Dear Sir,

Your favour with book came duly to hand, and I am glad to hear that some of the papers have been of a little use to you.¹ I am very much pleased to learn that you consider the facts in distribution favourable to some of the views expressed in my paper on climate²

I have not as yet been able to overtake that part of the question relating to the condition of the hemisphere whose winters occur in perihelion. I have no doubts that when this part of the subject has been fully discussed Sir Charles Lyell will agree with me;³ the facts in favour of a warm climate are so numerous and strong

It is a pity that Sir Charles should have made those remarks on the “secular loss of heat in the solar system” vol. II p 213.⁴ He must have done it without due consideration of that point

If there is one thing more than another in physics, regarding which we have absolute certainty, it is that the Solar system is losing its store of energy. We not only know this fact, but we have a means of determining the actual rate at which it is losing its power.

3,869,000 foot-pounds of energy in the form of heat is radiated off every square foot of the sun’s surface per second. In other words the quantity of energy thrown off into space by the sun is equal to a 7000 horse power engine working on every square foot of its surface. And when we reflect that all this prodigious expenditure has been going on during countless geological ages we may well ask the question what is the secret of the sun’s great strength? Gravitation only affords up to the present time 20,000,000 years heat

There must be some other source in addition to that of gravitation. It is strange that that other possible sources did not suggest itself to Sir William Thomson and other physicians when working at this question.⁵ It is perfectly obvious that the sun or rather the matter which composes the sun might have been in possession of heat prior to condensation

In this case it is difficult to say how old the sun may be for we do not know what this original store may have amounted to. In my paper I assumed a certain relation between the amount of original heat and that produced by gravitation viz 234 to 95 (Phil. Mag May 1868)⁶ but as I stated, I may be wrong   It may be more than this, or it may be less. This proportion some 70 000 000 years

The introduction of this new element, changes the entirely the conditions of the problem and I have no doubt that the whole matter will have to be re-considered. And it is quite possible that we may yet be able to get considerably more than one hundred million of years although very much beyond this we are brought to a limit by other considerations.

As regards determining the age of the earth’s crust from the secular cooling of the globe I am not altogether satisfied with the plan.⁷ It would no doubt do if we had proper data to go by, but I don’t think we have got that yet.

I think that you may quite fairly assume a very long period before the Cambrian formation, even according to Sir William Thomson’s theory for supposing the earth to have orginally been in a molten condition, a solid crust would very rapidly form, and if this crust would not break up and sink, the globe at the surface would be cool and suitable for life although a short way down below the surface the heat was intense. This results from the slow rate at which the crust is able to conduct the heat from within

It is some years since I read Sir William’s paper on the Secular cooling of the globe,⁸ but I think he states the above as his opinion, at all events I heard him once say in a lecture on the subject that supposing the earth to be in a molten state, in a few thousand years you could walk on its surface and hardly be sensible of the heat from within

Electricity and Magnetism used to be my favourite study, but for the past four years I have been paying little attention to what was going on in that department.

A relation between the spots of the sun and the manifestation of electric phenomena on the earth does not necessarily imply any transmission of electric forces from the one body to the other

One thing is certain that it is but an infinitesimal quantity of the forces of nature that ever assume the electric or magnetic form. Electrical phenomenon is very imposing and this is the reason why so much is attributed to it. A thunder storm is something very striking but Faraday has shown that more electricity is evolved in the silent decomposition of a few grains of water in the cell of a battery than would be required to produce the most violent flash of lightning⁹

It is owing to high tension that electricity makes such a display in passing from the statical to the dynamical state    But when you estimate the amount of energy thus displayed in foot-pounds it is often very little

The quantity of energy in the form of electricity coming from the sun (if there be any at all) is certainly trifling compared with what comes in the form of heat.¹⁰ I believe that no physicist will call this in question.

Your suggestion as to to the possibility of a cosmical cause for the ups and downs of the crust never occurred to my mind. I can see no possible way at present how the thing can be; but I shall ponder certainly over it.

I have never heard of M^r Moseley’s paper.¹¹ My curiosity is very much excited and perhaps you will be so kind as to let me know when the paper appeared.

Edinburgh with all its books and learning is miserably behind in scientific literature. Since I came here I hardly know what is going on in the scientific world around. One can get plenty of good solid books on science, but the current news and literature of the subject is not to be found anywhere.

Edinburgh I fear is falling behind.

I need hardly say that a present of a copy the Origin of Species from its Author will be esteemed worth a dozen of copies out of a shop.

I trust you will make out to read this rather long affair, written hurriedly to catch the post.

I am, | yours very truly, | James Croll

Charles Darwin - Esqr. FRS

Keep the M.S. since it is of no use to me.