486 



SCIENCE. 



[N. S. Vol. U. No. 41. 



from other solid radiators, the lunar beam has 

 the greatest quantity of energy in rays of greater 

 wave-length than those of its maximum energy 

 (A. J. S., 435), and this is a matter of great im- 

 portance in the present connection. The air is 

 practically opaque to the strongest of these rays, 

 of wave-length about seven /', for it is here that 

 the ' great cold band ' of the lunar spectrum oc- 

 curs ; yet in the region of the great body of 

 coarser but weaker lunar rays the percentage of 

 transmission rises to 70 or 80 (Mem. N. A. S., pi. 

 6 and 7 ; A. J. S., pi. X. and XI.). The average 

 transmission of the whole lunar beam of emitted 

 rays is estimated at 40 Jio (Mem., 189). A valve 

 with a leakage of 40 fo should not be spoken of 

 as ' absolutely refusing ' to let a current pass ; it 

 should hardly be called a valve at all. Assum- 

 ing that terrestrial and lunar radiations are 

 much alike, Langley's results seem to show that 

 something more than half of the solar beam 

 comes in through clear air, and something less 

 than half of the terrestrial beam goes out. It 

 can be only by the small amount of energy thus 

 saved to our use that the temperature of the 

 earth is maintained. 



Regarding the process by which the earth 

 maintains its existing temperature, there is an 

 interesting suggestion by Prof. Arthur Searle, of 

 the Harvard College Observatory, in a brief es- 

 say entitled 'Atmospheric Economy of Solar 

 Radiation' (Proc. Amer. Acad. Arts and Sci., 

 XXIV., 1888, 26-29), to which those who are in- 

 terested in this problem may refer, and from 

 which I quote the following sentence in or- 

 der to show how others than myself read Lang- 

 ley's conclusions : " The hypothesis which has 

 been current until recently with regard to this 

 protective action of the atmosphere depended 

 upon a supposed effect of selective absorption, 

 which has now been largely, if not entirely, 

 disproved by Laugley's experiments" (p. 26). 



The following quotations may indicate Lang- 

 ley's position regarding the ' valve ' or ' trap ' 

 or 'hot-bed' action of the atmosphere: "Alle- 

 gheny observations * * * show that a consider- 

 able part of this radiated [lunar] heat does 

 pass through our atmosphere along with that 

 reflected" (Science, 1st Ser., Jan. 1, 1886, 8). 

 "Contrary to all previous experience, it [the lu- 

 nar beam] nevertheless reaches us, thus bringing 



evidence of the partial transparency of our ter- 

 restrial atmosphere even to such raj's as are 

 emitted by the soil of our planet " (Mem. N. A. 

 S., 193). " To see how the question of the lu- 

 nar heat affects our knowledge on the whole 

 subject of our planet's temperature, we must re- 

 member that until a few years past it had been 

 assumed by all writers of repute that the earth's 

 atmosphere acted exactlj' like the glass cover of 

 a hotbed, and kept the planet warm, in ex- 

 actly the same way that the hotbed is warmed, 

 by admitting the light-heat of the sun, which 

 was returned by the soil in the invisible radia- 

 tions of greater wave-length to which the at- 

 mosphere was supposed to be impervious, and 

 that thus the heat was stored " (Mem., 110). 

 Although no equally explicit announcement is 

 made of the belief with which Langley would 

 replace this assumption made by ' all writers of 

 repute,' these extracts and quotations give rea- 

 son to think that Professor Hallock's brief state- 

 ment is over-strong in using such phrases as 

 ' almost undiminished ' and ' refusing abso- 

 lutely.' The analogy of the valve needs so 

 much qualification that it is not very serviceable 

 in the way of explanation. 



In this connection, a few words maybe added 

 on the matter of terminology, which I find very 

 embarrassing. We all recognize that the care- 

 ful definition of scientific terms is an important 

 aid to careful thought, and that the use of a 

 term loosely and vaguely is apt to tangle up the 

 ideas of the hearer or reader, if not of the 

 speaker or writer. The introduction to Max- 

 well's 'Theory of Heat' offers an excellent il- 

 lustration of the care with which that masterful 

 physicist used his terms (Appleton's Edition, 

 1883, p. 8, 9. He afterwards says: "Heat is 

 certainly communicated from one body to an- 

 other by a process which we call radiation, 

 which takes place in the region between the two 

 bodies. We have no right, however, to speak 

 of this process of radiation as heat. We have 

 defined heat as it exists in hot bodies, and we 

 have seen that all heat is of the same kind. 

 But the radiation between bodies differs from 

 heat * * * in being of many different kinds * * 

 When we speak of radiant heat we do not mean 

 to imply the existence of a new kind of heat, 

 but to consider radiation in its thermal aspect " 



