13° 



NATURE 



\Dec. ii, 1879 



and water, we shall see that there is no reason whatever why 

 the air at the equator should be hotter in January than in July. 



It is well known that, notwithstanding the nearness of the sun 

 in January, the influence of the present distribution of land and 

 water is sufficient to make the mean temperature of the whole 

 earth, or, what is the same, the mean temperature of the air 

 over the surface of the earth higher in July than in January. 

 The reason of this is obvious. Nearly all the land is in the 

 northern hemisphere, while the southern hemisphere is for the 

 most part water. The surface of the northern or land-hemi- 

 sphere, for reasons to which I need not here refer, becomes 

 heated in summer and cooled in winter to a far greater extent 

 than the surface of the southern or water hemisphere. Conse- 

 quently when we add the July or midsummer temperature of the 

 northern to the July temperature of the southern hemisphere, we 

 must get a higher number than when we add the January or 

 midwinter temperature of the former to the January temperature 

 of the latter. For example, the mean July temperature of the 

 northern hemisphere, according to Dove (" Distribution of Heat 

 on the Surface of the Globe ") is 7o° - 9, and that of the southern 

 hemisphere $$"6 ; add the two together and we have I24°'5, 

 which gives a mean for both hemispheres of 62 0- 3. The mean 

 January temperature of the northern hemisphere is 4S '9, which, 

 added to 59°'S, the mean January temperature of the southern 

 hemisphere, gives only loS°'4, or a mean of 54°'2. Conse- 

 quently the air over the surface of the globe is hotter in July by 

 8" than in January, notwithstanding the effects of eccentricity. It 

 is obvious that, were it not for the counteracting effects of eccen- 

 tricity, the difference would be much greater. Ten thousand 

 years ago, when eccentricity and the distribution of land and 

 water combined to produce the same effect, the difference must 

 have been far greater than 8°. 



But it will be asked, How can this affect the air over the 

 equator, which is not situated more on the one hemisphere than 

 on the other? It is trite that those causes have but little direct 

 effect on the air at the equator, but indirectly they have a very 

 powerful influence. The air is continually flowing in to the 

 equatorial regions from both hemispheres. In fact, the air 

 which we find there is derived entirely from the temperate re- 

 gions. In July we have the northern trades coming from a 

 hemisphere with a mean temperature as high as 7o 0- 9, and the 

 southern trades coming from a hemisphere with a mean tempera- 

 ture not under $3°, while in January the former trades flow from 

 a hemisphere as low as 50°, and the latter from a hemisphere no 

 higher than 6o°. Consequently the air which the equatorial 

 regions received from the trades must have a higher temperature 

 in July than in January. The northern is the dominant hemi- 

 sphere ; it pours in hot air in July and cold air in January, and 

 this effect is not counterbalanced by the air from the opposite 

 hemisphere. The jiean temperature of the air passing into the 

 equatorial regions ought therefore to be much higher in July 

 than in January, and this it no doubt would be were it not, let 

 it be observed, for the counteracting effects of eccentricity. The 

 tendency of the present distribution of land and water, when 

 our northern winter occurs in perihelion, is to counteract the 

 effects of eccentricity. But ten thou-and years ago, when our 

 winters were in aphelion, that cause would co-operate to intensify 

 the effects of eccentricity. In fact, it would actually more than 

 double the effects then produced by eccentricity. Now if the 

 influence of the present distribution of land and water is so 

 great as not merely to counteract but to reverse the effects of 

 eccentricity to the extent of making the mean temperature of the 

 earth 8° warmer in July than in January, it is not surprising that 

 it should be sufficient to make the equatorial regions at least as 

 ■warm in the former as in the latter period. 



The fact that the equator at present is not hotter when the 

 ■ earth is in perihelion, instead of being an objection to the theory 

 that the glacial period was due to an increase of eccentricity, as 

 Mr. Fisher supposes, is in reality another strong argument in its 

 favour, for it shows that a much less amount of eccentricity 

 would suffice to induce a commencement of glacial conditions in 

 the northern hemisphere than would otherwise be required, were 

 it not for the circumstance to which Mr. Fisher refers. This 

 objection, like many others which have been urged against the 

 theory, arises from looking too exclusively at the direct effects of 

 eccentricity. 



There is another cause which must also tend to lower the 

 January and raise the July temperature of the equator, viz., the 

 northern trades pass further south in January than in July, and 

 consequently cool the equatorial regions more during the former 

 than the latter season. This general tendency of the trades to 



lower the temperature of the equatorial regions more in January 

 than in July is of course subject to modifications from the mon- 

 soons, the rainy seasons, and other local causes ; nevertheless, 

 so long as the present distribution of land and water endures, so 

 long will eccentricity have a counteracting effect upon the tem- 

 perature of the air at the equator, which but for that would be 

 hotter in July than in January. 



Mr. Fisher somewhat misapprehends what he designates my 

 " fundamental proposition." What I stated was " the tempera- 

 ture of a place other things being- equal is proportional to the heat 

 received from the sun." Those who have read what I have 

 written on this point will remember that what I mean is, that if 

 the temperature of any place depended alone on the direct heat 

 of the sun that temperature would be proportional to the amount 

 received. But then there is no such spot on the face of the 

 globe — there is no place where heat or cold distributed by ocean 

 or aerial currents does not affect the temperature — and I have in 

 " Climate and Time," pp. 41-44, proved that, with the exception 

 of the Arctic regions, there is no part where the temperature is 

 so much affected by those currents as the equator. Were it not 

 for the cooling effect produced by them the equator would be 

 uninhabitable. No knowledge whatever as to the intensity of 

 the sun's heat can be obtained from observations on the tempera- 

 ture of the air at the equator. The comparatively cold air 

 flowing in from the temperate regions has not time to be fully 

 heated by the sun's rays before it rises as an ascending current 

 and returns to the temperate regions from whence it came. More 

 than this these trades prevent us from being able to determine 

 with accuracy the intensity of the sun's heat from the tempera- 

 ture of the ground ; for the surface of the ground in equatorial 

 regions is kept at a much lower temperature by the air blow ing 

 over it than is due to the intensity of the sun's heat. It thus 

 becomes a very intricate problem to determine how much the 

 surface of the ground is kept below the maximum temperature 

 by the heat absorbed by the moving air. 



I may add that although my estimates of the low-ering effect 

 resulting from the decrease of the sun's heat arising from increase 

 of distance were computed according to Newton's law, yet I 

 distinctly stated that this law holds only approximately true, but 

 that nevertheless, for reasons given at p. 34 of "Climate and 

 Time," it would be found near enough for my purpose. 



James Croll 



A Possible Consequence of our Present Weather 

 I have observed on several occasions that abnormally cold 

 weather in November has been followed by an unusually mild 

 mid-winter and January. These may possibly have been mere 

 accidental coincidences, or they may be connected by a link of 

 causation thus. Our climate, and more especially our winter 

 climate, is largely influenced by the Gulf Stream, and whatever 

 augments this raises our winter temperature, and vice versa. 



How, then, is the Gulf Stream likely to be affected by an 

 unusual prevalence of Arctic winds and unusual cold in these 

 latitudes ? Such winds, must, to some extent, drive the waters 

 of the Atlantic towards the source of the Gulf Stream, and tend 

 to heap them there, and if there is any truth in the theory which 

 attributes ocean currents to differences of oceanic temperatures, 

 the present unusually cooled waters of the temperate zone will 

 co-operate with the winds and augment this accumulation by 

 their underflow. I do not mean that these combined actions are 

 reversing the Gulf Stream at the present time, but simply that they 

 are exerting a counter action or retarding influence which must 

 result in augmenting the normal magnitude of the reservoir, or 

 tropical accumulation, the outflow of which constitutes the Gulf 

 Stream, and that thus the volume and velocity of the tropical waters 

 which usually flow towards our coast will be augmented when 

 the pressure of the present Arctic winds shall cease, and that our 

 climate will be influenced accordingly. If I am right we may, 

 in spite of present symptoms, or rather on account of them, have 

 an unusually warm Christmas sea-ron and January. 



This idea is not thrown out as a mere weather prophecy, but a 5 

 a suggestive hypothesis and an incentive to what appears to me to 

 be a very important and a much neglected branch of meteoro- 

 logical research, viz., systematic observation and record of the 

 variations of the Gulf Stream. The countries whose coast is 

 washed by this beneficent river of ocean are deeply interested in 

 its movements. The Norwegians have already done something 

 towards recording its variations, but 50 far as I can learn we, 

 who are almost as deeply concerned as they are, have done 

 little or nothing. 



