Hail and Snoiv. 429 



presently collect to large drops. It is the property of elec- 

 tricity to reside only upon the exposed surfaces of excited 

 bodies, and hence the electricity, originally imparted to the 

 vapour by evaporation from the earth and sea, acquires rapidly 

 a high state of tension in exact ratio to the diameters of the 

 drops. The latter relieve themselves of their charge by flashes 

 of electricity to the neighbouring bodies of mist, or vapour less 

 advanced in concentration, and lightning and thunder are the 

 results. The drops so formed continue a rapid descent to the 

 earth. 



If we take four miles as a usual height for such flashes as 

 precede a storm of hail, we have a cold of 60 degrees below 

 the temperature upon the earth, or 30 degrees below freezing 

 in our ordinary summer temperature, for the condition of the 

 atmosphere where the drops are formed. That they are not 

 frozen upon the spot is partly due to the warmth of the gaseous 

 vehicle that accompanies the vapour to this height, but more 

 especially to the latent heat evolved in the condensation, which 

 maintains the whole at the dew-point of the existing vapour 

 until its last portions are condensed to the fluid form. With 

 our knowledge of the low intervening temperature, we need not 

 however, be surprised that the drops so formed in a moderate 

 temperature should be converted into pellets of ice before their 

 arrival at the earth. 



Why should we not then experience the very same processes 

 in winter that are so frequent in summer ? 



The mean temperature of the former season approximates 

 to the freezing-point of the scale and is more than 80 degrees 

 Fahrenheit below our summer mean. In consequence, the snow 

 line of the atmosphere has descended as it were two miles upon 

 our heads, and the source and fountain-head of our hail-storms 

 is extinguished. These are the very two miles of substratum 

 which in summer despatch vapour of high pressure and dew- 

 point in copious quantities to the colder regions above, and 

 whose most violent upheavals produce the phenomenon of hail. 



In winter such an elevated dew-point and pressure are rarely 

 attained, and never in any considerable quantities. The snow 

 that we may watch gradually wasting away upon the ground, 

 in frosts that are long and unbroken, even when severe, be- 

 speaks clearly the low dew-point and pressure of the aqueous 

 vapour at this season of the year. Let us consider what occurs 

 to such a body of vapour elevated suddenly to an unusual height 

 above the earth. 



An. experiment is easily performed, which is highly in- 

 structive in this inquiry. Conceive ordinary alum to be boiled 

 to saturation in a consistent cream of pipeclay and water, and 

 to be set aside to cool. The deposition in crystals is found 



