Equilibrium of Ether in Ponderable Matter. 287 



exposed in the open air under sunlight, or under the light o£ 

 a more or less cloudy sky, or under moonlight or starlight, 

 or in the darkest attainable cellar. 



§ 7. Not being at present able to undertake experiments 

 of the kind, I asked "Dr. Glazebrook a few weeks ago if he 

 could conveniently allow some such experiments to be made 

 under the auspices of the National Physical Laboratory. He 

 kindly consented, and asked Dr. Chree to commence an in- 

 vestigation of the kind. I have to-day (28th July, 1905) 

 received the annexed description of his work, and statement 

 of results. 



§ 8. It is very interesting to see in Dr. Chree's results 

 how large are the differences in the temperatures of the 

 thermometers under black and under white cloth, ranging 

 from *5° to '6° cent., even at times when the sky is covered 

 with dark clouds ; and how comparatively moderate are the 

 differences ranging from 1° # 1 to 3°*6 cent., at times of 

 exposure to direct sunshine. 



§ 9. Returning to § 4 with one of the globes black over its 

 whole surface and the other white : suppose the two to be 

 taken to 1000 times the earth's distance from the sun ; and 

 suppose, all at about the same distance (for simplicity of 

 calculation), 999 stars, each equal to our own sun, to be 

 scattered through space, round the place of our ideal ex- 

 periment. The total of radiational energy coming from all 

 these suns to the place of observation per unit area, will 

 be one one-thousajidth of the amount coming from our 

 own sun in the case o£ §§ 1, 2, 3 ; and the difference of 

 steady temperature between the white globe and the black 

 globe may be about one one-thousandth of that which it 

 would be in §§ 1, 2, 3. This last arrangement would be some- 

 what similar to an exposure to starlight on a cloudless night, 

 at the top of a high mountain of our earth, with two or three 

 polished silver screens between the tested globes and the 

 mountain top. It does not, however, seem probable that any 

 differences of temperature will be perceptible on the two 

 thermometers exposed only to stellar radiation from the sky. 

 Even less of difference may be expected when the two 

 thermometers are placed in the darkest attainable cellar. 

 The bolometric method would of course be much more sensi- 

 tive than the comparison of two ordinary thermometers : 

 even of the most extreme sensibility : and it will, I think, 

 be worth while to try it in cases in which the thermometric 

 method fails, or almost fails, to show any difference between 

 the temperatures in the two cases. 



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