Lyons — The Distribution of Mean Annual Rainfall. 373 



rain-making, viz. the development of rain-drops from the first minute particles 

 of condensation. The vertical air-currents in the vicinity of mountains would 

 impel upwards any approaching cloud-masses, the products of previous and 

 independent condensation. This upward impulse would cause cooling and 

 produce further condensation on the already formed cloud-particles; further- 

 more, the eddying currents likely to occur in mountain systems would similarly 

 help in the growth of rain-drops. If, as is probable, coalescence plays a part 

 in rain-making, it is easy to see that mountains would tend to increase the 

 frequency and intensity of collisions by the aerial disturbances set up, and 

 the unequal velocities such disttirbances would impart to different-sized 

 drops. Moreover, if tlie coalescence process is affected by electrical con- 

 ditions, and if, as suggested by Lord Rayleigh's^ experiments on colliding 

 water-jets, coalescence is induced by differences in the electrical iield, it is 

 possible that the well-known inequalities" in the atmospheric electrical field 

 near mountains play a part in the formation of rain in such places. It is 

 proposed to develop these suggestions in a subsequent communication. 



In conclusion the author would express his obligations to the various 

 rainfall observers whose conscientious daily returns formed the groundwork 

 of this investigation, and to the British Rainfall Organization for rendering 

 the data so readily available for study. Indebtedness must also be expressed 

 to Dr. H. R. Mill and Sir John Moore, M.D., D.Sc, for help in the matter of 

 references. Acknowledgment should also be made of the special facilities for 

 visiting the gauges and districts afforded by the Department of Agriculture 

 and Technical Instruction for Ireland. 



1 loc. cit., p. 371. 



'^ Chwolson, " Traite de Physique," tome iv., ler fascicule, 1910. 



SCIENT, PEOf. R.D.S., VOL. XII., NO. XXX. 3 P 



