From William Hopkins   March 16 1858

Cambridge | March 16 1858

My dear Sir,

I sent by last evening’s post a copy of my memoir on the temperature of the earth and planets.¹ I may mention a point which may be liable to be mistaken. You will find two kinds of heat spoken of and designated respectively by H_o and H_i and radiating from a spherical envelope, which represents the external sources of radiant heat falling on the earth. H_i is supposed to have the power of radiating completely thro’ a certain imaginary shell (which may represent the earth's atmosphere) so that the shell is supposed completely diathermanous² for this kind of heat; but it is not diathermanous for H_o which represents in fact all the heat from whatever source which is absorbed by the imaginary shell. In the actual case it represents the portion of the solar and stellar heat which is absorbed by the atmosphere. My reasoning and conclusions do not depend on the ratio of H_i to H_o so far at least as regards the nature of these conclusions. It is of no consequence whether it be 100: 5 or 100: 25.

Yours very truly | W. Hopkins.

RI MS JT/1/TYP/2/613

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