466 Lord Rayleigh's Notes on some 



of air, which have acquired a tendency to descend. We must 

 thus have an outer descending current formed around the 

 ascending one, by means of which as much air descends as 

 has been carried up by the ascending current. If the dis- 

 turbance of equilibrium embraces extended upper and lower 

 strata of air, the descending masses will produce an increase 

 of pressure in the neighbourhood of the cyclone, gradually ex- 

 tending to the surface of the earth, and on the other hand into 

 the highest regions of the air, and transfer its vis viva con- 

 tinually to new superheated masses of air which ascend in 

 the cyclone, whilst a part of the descending external cyclone 

 circulating in the same direction ascends again with the inner, 

 and transfers to it a part of the vis viva gained in the higher 

 regions of the air. The course of the centre of the cyclone 

 is then determined by the direction of the mean velocity of 

 all the air-masses forming the cyclone, and its duration is that 

 of the disturbance of the neutral equilibrium of the atmosphere 

 which called it forth and maintains it. 



In conclusion I will only mention that the suspicion which 

 I have previously expressed that aqueous vapour can be cooled 

 beyond its usual condensing-point without condensation 

 taking place, in the same way as water can be cooled below 

 its freezing-point without solidifying, has been confirmed by 

 recent investigations of Robert von Helmholtz. We have 

 thus the explanation of the remarkable fact that the ascent of 

 air containing so much aqueous vapour from the tropical seas 

 is not followed by ceaseless rain. We may now assume that 

 the aqueous vapour in the absence of dust and particles of 

 water reaches the higher regions of the air without con- 

 densing. 



It follows, further, that a local ascent, like a solar pro- 

 minence, which must reach the higher regions and carry 

 with it dust and particles of water, may, by condensation of 

 the aqueous vapour of these strata of air, bring about the 

 tremendous rainfall which we observe. Further, we have the 

 explanation of the quantity of water which the equatorial 

 current conveys to the temperate zones. 



LXIV. Notes, chiefly Historical, on some Fundamental Propo- 

 sitions in Optics. By Lord Rayleigh, Sec, R.S., DC.L* 



IT is little to the credit of English science that the funda- 

 mental optical theorems of Cotes and Smith should have 

 passed almost into oblivion, until rediscovered in a somewhat 



* Communicated by the Author. 



