WCP2876

[1]¹

Dunsink Observatory,

CO. DUBLIN

July 22nd, 1907

Telephone — Castleknock, No. 14.

Telegrams – Blanchardstown.

My dear Barrett²

At last our guests have left us & I can settle down to work: I am exceedingly sorry for the delay in discussing Dr Wallace's suggestions.

An extreme degree of uncertainty is attached to all estimates of planetary temperature. One realises this e.g. when one walks from the outer air into a greenhouse on a perfectly calm day in summer, when there is no artificial heat applied: the greenhouse & the outer air receive exactly the same amount of solar radiation, & yet are at quite different temperatures — owing [2] 2. of course largely to the circumstance that glass is comparatively transparent for the incident solar radiation & comparatively opaque to the radiation ^{of longer wave-length} emitted by the heated walls & plants inside — so that it acts as a radiation-trap. If there is anything on Mars which can act in this way, the temperature of Mars might be almost anything short of the Sun's own temperature!

But if we decide to ignore all considerations of this kind, I suppose we should argue as follows.–

The amount of radiant energy received from the Sun in one second is proportional to the inverse square of the distance r from the Sun. This energy received must exactly balance the energy radiated away, in order [3] 3 to keep the temperature steady. But the energy radiated away is ^{by Stefan's law} proportional to [illeg.] T⁴, where T is the absolute temperature of the planet. So T⁴ must be proportional to 1/r², & therefore T must be proportional to 1/√r. This would give -44°C. for as the temperature on Mars.

Dr Wallace in his calculation appears to assume that T is proportional to the radiant energy received: but I can't follow the ^reasoning grounds on which this assumption is based.

However, I make no secret of my own belief that we know nothing whatever about the temperature of Mars.

It is very kind of you & Miss Barrett² to think of inviting Mr T. W. Russell.⁴We [4] 4 should much like to meet him, as I think he knows various relatives of mine who are engaged in Temperance & Political work. Any day would suit us.

The balloon ascent this morning was a failure, owing to the cloudy & foggy weather. Better luck next time.

Ever your sincerely | Edmund T Whittaker³ [signature]