Page:Proceedings of the Royal Society of London Vol 4.djvu/415

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Thomas Chapman, Esq., was balloted for, and duly elected into the Society.

Richard Quain, Esq., was also balloted for, but was not elected into the Society.

A paper was in part read, entitled, *' On the Transparency of the Atmosphere, and the Law of Extinction of the Solar Rays in passing through it." By James D. Forbes, Esq., F.R.S., Sec. R.S. Edinb., Professor of Natural Philosophy in the University of Edinburgh.

The reading of a paper, entitled, " On the Transparency of the Atmosphere, and the Law of Extinction of the Solar Rays in passing through it." By James D. Forbes, Esq., F.R.S., &c., was resumed and concluded.

This paper is divided into seven sections. In the first, the qua- lities of heat and light are considered in as far as they modify the comparability and absolute nature of our measures of the influence of the solar rays. All instruments, whether called Thermometers^ Photometers^ ov Actinometers, measure but the peculiar effect to which their construction renders them sensible, but are incompetent to give absolute measures of either heat or light.

The second section treats of the history of the problem of the law and measure of extinction of the solar rays in passing through the atmosphere of the earth in clear weather. The labours of Bouguer, Lambert, De Saussure, Leslie, Herschel, Kamtz and Pouillet are successively passed under review, and their instrumental methods considered.

In the third section, a mathematical problem of considerable difficulty and interest is investigated ; principally after the manner of