320 



SCIENCE. 



[Vol. V., No. 115. 



To a certain extent, the principal results have 

 already been given in various papers read be- 

 fore the National academ}' of sciences, and 

 printed more or less fully in the different 

 scientific journals ; but we now have, for the 

 first time, the details of the observations and 

 computations from which the results have been 

 derived, and are put in possession of the facts 

 necessary to a due appreciation of their weight. 



The first of the twent3'-one chapters of which 

 the report consists, is occupied with the prelim- 

 inary observations at Allegheny during 1880 

 and 1881, — observations which brought out 

 clearly the fallacy of most of the methods and 

 conclusions previously adopted, and the neces- 

 sity of a careful series of observations at some 

 elevated station. 



The second chapter contains an account of 

 the organization of the expedition under the 

 auspices of the signal-service, and gives the 

 story of the journey, with a description of 

 the stations. It is made quite clear that Mount 

 Whitne}' is a station every way adapted to the 

 purposes for which it was selected ; and every 

 one interested in science will most sincerely 

 join in the author's hope " that something more 

 than a mere ordinary meteorological station will 

 be erected here, and that the almost unequalled 

 advantages of this site will be developed by 

 the government." 



The third chapter contains a brief historical 

 summary of the actinometric work done by 

 various observers previous to 1880. We miss 

 in it, however, any allusion to the labors of 

 Secchi, Rosetti, and Waterston. 



The next five chapters are devoted to the 

 pyrheliometric and actinometric observations 

 made by the expedition, with all necessary 

 details as to the apparatus and methods of 

 reduction. Professor Langley condemns the 

 pyrheliometer of Pouillet as liable to give a 

 ^ very inaccurate determination of the quantity 

 of heat actually brought by a given sunbeam 

 under given circumstances ; and he appears 

 to consider the globe actinometer of Violle as, 

 on the whole, the best when the constants of 

 the instrument have been determined with suf- 

 ficient care. The summary of results in chap- 

 ter ix. makes it very clear, however, that the 

 mere inaccuracies of observation are not so 

 prejudicial to the satisfactory determination of 

 the ' solar constant ' as the use, in the reduc- 

 tions, of the fallacious assumption that the 

 amount of radiant energy transmitted through 

 an imperfectly transparent medium is given by 

 the long-accepted formula, C = Ea e , in which E 

 is the ' solar constant,' a a constant ' coefficient 

 of transmission,' and e the ' thickness ' of the 



air-stratum through which the ra3's penetrate. 

 To bring out this fallacj- is one of the author's 

 main objects ; and he sets it in a striking light 

 by certain comparisons, given on pp. 69 and 

 119, between the results obtained at Lone Pine 

 and at Mountain Camp, eight thousand feet 

 higher. We note, however, that, by a sort of 

 impish perversity of typographical luck, 1.797 

 is printed for 1.707 on the ninth line of p. 119, 

 making the printed figures egregiously con- 

 tradictory of the conclusions asserted in the 

 text. 



The fallacy consists in neglecting the fact 

 that the solar radiation is not homogeneous, 

 and in assuming, that, while such is the fact, 

 the formula given above is applicable, provided 

 one determines with care a sort of mean value 

 for a by the comparison of observations made 

 at different altitudes of the sun. In chapter x. 

 the author discusses the matter fully, and shows 

 mathematically that values of the solar con- 

 stant, obtained by reducing, according to this 

 formula, any possible actual observations, will 

 inevitably be too small, and probably very much 

 too small. 



Chapters xi., xii., and xiii. are taken up 

 with the description of the special apparatus 

 devised by the author to meet the difficulty, 

 and with an account of the observations made 

 with the spectrobolometer at Mount Whitney 

 and Allegheny ; other chapters are devoted to 

 the ' transmissibility ' of our atmosphere for 

 light, and to sky and nocturnal radiation ; and 

 others yet, include an interesting summary and 

 discussion of the hygrometric and barometric 

 observations. The report proper closes with 

 a general summary of results. As regards the 

 ' solar constant ' itself, the author's conclusion 

 is, that " at the earth's mean distance, in the 

 absence of its absorbing atmosphere, the solar 

 rays would raise one gram of water three 

 degrees Centigrade per minute for each nor- 

 mally exposed centimetre of its surface." Ac- 

 cording to this, the ' solar constant ' is three 

 (small) calories (gram degrees) per minute 

 per square centimetre, — equivalent, of course, 

 to thirty large calories (kilogram degrees) 

 per minute per square metre. The hitherto 

 received values range from twenty to twenty- 

 five. Other results of great importance are 

 also indicated, relating to the wave-length of 

 8 dark-heat/ the theory of the maintenance 

 of the earth's temperature by its overlying 

 atmosphere, the amount of absorption by this 

 atmosphere, and a number of other related 

 subjects. We have not room to quote them, 

 and they would better be read in their con- 

 nection. 



