Observations made with a Black Bulb Thermometer. 87 



But we have to remember that the values given in Table V. are 

 complicated by the radiation of dark heat from the bulb of the ther- 

 mometer. The fact that the relative humidity falls on the whole 

 (see the last line of the Table) as the dew-point falls bears directly 

 upon this fact. For if the rate of radiation of dark heat is governed 

 by moisture at all, whether it depends upon the relative or the abso- 

 lute humidity, or both together, it is certain that it will be greater 

 for the lower temperatures of the Table than for the higher (apart 

 from the greater difference between the temperature of the black 

 bulb and that of the surrounding air). The maxima in the sun 

 are hence relatively lower in the drier air than they would be if the 

 radiation of dark heat were independent of the moisture present. 

 That is, it is not inconceivable that the difference of maxima would 

 increase faster as the temperatures decreased if the radiation of dark 

 heat were constant. 



In Table VI. the matter is examined in another way by arranging 

 the maxima in sun and shade, under clear skies, in the order not of 

 dew-point but of relative humidity. According to this arrangement 

 the maxima in the sun and shade both fall as the humidity increases ; 

 between ratios of 18 per cent, and 48 per cent, the latter falls 16° 

 while the former falls 12° : consequently the " radiation" increases 

 as the humidity increases. That is to say, a damp air seems at first 

 sight to have the same influence upon the action of the black bulb as 

 air with a small quantity of moisture. And the result is the more 

 remarkable because, as it happens in Table VI., the dew-point shows 

 a disposition to rise, albeit not very rapidly, as the ratio of humidity 

 rises. In fact, if Table VI. stood alone the conclusion to be drawn 

 would seem to be exactly the opposite to that of Stow mentioned 

 above ; but at the same time to perhaps conform to that of 

 H. von Schlagintweit. 



to prevent it from losing heat as fast as it receives it " ; but he does not explain 

 how this can be, and yet that the maximum temperature is attained three days 

 after full moon (" General Astronomy," 1888, p. 162). A favourite argument with 

 those who hold that no part of the moon's surface is ever very warm is that on the 

 top of our highest mountains, where of course the air is rare, there is perpetual 

 snow. Wherefore, by analogy, the lunar surface must be colder still. But it is 

 not an analogy at all. The air at high levels is cold because it intercepts little of 

 the solar radiation. The snow does not melt because it reflects, instead of absorb- 

 ing, a very great proportion of the incident heat. As for the supposed important 

 fact that in the lunar radiations there is a considerable quantity of heat having a 

 wave-length greater than that of the heat radiated from a block of ice, it may be 

 suggested that it comes, in large measure, from high lunar latitudes. Some 

 remarks by E. Nevill (Neison) on this subject in his great work, " The Moon," 

 1876, p. 37 et. seq., are worth attention. 



