480 



this report will be printed I shall refer you to it, feeling that I could 

 not, by anything that I might say, add to its effect on your minds. 

 I must, however, be allowed to congratulate Mr. Forbes on these 

 researches, as one of the fruits of his arduous and meritorious la- 

 bours amid the eternal ice and snows of the loftiest region of Europe. 

 Mr. Forbes is now fairly enlisted in that enterprising scientific band 

 which looks up to De Saussure as its leader. His researches into 

 the law of extinction of the sun's rays is but a poition of the valu- 

 able results that he has obtained among the mountain solitudes, 

 where, though vegetation scarcely exists, and animal life is equally 

 rare, the eternal glacier itself is ever pursuing its gradual and silent 

 course : — silent, till it is interrogated by a philosopher endowed v*dth 

 the energy and perseverance of a De Saussure in the eighteenth, or 

 a Forbes in the nineteenth century. 



Mr. Wheatstone, 



I now present you with this Medal, one of those entrusted to the 

 President and Council of the Royal Society by Her Most Gracious 

 Majesty, for your paper entitled " An account of several new Instru- 

 ments and Processes for determining the Constants of a Voltaic Cir- 

 cuit." This is not the first time that I have had the pleasing task 

 of acknowledging, on the part of the Royal Society, the great inge- 

 nuity as well as knowledge that you bring to the increase of science. 

 You not only add to our store of knowledge, but you give to others 

 the means of doing so too. You not only set the example of 



varies in geometrical progression as the mass of air traversed varies in 

 arithmetical, Mr. Forbes, calculating on the whole series of observations 

 in question, concludes an extinction of 31^ per cent, of the incident heat- 

 ing rays in passing vertically through the atmosphere under the conditions 

 of mean barometric pressure, and a dampness such as prevailed on the 

 average during the day of observation, thus appearing to afford a con- 

 firmation at once interesting and unexpected of the results of Bouguer and 

 Lambert, as deduced on a similar hypothesis, from their experiments on 

 the extinction of light, though properly speaking it is impossible to argue 

 from one case to the other. 



" But Mr. Forbes adduces a great many considerations, both theoretical 

 and practical, in proof that such a law of extinction cannot be that of 

 nature — the incident heat being analysed in its progress, and so rendered 

 relatively more transmissible after passing through a certain thickness of 

 the medium than before it (a conclusion grounded on the discoveries of 

 M. De la Roche, M. Melloni, and his own) ; and secondly, laying aside every 

 theoretical consideration and obtaining from the series of obseiTations 

 under discussion an empirical formula, by means of an interpolating curve, 

 expressing the rate of loss of intensity of a solar ray which has been 

 transmitted through a varying atmospheric thickness, in traversing the 

 stratum immediately subsequent, he finds for the result of this inquiry a 

 rate corresponding to the ordinate of a logarithmic curve, having its 

 asymptote not passing through the origin of the coordinates, and thence 

 deduces the following remarkable conclusions, which, as a result of experi- 

 ment and direct observation, I conceive to be of great interest, viz. — 



" 1st. The extinction of solaa' heat in traversing vertically an atmosphere 

 mechanically pure and of mean barometric pressure, amounts to 0*466 of 



