NO. 4 SOLAR RADIATION ABBOT, FOWLE, AND ALDRICH 20, 



has confused that discussion with our definitive determination of the 

 solar constant of radiation, of which it forms no part at all. We do 

 not care to discuss, at the present time, the coefficients for terrestrial 

 radiation, as we are engaged in investigations of this matter which 

 are not as yet completed. It has no bearing upon the definitive values 

 of the solar constant obtained by us. 



As for the dependence of the transmission of solar rays upon 

 atmospheric water vapor, we have employed the hypothesis of 

 Langley, namely, that there will be no water vapor outside the 

 atmosphere. This gives us the highest results which can properly 

 be reached. As we shall see in the conclusion of this article, our 

 results obtained in this manner are supported by another line of 

 investigation. 



Fourth objection. — We perhaps do not understand just what Mr. 

 Very has in mind in regard to this. Certainly there is no sheet of 

 ice or anything of a continuous surface to be found in the air, so far 

 as we know, which would answer to the description of the conditions 

 referred to in the fourth objection. Some approach to it may be 

 found in the case of a cloud. But we have repeatedly ascended from 

 Pasadena to Mt. Wilson through clouds, and even in this case we 

 always perceived that the upper edge of the cloud had a gradual 

 thinning out for at least many meters. We do not conceive that 

 there is any other layer in the atmosphere for which this is not true. 

 A transition extending through at least many meters is all that we 

 require when we speak of a " gradual " change of transparency from 

 one atmospheric layer to another. 



As Mr. Very hints, there are irregularities in the distribution of 

 the various bodies of air. For instance, in the neighborhood of a 

 mountain there are currents of air of different temperatures rising 

 and falling along the slopes. These, to be sure, do not fall into the 

 horizontal layers postulated in our hypothesis of the atmospheric 

 transmission, but they disturb the regular distribution in altitude 

 so little relatively to the whole thickness of the atmosphere, and 

 furthermore, the differences of atmospheric transmission of these 

 different bodies of air from their immediate surroundings are so 

 slight, that their influence on the transmission coefficients which we 

 obtain may be neglected. 



Fifth objection. — We understand that it is here claimed that the 

 general, apparently non-selective, losses to which the solar beam is 

 subject in passing through the atmosphere are due not only to the 

 scattering of radiation by particles small as compared with the wave 

 length of light as indicated by Lord Rayleigh's theory, but also to a 



