132 W. RAMSAY ON THE PRECIPITATION 



these phenomena. It is generally agreed that the molecules of a 

 fluid acquire, when heat is applied, a greater amplitude of vibra- 

 tion, hut that the duration of each vibration remains constant. 

 To take the simplest case, suppose a particle a vibrating in 

 a horizontal plane, and that it performs each vibration in equal 

 times, whether the amplitude of the vibration remain constant or 

 not. Another particle b is descending at right angles to the piano 

 of vibration of a. If the amplitude of the vibration of a be doubled, 

 the particle b will have twice as great a chance of avoiding collision 

 with a as before ; for the particle a will traverse a given space in 

 half the time it formerly took, leaving it vacant the other half. 

 The particle b will therefore be at liberty to fall through this space 

 for twice as long a time as was at its disposal before the amplitude 

 of vibration was doubled. 



Of course the problem is infinitely more complicated than the 

 very rough and ready instance I have given; but I think it is 

 simply an extension of the phenomenon, and can therefore be ac- 

 counted for on the same principle. 



Note by Prof. A. C. Eamsay, F.E.S., P.G.S. 



Though the author docs not draw any geological inferences from 

 the foregoing data, it is yet obvious that they have geological 

 bearings. Thus, given equal amounts of mud in suspension in a 

 salt lake or in the sea, and in a freshwater lake, the water of each 

 being still, a greater quantity of sediment in a given time will be 

 precipitated to the bottom in the salt water than in the fresh water, 

 and the amount of salt in solution will relatively retard or hasten 

 precipitation. But supposing mud in suspension to be carried into 

 the sea by a river, tho mixture of fresh and salt water, the current 

 at its mouth, and the movements of the sea beyond that current, 

 due to winds, tides, and ocean currents, will also affect the question 

 to such an extent that under these circumstances no absolute rate 

 of precipitation can be estimated in a given time, over a given area, 

 for a given quantity of mud. Still salt water y as compared with 

 fresh water, must under like circumstances have an effect — as, for 

 example, in the case of the mud carried by rivers into the great 

 lakes of America and into Hudson's Bay, or the mud carried into 

 the salt lakes of Central Asia, and that carried into the large lakes 

 of Switzerland and the north of Italy. Examples of fresh and salt 

 lakes occurred in old geological times ; take the case of the Swiss 

 Miocene deposits for the first, and of the British Permian and 

 Triassic (and some other areas) for the second ; and the rates of 

 deposition of mud in such instances may possibly have been modi- 

 fied by the freshness or saltness of the water. 



Discussion. 



Prof. Hughes inquired whether the observations had been suf- 

 ficiently carefully made to determine the amount cf precipitation in 



