NATURAL PHILOSOPHY. 179 



frigorific actions, successively excited in the radiating body, and the 

 medium which envelopes it, is the congealation of water, produced 

 artificially in Bengal, during the calm and clear nights. And it is 

 also in consequence of these same frigorific actions that the buds of 

 plants, and the shallow waters of ditches and ponds, scattered here 

 and there over the country, often freeze during the calm, clear nights 

 of spring, whilst the thermometer marks several degrees higher than 

 the freezing point. 



OX THE CAUSES OF THE EXCESS OF THE MEAX TEMPERATURE 

 OF RIVERS ABOVE THAT OF THE ATMOSPHERE. 



MR. RAXKIXE, at the Belfast meeting of the British Association, 

 stated : M. Renou having for four years observed the temperature of 

 the river Loire, at Vendouie, as compared with that of the atmos- 

 phere, has found that the mean temperature of the river invariably 

 exceeds that of the air, by an amount varying from l^ to 3 

 Centigrade, and averaging 2.24 Centigrade, and a similar result has 

 been deduced from observations made by M. Valin on the Loire at 

 Tours. M. Renou and M. Babinet account for this fact by the re-ra- 

 diation from the bed of the river of solar heat previously absorbed by 

 it. Mr. Rankine thinks this supposition inadequate to account for 

 the facts ; because the excess of temperature of the river over the 

 air was considerably above its mean amount in November, and very near 

 its maximum in December ; and because the mean diurnal variation 

 of temperature of the river was much less than that of the air. He 

 considers that friction is more probably the principal cause of this 

 elevation of temperature ; for if water descends in a uniform chan- 

 nel, with a uniform velocity, from a higher level to a lower, the whole 

 power due to its descent is expended in overcoming friction ; that is 

 to say, is converted into heat, as the experiments of Mr. Joule have 

 proved. This must cause an elevation of temperature, which will go 

 on until the loss of heat by radiation, conduction, and evaporation, 

 balances the gain by friction, and at this point the temperature of the 

 river will remain stationary. 



OX THE REFRACTIOX OF SOUXD. 



THE unequal rapidity of the propagation of sound in different 

 mediums, enables us to show the refraction of the sonorous undulations 

 when it passes fr om one medium to another. The existence of such a 

 refraction, though placed beyond doubt by the theoretical researches 

 of Poisson and Green, was never shown by direct experiment before 

 the investigations of M. Sondhauss. Upon examining the action of a 

 lens upon sonorous waves, M. Sondhauss verified the existence of this 

 refraction. As solid lenses were not suitable for the experiment, he 

 was led to prepare gaseous ones, by filling a membranous envelope in 

 the form of a sphere, or lenticular shaped, with gas. At first he pre- 

 pared a globe of golcl-beater's skin, about one foot diameter, filled with 

 18* 



