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"There is no relation, at the periods in question, between the intensity of the sun's heat and the temperature of the summer. One is apt to suppose, without due consideration, that the summers ought then to be as much warmer than at present as the winters are colder. Sir C. Lyell in his 'Principles' has given a column of summer temperature calculated from my table on this principle. Astronomically this is correct, but physically, as shown in ch. iv., it is wholly erroneous, and would convey a wrong impression on the whole subject of geological climate. The summers of that period, instead of being much warmer than at present, would in reality be much colder, notwithstanding the great increase in the sun's heat from her diminished distance."

36. I think there is not the least solid ground for the contrast here affirmed, and that the want of due consideration is on the other side.

First, let us inquire what will be the summer temperature, if the principle in the previous calculations of midwinter heat is maintained. The contrast will then be between the present heat, when the sun is near the aphelion, and the perihelion heat with the increased excentricity. Adopting the three corrections already introduced, first, of the value of the excentricity, secondly, of the law of radiation, and thirdly, of the maximum heat or cold a month after the solstice, the increase of summer heat would be 34°.88 and 28°5 at the two eras proposed. Thus, instead of 39° and 64°, the present midwinter and midsummer heat in our island, the temperatures would be, by the corrected rule, 5°.8 and 98°.88 for the earlier, and 10°3 and 92°.5 for the later date.

37. The reasons assigned, why glaciation should have resulted indirectly from the increased excentricity about 200,000 years ago are these: First, the midwinter temperature would be lowered to an enormous extent. I have just shown that this is not correct. The decrease would be only 28.7 instead of 37°.7, and the resulting temperature 10°.3. This is nearly the same as that of Canada, near Quebec, while the summer temperature, by the previous estimate, would be almost 30° higher. This is wholly different from the conditions of a glacial period.

The winters, it is said, would be longer as well as colder. Instead of being 8 days shorter than the summer, as now, the excess would be 36 days. But for the period mainly in question the difference is 26 days, or 13 days is the excess of the winter over half a year. The mean rainfall of our island is 32 inches. Without some unproved change in the physical conditions, the rainfall of the winter months would be less than 20 inches, or if snow be reckoned six times lighter than water, this would amount to a depth of 10 feet only. But the latent cold of ice is 140°, and water has four or five times the specific heat of

most solids. The formation of ice is thus a most powerful means of arresting a decline of temperature, as evaporation is the great natural remedy for excessive heat. The heat required to melt 20 inches depth of frozen water over the whole surface of any portion of land is equal to that of 37 hours of vertical sunshine, if we adopt the datum of Sir J. Herschel, that vertical solar heat on a square foot in one second would raise one pound about one-ninth of a degree. The total summer heat, reckoned roughly, would be equal to 1,300 hours of vertical heat at the equator, or lat. 54°, and 900 hours at the pole hence, if the whole winter rainfall were deposited in snow or ice, the heat needed to melt the whole would be that of four days only nearest to the summer solstice, or onethirtieth of the whole summer heat in our latitude.

38. The reasoning in " Climate and Time," pp. 58, 59, seems to assume that ice and snow are the cause and not the effect of a cold climate, and tend to aggravate not to mitigate its severity. But the exact opposite is true. As ocean currents tend to equalize the temperature of different parts of the earth, so the formation and melting or evaporation of ice and snow are the chief natural means of lessening the difference of sensible heat in different seasons of the year. When the radiation is in excess of the supply of solar heat, the freezing of water sets free 140° of heat to repair the loss; and when the summer returns, all the ice and snow must be melted before the temperature can have a sensible rise above the freezing-point. A pound of water, with a sensible difference of 180° only from its frozen state to its evaporation at the boiling-point requires 1,320° of heat, and this will be equivalent to 5,280° or 6,600° degrees for a pound of rock or of earth, the specific heat being one-fourth or one-fifth of that of water. Or, taking the interval from zero to 70°, a pound of water, in virtue of the process of freezing and its great specific heat, serves to reduce the sensible change of temperature from twelve to fifteen times.

39. There are three ways in which snow and ice are said to lower the summer temperatures. First by direct radiation. Whatever the heat of the sun, the snow and ice can never rise above 32°, and their radiation lowers all surrounding bodies to that level. Next, the rays which fall on them are to a great extent reflected into space, and those which are not reflected, but absorbed, disappear in the mechanical work of melting the ice. Thirdly, they chill the air, and condense the moisture into fogs, and these prevent the sun's rays from reaching the earth; thus the snow, in these aphelion winters, would remain unmelted the whole summer.

Now of these causes the first and third exclude each other. If fogs hinder the sun's rays from reaching the earth, they must also prevent the ice and snow from radiating heat away into empty space. The dull, cloudy surface above must receive and absorb all the heat of the summer sun, and can allow little heat to radiate into space, except at night; even then much less than under a clear sky. Of course, till the ice and snow are all nearly melted, they effectually hinder a sensible rise of heat above 32°; but this is only the converse of their previous effect, in their formation, to hinder a lowering of the temperature till the whole has been frozen. All the heat of the sun which falls on the earth must produce its full effect, either in raising the ice, snow, and the ground itself, up to the freezing-point, or in melting them, and turning them into water or aqueous vapour. The same amount of cold which would depress a stratum of chalk ten feet deep to the zero of Fahrenheit would spend itself in turning 7 inches of rainfall into ice and snow. Thus the presence of moisture, whether in the air or the soil, or lakes and rivers, is the most effectual hinderance to excessive lowering of the winter temperatures, so long as the total annual heat received from the sun is not diminished. But in the imagined glacial epoch, this total amount is increased per cent. for the whole globe, and 3 per cent. for the northern hemisphere.

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40. Even with the corrections before named, the calculation cannot lead to a precise result, but shows at the most a limit towards which the temperature would tend, if the solar heat and radiation into space maintained the given proportions for an indefinite period of time. If the rule were sound, some very unnatural conclusions would follow. Each pole, during its winter of half a year, when it receives no heat at all from the sun, would sink to the temperature of space, or -239° F. Again the heat which the pole receives from the sun at midsummer, exceeds that received by an equal surface at the equator in the ratio of π. sin. to cos., or 1·3638 to 1. But since the summer heat of the equator is 79°, or 318° above that of space, the midsummer heat of the pole, by Mr. Croll's mode of reckoning, should be 115° higher, or 194°, little short of the heat of boiling water. Each conclusion is plainly very wide. of the truth.

41. Again, Mr. Croll insists forcibly on the vast amount of heat transferred northward by the Gulf Stream. He reckons it equal to one-fourth part of the whole amount received from the sun by the Atlantic area or basin, from 25° N. up to the Arctic Circle. The consequent increase of the mean temperature of Great Britain is not less, he thinks, than 30°; but in

estimating the temperature for his glacial epoch this element is omitted altogether. It is plain, however, that it must then have been not much less than it is now. The contour of land and sea was nearly the same as at present in the Boulder Drift period, and the Atlantic basin had nearly its actual outline, and reached as far to the north. The strength of the current must depend on the contrast between the heat of the southern summer and the cold of the northern winter, so far as these were directly dependent on the sun. This would be only 8 per cent. less than it is now. On the other hand the current would be greater in the summer half of the year, and serve more fully to blot out the traces of the cold of the previous winter. The general result would be an increase of summer heat and winter cold, each about 28° at the most, but probably much diminished by the equalizing effects of aërial and ocean

currents.

42. Another element has still to be considered. In Mr. Croll's Table, p. 320, vol. iii., the longitude of the perihelion at the date B.C. 210,000 is stated to be 144° 55'. From the last entries it seems plain that this amount has reference to a fixed and not a movable solstice or equinox, and is the change resulting from the progression of the apsides alone. The change from precession for this same period, at the present rate, would be eight complete circuits and 46° 56'. Hence the true longitude of the perihelion, on this view, would be 144° 55′

46° 56', or just 98°. Thus the northern summer solstice, ast it is now, would be nearly in aphelion. This is precisely the opposite condition to that which forms the basis of Mr. Croll's theory. We need to go backward or forward 10,000 years, to have the winter solstice in aphelion, when the excentricity is ·0497 or 0569. In the former case the midwinter increase of cold would be only five-sixths of Mr. Croll's estimate, when his other data are retained, or the decrease, which has been reduced from 37°.7 to 28°.7, would be further reduced to 23o.9, or the midwinter temperature by the rule be 15°1, which is higher than the temperature of Canada.

43. The main principle involved in Mr. Croll's theory is that the cold or hot state of each hemisphere is determined chiefly by its midwinter temperature, and this in turn by the simple ratio of the direct solar heat then received, the excess over the mean temperature of space, or - 239° F., being determined by a simple rule-of-three calculation. And since the winter northern solstice is now very near the perihelion, the present excess above the average value, when combined with the deficit at other periods, results in a very considerable disproportion. The ratio, according to Mr. Croll, 850,000

years ago, is about five-sixths; and hence, one-sixth of 278°, or 45°, will be the aggravation at that date of the winter cold. But if this mode of reckoning were sound, it ought to apply to the northern and southern hemispheres with the present excentricity. In this case the southern winter should be colder than the northern in the amount answering to the ratio •93507, or 18° F. But in fact there is no such inequality, and it would almost appear that the climate, in answering latitudes, is slightly warmer than in the northern hemisphere, except in the immediate neighbourhood of the pole.

44. The following extract from Mr. Croll's table gives his conclusions with regard to his two proposed glacial periods, and the midwinter temperature of Great Britain at the answering periods :-

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45. In the following table, the excentricity is reduced by the formula ee+024 to correspond with Mr. Stockwell's corrected maximum, 0693888, instead of Leverrier's 077747. The equilibrium of solar heat and radiation is assumed to be 30° after the winter solstice, and the law of radiation is taken from Dulong and Petit's experiments. The ratiol 165 log.=0663259 answers to a change of 36° F., or a change of solar distance to that amount to 72°. Hence log. radius vector +1-200 will give the answering change in degrees. The precession at the rate of 50"-3405 a year, or 139° 50' for 10,000 years, is combined with the perihelion places of Mr. Croll's table, to give the anomalies at 30° after the solstice. The columns are the date (in 10,000's of years B.C.), excentricities, anomalies,

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