254 



river, if it flows off witii half its usual rapidity. This is the ordi- 

 nary \vay in which frost acts on the rivers in this country. But 

 when, as on the night of the 26th November, the frost is accom- 

 panied with a strong and keen wind, which lasts for only a few 

 hours, it freezes the water in the small rivulets near the sources of 

 the rivers in high and exposed sitnations, whilst it has not time to 

 freeze even the surface of the deeper and more rapid currents flow- 

 ing in the lower parts of the rivers. 



The easterly gale which, by its low temperature, produced this 

 phenomenon, continued to blow until about 7 or 8 a. m. on the 

 morning- of tlie 27ih November. The temperature of the at- 

 mosphere then underwent a sudden change, as indicated both by 

 the barometer and the thermometer. This change was brought 

 about by the advent of two storms, which came from southern lati- 

 tudes, and one or perhaps both of which had, on the morning of the 

 37 th, begun to aflr'ect the upper regions of the atmosphere, and load 

 tbem with warm vapour. 



2. Memorandum on the Intensity of Reflected Light and 

 Heat. By Professor Foi"bes. 



"At the meeting of the Society on the 4th February, I remarked, 

 on the occasion of Professor Kelland's paper on the Intensity of 

 Reflected Light, that it was almost without a parallel in science, 

 that a quantitative physical law like that of the intensity of the re- 

 flection of light at diff'erent angles, should have first been divined 

 by the rare sagacity of Fresnel, and confirmed by the very diflrerent 

 but elaborate mathematical investigations which Mr Green of Cam- 

 bridge and Professor Kelland have applied to the subject, whilst 

 scarcely any attempt has been made towards its verification by di- 

 rect experiment. 



" Some critical cases for polarized light were indeed assumed as 

 the basis of the original formula ; and M. Ai*ago has confirmed 

 it by one or two intermediate photometrical experiments : but the 

 chief evidence for the truth of this remarkable law rests on the in- 

 direct observation of the change of tlie plane of polarization of an 

 incident ray after reflection. 



" It occurred to me, about the end of 1837, that the anomalies of 

 photometrical observations being nearly as unsatisfactory as ever, 

 some light might be thrown upon this important subject by ascer- 

 taining the law in the case of heat, the intensity of which we have 



