HARD WICKE 'S S CIENCE- G OSSIF. 



i53 



Sir \V. Snow Harris, in his treatise on electricity,* 

 suggests the following as a brief explanation of the 

 discharge between the clouds and the earth: "If,'' 

 says this writer, "we consider attentively the elec- 

 trical conditions of a thunderstorm, we may observe 

 in them all the elements of the Leyden experiment : 

 the atmosphere in fact becomes a great coated pane, 



regard to the air which forms their base. Thus one 

 large, leading cumulus may become a centre of 

 force ready to operate, not only on the earth beneath, 

 but on various collateral masses of the surrounding 

 vapour, when the general equilibrium is disturbed. 



The tension, then, becoming too great, and the 

 balance of forces being disturbed, the discharge 



Fig. 123. — Approach of Electrical Cloud-masses. 



Fig. 124. — Approach of Electrical Cloud-masses, seen in perspective. 



or fulminating square, of which the charged cloud is 

 the insulated and the surface of the earth the unin- 

 sulated, terminating conducting planes ; the phenomena 

 of thunder and lightning are neither more nor less than 

 disruptive discharges through the intervening air." 



In explaining the thunderstorm by reference to the 

 principles of electrical induction, and of the disruptive 

 discharge, the reader will observe that much depends 

 upon the condition of the air contiguous to the earth and 

 subjacent to the cloud — in regard to conductive power. 



And it may well happen that large masses of cloud, 

 separated, perhaps by intervals of several miles, may 

 be very differently situated in this respect, with 



* Sixth ed. Virtue & Co. 



ensuing would connect together the different points — • 

 centres of the masses — and these again with the earth. 

 The instantaneous dissolution of a polygon of forces 

 in this way, would, through the electric current, 

 darting from point to point, involve an interlacing or 

 net-work pattern in the lightning flash. 



Vast, however, as is the force which the lightning 

 wields, an apparently slight circumstance may direct 

 its course. The configuration of the earth beneath 

 as affecting the upward vapour currents ; the presence 

 of smoke or metallic dust might be responsible for 

 effects most disastrous to man ; so delicately balanced 

 are the forces of nature. 



Such atmospheric conditions may perhaps be 



