WCP6893

[1]

Waldron Edge, Duffas Hill, Croydon.

August 1st 1879

My dear Sir

In a rough way I arrived at pretty nearly the same result while preparing my paper as those you sent me. The reason I did not refer to them was that I could not find any data by which to test them, & they would have led into discussions unfitted for a popular article and which was already sufficiently crowded with facts & arguments within a strictly limited space. I am glad however to have the opportunity of making a few remarks on them.

First as to the [illeg.] results which ought to appear from <sun's> ^excentricity; I could find no good observations in places of equal North & South latitudes to compare,— though no doubt there are good Cape observations. The meteorological observations however never (so far as I know) observe [2] the amount of the sun's radiation, which is what is required by us. The temperature of the air depends so much on local causes and may so greatly diverge from that due to the sun's radiation at the place as to be quite valueless. I have however somewhere seen a statement by Sir John Herschel that the direct ^maximum sun-heat at the Cape of Good Hope was greater than in the same latitudes in the Northern Hemisphere. Also as to the temperature of the equator in July and January I could find no recorded difference,— but this air-temperature will depend almost wholly on general meteorological causes as direction of winds, amount of rain, of cloud, &c. &c. Observations of direct sun-heat on the equator with the sun at equal attitudes, at different times of the year should certainly show the difference between July & January if there is any truth in the theory of heat at all. [3]

The radically different distributions of land and water in the northern & southern hemispheres renders any comparison of climates as due to summers or winters in aphelion quite useless, as the differences caused by these peculiarities of distribution are far greater than those theoretically due to the latter causes. In the northern hemisphere we know, in the same latitudes, enormous divergences of summer & winter temperatures due to sea and land distribution, ocean currents, wind &c. The only places I can think of that might be fairly compared are Owyhee [Hawai'i] & Tahiti in nearly the same latitudes N. & S. of equator, & both in mid-Pacific about equally remote from continents. The Azores & Tristan da Cunha would also compare well — but of course there are no reliable observations in the latter place.

I do not think therefore that the apparent non-existence of the differences of temperature due to present excentricity¹ on the theory that the [4] temperature of space is -239° F. is any proof that that temperature is wrong. But even if we halve it and call it only -120° F the resulting differences that now ought to occur in the July & Jan. temperatures of equator are still larger (I make it 13° F); and it is almost impossible that it can be more than this seeing the actual temperatures of -80 F. (I think) have been observed.

As to the glacial epoch the ^calculated differences of summer & winter temperatures die to increased excentricity are so enormous on the supposition of -239 F. that we may easily reduce it & still have an ample margin left. I do not think therefore there any uncertainty about the temperature of space much effects Croll's explanation of the glacial epoch, for as [5] we assume it at present the ^maximum summer & winter temperatures come out too excessive. I make, for example, during the Miocene warm period the winter temp. of Britain (January) 72° F. and in July only 37°F.! due to ^direct sun-heat only in proportion to present amounts,— while during the Glacial Epoch the mean January temp. of Britain only 3° F. or 29 below the freezing point script ^{(or 1½ degrees as you make it)} and as the winter temperature would be still lower further north and further inland, we have cold enough and to spare for all the required phenomena of glaciation.

The winter temperatures of places in S. temperate zone are certainly much below those of corresponding places in N. temperate [6] zone, which is as it should be; but then as soon as we get towards 40° 50° S. the summer temperatures are also much lower, which ought not to be. This is I think satisfactorily accounted for by the great mass of Antarctic ice and the greatest strength of the south polar wind, and it affords a good example of the kind of influences that would be brought to bear by a higher excentricity and winter in aphelion, so as to produce a glacial epoch when aided by high land and ample precipitation.

I hope Mr Croll will take up the subject of the influence of present amounts of excentricity on solar heat, & temperature generally. No doubt some available records of observation exist but I have no time [7] to search for them. I have voluminous meteorological reports of the Batavia, & Bombay observatories, but neither gives a single observation on solar radiation.

I have no objection whatever to your informing Mr. Croll that I wrote the article and I shall be greatly interested by hearing his candid opinion of it. I have a great admiration for his work, and have only ventured to differ from him on a few points which I think needlessly encumber his otherwise admirable theory with difficulties, and, I think, errors.

Excuse this very hasty & disjointed & badly expressed letter, but I dare say you will understand the points [8] I have meant to insist on,— briefly, that air-temperatures are so affected by geographical causes as to mask the ½-yearly differences of sun heat ^{due to excentricity} & sometimes to reverse them,— which we have no sufficient [illeg.] of solar radiation on the equator or in corresponding N. and S. latitudes to enable us to test theoretical conclusions.

Believe me | Yours very faithfully | Alfred R. Wallace [signature]

Revd O. Fisher