M. De la Rive on the Formation of Hail. 281 



tion produced by the heating of the cloud by the action of the 

 solar ray can be the real cause of greater cold, when this evapora- 

 tion takes place only in virtue of the larger quantity of heat 

 which is supplied to the liquid ? Who, again, can conceive that 

 the electrical power exercised by bodies so light as the clouds can 

 sustain and neutralize the action of the weight of the hailstones, 

 amounting sometimes to half a pound ? or, finally, how can we 

 suppose that two clouds can continue so strongly electrical that 

 they can move heavy masses when they are so near each other, 

 and separated only by an extremely humid stratum, through 

 which the electricity might freely pass from the one cloud to the 

 other ? 



Such are some of the objections to which the theory of Volta 

 is liable, and which M. Arago points out in the article just 

 referred to. It was the difficulties in which the theory is in- 

 volved that led the Academie des Sciences in the year 1830, to 

 appoint the best explanation of the phenomena of hail as the 

 subject of the great prize which fell to be delivered in 1832. 

 The conditions on which it was to be conferred were severe. 

 The competitors were to supply a theory supported by direct 

 experiments, and upon varied observations made, if possible, in 

 the very regions in which the hail was formed, and which 

 might replace the vague hypothesis with which we have been 

 compelled to be satisfied up to the present time. The essayists 

 were also recommended to avail themselves of all the accu- 

 rate information which had hitherto been collected on the radia- 

 tion of caloric, on the temperature of the atmosphere at different 

 elevations, on the cold produced by evaporation, and upon elec- 

 tricity, &c. : finally, they were required, whilst treating of the 

 formation of hailstones, to follow out the consequences of the 

 theory they should adapt to its numerical applications, regard- 

 ing the physical constitution of these hailstones, also respecting 

 the enormous bulk they sometimes acquire, and as to the season 

 of the year, and the times of the day in which they were most 

 commonly observed. But in 1832 the prize was not conferred, 

 because none of the memoirs presented were considered worthy 

 of the honour ; and the Academie again appointed this subject 

 as the question for competition for the year 1834. Again, how- 

 ever, none of the essayists fulfilled the conditions proposed, the 



