator much colder than it went thither." * "During all this time.

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[while the upper current is traveling from the equator toward the poles] the air is in a region below the freezing point; and it is perfectly obvious that by the time it begins to descend it must have acquired the temperature of the region in which it has been traveling."

This passage is quoted as showing the weakness which everywhere marks Mr. Croll's reasoning on the subject of temperature. With all the care and study he has devoted to the subject, we are entirely unable to reconcile his views with the known laws of heat. The facts that the same amount of heat is given off when water freezes or vapor condenses which is necessary to melt the ice or to evaporate the water; that the amount of heat developed by the compression of air is equal to that absorbed by its expansion; that if, from any cause, heat passes very slowly from a warm body A to a cool body B, it will also pass slowly from B to A when B is the warmer; that a body cannot abstract heat from another without itself becoming warmed, belong to a class which he does not seem to bear in mind. In the passage we have quoted, he speaks of the hot air rising from the earth and becoming exposed to the intense cold of the upper regions of the atmosphere. But, what can this cold be but the coldness of the very air itself which has been rising up? If the warm air rises up into the cold air, and becomes cooled by contact with the latter, the latter must become warm by the very heat which the former loses, and if there is a continuous rising current, the whole region must take the natural temperature of the rising air. This temperature is indeed much below that which maintains at the surface, for the simple reason that air becomes cold by expansion according to a definite and well known law.

Having thus got his rising current constantly cooled off by contact with the cold air of the upper regions, it has to pass on its journey toward the poles"in a region below the freezing point." Here again the question arises whether Mr. Croll conceives that the temperature of a region can be anything materially different from the temperature of the air or other substance which fills the region. Apparently he does, for he speaks of the air "acquiring the temperature of the region," but what the difference is, or can be, he does not explain. There is such a thing as temperature expressive of the amount of radiant heat passing through a diathermanous region, but the "upper regions" are exposed to the radiation of the sun. on the one side, and of the earth's lower atmosphere on the other,