258 ELEMENTARY LESSONS ON [CHAP. iv. 



that the conductor between the apex and the ground be 

 perfectly continuous and of sufficient conducting power ; 

 (4) that the leads and any iron work or metal work about 

 the roofs or chimneys be connected by stout wires with 

 the main conductor. Too great importance cannot be 

 attached to the second and third of these essentials. 

 A copper rod of one square centimetre of sectional area 

 would probably form a trustworthy conductor. Maxwell 

 has proposed to cover houses with a network of con- 

 ducting wires, without any main conductor, the idea 

 being that then the interior of the building will, like 

 Faraday's hollow cube (Art. 31), be completely pro- 

 tected from electric force. Preece has lately calculated 

 that a lightning-conductor of a given height above the 

 surface of the ground will protect from the external 

 action of electricity a conical space the radius of whose 

 base is equal to the height of the rod, but whose side is 

 hollowed in the form of a quadrantal arc. 



3O6. Atmospheric Electricity. In 1752 Le- 

 monnier observed that the atmosphere usually was in 

 an electrical condition. Cavallo, Beccaria, Ceca, and 

 others, added to our knowledge of the subject, and 

 more recently Quetelet and Sir W. Thomson have 

 generalised from more careful observations. The main 

 result is that the air above the surface of the earth is 

 usually, during fine weather, positively electrified, or at 

 least that it is positive with respect to the earth's 

 surface, the earth's surface being relatively negative. 

 The so-called measurements of " atmospheric electricity " 

 are really measurements of difference of potential between 

 a point of the earth's surface, and a point somewhere in 

 the air above it. In the upper regions of the atmosphere 

 the air is highly rarefied, and conducts electricity as do 

 the rarefied gases in Geissler's tubes (Art. 292). The 

 lower air is, when dry, a non-conductor. The upper 

 stratum is believed to be charged with + electricity, 

 while the earth's surface is itself negatively charged ; 



