THE METEOROLOGY OF TEE FUTURE 29 



now enlarge the lower chamber so that the expansion may be in ratio of 

 500 to 760 or 1 to 1.5. Moreover, I will not allow any dust to enter the 

 upper globe, but will draw into that globe dustless air filtered through 

 this bit of cotton wool. If now I allow moist or saturated dustless air to 

 expand into the lower chamber from the pressure 760 to that of 500 or 

 an expansion of about 1.5 in volume then I shall form not a cloud of 

 small particles, but a few larger drops of water. This is the process that 

 must be going on within the thunder-cloud, or in fact inside any rain- 

 cloud. Out of the great mass of moist air that makes up the whole 

 cloud only a small proportion is free from dust and of that only a small 

 portion expands rapidly enough to form drops of rain-water. 



I think you will see that the firing of cannon or d3mamite in order 

 to make a great noise is not likely to form rain and in fact can not pos- 

 sibly bring it down. Neither can it prevent the formation of hail or 

 rain. If we wish to avert heavy rain or hail we must either cut off the 

 supply of moisture, or else prevent the rapid expansion, or else throw 

 dust upward into the air to cause cloudy condensation instead of rain. 

 Apparently this latter process is carried out for us in nature when great 

 forest fires afford enough particles of smoke to provide for the cloudy 

 condensation of the free moisture. From the great clouds of smoke that 

 attend these forest fires we get no rain until after a long time the heat 

 that is in the cloud is lost by radiation, or until larger drops are formed 

 by further expansion. 



Even the bombardment of a cloud by the explosion of dynamite 

 within it is inefficient to produce rain, in part because no violent con- 

 cussion can drive the cloud-particles together into large drops of rain, 

 and in part because the explosive itself furnishes more dust particles 

 and more nuclei of condensation and therefore produces clouds instead 

 of rain. In the same way the bombardment of the clouds by means of 

 vortex rings is ineflBcient. 



I have here an apparatus for making vortex rings of air ; you notice 

 that a slight stroke on the rear side of this box drives forward a vortex 

 ring of smoky air. It is a beautiful sight and very instructive in 

 many ways, but the special form of cannon devised in Italy to send such 

 rings of gunpowder smoke up into the clouds and break up the forma- 

 tion of hail does not usually send them higher than 1,000 feet; they 

 break up long before they reach the clouds. We have no evidence that 

 they ever reach them, or that they could have any effect if they did so. 

 If they carry up much dust they ought to have a slight effect in pro- 

 ducing cloudy condensation, and thus cutting off the formation of rain 

 or hail. But this effect is certainly too slight to be appreciable in our 

 statistics. I regret to think of so many thousands of farmers wasting 

 time and money on this delusion. You know that De Morgan after 

 spending much time in combating analogous delusions wrote an inter- 



