Page:Science vol. 5.djvu/346

 To a certain exteut, ibe principiil results have already been given in vaiioua papers read be- fore the National academj' of sciences, and printed more or less fully in the different scientific journals; l>ut we now have, for the first time, the details of the observations and computations from which llie results have been derived, and are put in [possession of the facts necessary to a due appreciation of their weigbt.

The first of the twenty-one chapters of which tbe reiiort consists, is occupied with the prelim- inary obser%-ations at Allegheny during 18HU and 18SI, — observations which brought out clearly the fallacy' of most of the methods and coDclusions previously adopted, and the nece.s- aitj' of a careful series of obsei-vations 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 Whitney is a station every way adapted to the purposes for which it wiis 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 uiietjualled advantages of this site will be developed b>' 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, Hosetti, and Wateraton.

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 pyrhcliometer 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 inaociiracies of observation are not so prejudicial to the Batisfactory determination of the 'solar constant' as the use, in the reduc- tions, of the fallacious assumption that the amount of radiant energy transmitted throngli an imperfectly transparent medimn is given by the long- accepted formula, = Ea', in which ^ is the ' solar constant.' a a constant * coeflicieut of transmission,' and c the ' thickness ' of the

���air-stratum through which the rays penetrate. To bring out this fallacy 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 FHne and at Mountain Camp, eight thousand feet higher, Wc note, however, that, by a sort of impish perversity of typc^raphical luck, 1.797 is printed for 1.T07 on the ninth line of p. 119, making the printed figures egregiouslj' con- tradictory of the conclusions asserted in the test.

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 bj' the comparison of observations made at liifferent 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, arty possible acCuai observations, uiill ineoUabljt 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 speclro bolometer 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 ob.servations. The report proper closes with a general summary of results. As regaMa 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 raj's would raise one gram of water three degrees Centigrade pier 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 [jer square centimetre, — equivalent, of course, to thirty large calories (kilogram degrees) [>er 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 'dark-heat,' the theory of the maintenance of the earth's temperature by its overlying atmosphere, the amount of absorption bj' this atmosphere, and a number of other related subjects. We have not room to quote them, and they would better be read in their oou- nection.

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