Lord Rosse’s later observations modified his conclusions, to some extent, showing that he had at first underestimated the percentage of simple reflected heat, but without causing him to make any radical change in his ideas as to the maximum heat of the moon’s surface.

For some time, however, there has been a growing skepticism among astronomers, relating not so much to the correctness of his measures as to the computations by which he inferred the high percentage of obscure radiated beat compared with the reflected heat, and so deduced the high temperature of lunar noon.

Professor Langley, who is now engaged in investigating the subject, finds himself compelled to believe that the lunar surface never gets even comfortably warm—­because it has no blanket.  It receives heat, it is true, from the sun, and probably some twenty-five or thirty per cent. more than the earth, since there are no clouds and no air to absorb a large proportion of the incident rays; but, at the same time, there is nothing to retain the heat, and prevent the radiation into space as soon as the surface begins to warm.  We have not yet the data to determine exactly how much the temperature of the lunar rocks would have to be raised above the absolute zero (-273 deg.  C. or -459 deg.  F.) in order that they might throw off into space as much heat in a second as they would get from the sun in a second.  But Professor Langley’s observations, made on Mount Whitney at an elevation of fifteen thousand feet, when the barometer stood at seventeen inches (indicating that about fifty-seven per cent. of the air was still above him), showed that rocks exposed to the perpendicular rays of the sun were not heated to any such extent as those at the base of the mountain similarly exposed; and the difference was so great as to make it almost certain that a mass of rock not covered by a reasonably dense atmosphere could never attain a temperature of even 200 deg. or 300 deg.  F. under solar radiation, however long continued.

It must, in fact, be considered at present extremely doubtful whether any portion of the moon’s surface ever reaches a temperature as high as -100 deg..

The subject, undoubtedly, needs further investigation, and it is now receiving it.  Professor Langley is at work upon it with new and specially constructed apparatus, including a “bolometer” so sensitive that, whereas previous experimenters have thought themselves fortunate if they could get deflections of ten or twelve galvanometric divisions to work with, he easily obtains three or four hundred.  We have no time or space here to describe Professor Langley’s “bolometer;” it must suffice to say that it seems to stand to the thermopile much as that does to the thermometer.  There is good reason to believe that its inventor will be able to advance our knowledge of the subject by a long and important step; and it is no breach of confidence to add that so far, although the research is not near completion yet, everything seems to confirm the belief that the radiated heat of the moon, instead of forming the principal part of the heat we get from her, is relatively almost insignificant, and that the lunar surface now never experiences a thaw under any circumstances.