ON THE PROGRESS OF SCIENCE. XV 



the Smithsonian Institution weather reports every day. When these 

 are received, a man indicates the weather upon a large map of the 

 United States, hung in the public hall, by means of a system of 

 small cards and pins. For example, a green card was hung over a 

 point where it was snowing ; black, where it was raining ; brown", 

 where it was cloudy ; and white where it was clear ; and by this 

 means an observer was able to see at a glance the exact state of the 

 weather over nearly the whole of the United States, at the same 

 hour. As the storms of the United States generally travel east, they 

 were enabled, from the meteorological reports at 9 A. M., in Cincin- 

 nati and upon the Mississippi, to predict the state of the weather in 

 Washington twelve hours in advance, and could thus announce or 

 postpone their evening lectures, in conformity with the weather. 



Under the auspices of the Smithsonian Institution, Mr. Meech has 

 been for some time engaged in investigating the subject of the 

 partial absorption or extinction which the rays of solar heat experience 

 in passing through the atmosphere to the surface of the earth. The 

 phenomenon is one of special interest, and various instruments have 

 been devised for its measurement ; among which the pyrheliometer 

 of Pouillet, and the actinometer of Herschel, may be mentioned. 

 The observations with these instruments, says Mr. Meech, are cer- 

 tainly valuable and instructive, but, with one very doubtful exception, 

 they fail to exhibit any distinct law. The law of absorption not being 

 obvious directly from observation, the simple hypothesis has generally 

 been adopted that equal thickness or strata of the medium absorb 

 equal proportions of the light or heat incident upon each stratum. 

 Lambert, Laplace, Pouillet, and others, have expressed this assump- 

 tion in an analytic form, which applies very correctly at higher 

 altitudes and near the zenith. For low altitudes, Laplace combined 

 the same assumption with his theory of refraction, and derived an 

 approximate exprcs.-ion for the relative amounts. 



But the inquiry arises, how far the fundamental assumption is 

 sustained by experiments. During the trigonometric survey of 

 India, the astronomer, Jacob, observed the extinction of light reflected 

 through an extent of sixty miles of horizontal atmosphere. His 

 results were found to correspond very nearly with the law that "as 

 the first differences of distance increases in arithmetical progression, 

 the intensity of light diminishes in geometrical progression." The 

 experiments of Delaroche and Melloni also indicate that the hy- 

 pothesis of equal thicknesses, absorbing equal portions of the incident 

 heat, is only an approximation, which, in extended media, will 

 differ widely from the truth ; indeed, their experiments show an 

 increasing facility of transmission through equal strata in the direction 

 in which the rays proceed. 



The necessity of a change, therefore, in the theory of atmospheric 

 absorption, to render it conformable to such experiments, being 



