ELECTRICITY. 



75 



If, therefore, any of oar readers should be inclined to repeat 



very precaution most be taken. A very good 

 ii c<-nl ui>on a roller, contained in a metal 



a Htout glass rods. A glass handle should 

 bo fixed to a multiplying wheel, so as to pay out or coil tip 

 the cord, and a largo boll, connected with the ground by means 

 of a stout wire or chain, should be placed within an inch or an 



A n halt' of the metal case. If then the power becomes 

 80 groat as to bo unsafe, the electricity will dart to this ball, 

 and by it to the ground. 



iiuiootion with thin it must bo remembered that the 



sparks thus drawn from a kite have far greater power than 



those from an ordinary electrical machine. Sparks two or three 



inches long may bo token from the latter without any incon- 



.o, while oao only half an inch long from the kite will 



o oa jMJwerful as a shook from a Leydon jar ; the power, 



however, doos not always depend upon the length of the 



spark. 



-::il<irg frequent observations upon the electrical condi- 



the atmosphere, it is needful to have something more 



:iont and manageable than a kite. A tall polo erected on 



the top of a house or lofty building, or in somo elevated 



^vers well. On the upper extremity of this there 



is fixed a thick glass rod, surmounted by a number of points, 



or a largo metal fork, so as to attract the electricity. 



The lowor part of tho metal should be fitted with a kind of 

 cap, so as to keep tho glass rod as dry as possible, and thus 

 preserve the insulation in damp weather. A wire is then 

 attached to tho metal, and being carefully insulated is brought 

 down to tho wall of tho laboratory, into which it enters through 

 a gloss tube. It is necessary, however, to fix a large ball in 

 connection with tho ground, within about two or three inches of 

 the wire, before it enters the room, so that in cose of a flash of 

 lightning striking it the current may escape in this way. A 

 similar boll and rod should also be placed inside the room, so 

 arranged that the distance between it and the knob in which tho 

 wire terminates may be adjusted to any required distance. In 

 this way the current may be allowed to escape quietly to tho 

 ground when it is not required. A peal of bells may also be 

 attached, so as at once to call attention to the passing of a 

 highly charged cloud. 



The most complete apparatus for the collection of atmospheric 

 electricity was that fitted up by Mr. Andrew Crosse, at Broom- 

 field, near Taunton. This gentleman collected tho electricity 

 by means of insulated wires supported on poles fixed to lofty 

 trees, and carried quite round his grounds. The insulators were 

 so constructed as to be as little as possible affected by tho 

 dump, and the length of the exploring wire, as it was called, 

 was upwards of 2,000 feet. Theso wires terminated in a large 

 insulating conductor placed on his table, near which was 

 another boll, the distance of which could be regulated by means 

 of a screw. By this the fluid escaped to the ground. 



The effects produced by this apparatus during stormy weather 

 were said to be awfully grand. A brilliant stream of fire, 

 almost too bright to look at for any length of 'time, passed 

 between the balls, and the sound was compared by some to tho 

 rattle of small fire-arms. His battery consisted of fifty largo 

 jars, and required more than 200 turns of a 20-inch plate 

 machine to charge it ; yet, when connected with this conductor, 

 it has been charged and discharged as many as twenty times in 

 a minute. 



From observations made with apparatus of this kind, we 

 find that with a clear sky the air is usually charged with 

 positive fluid, the intensity being greatest at sunrise and sun- 

 set. When, however, a large thunder-cloud passes over tho 

 apparatus, a series of very strange and interesting phenomena 

 is seen. As soon as tho edge of the cloud is over the wires, 

 a .number of discharges take place ; there is then a short 

 pause, and another series of discharges, the electricity being of 

 the opposite kind ; then another interval ; and in this way 

 there appear to bo a number of zones in tho cloud, alternately 

 charged with positive and negative fluid.' These increase in 

 intensity as they approach the centre of the cloud, and when 

 this part is over tho conductor tho effect is almost fearful to 

 gaze upon. Wo must not, however, dwell further upon this here; 

 a fuller account can be found in somo works on tho subject. 



One very simple mode of collecting tho atmospheric elec- 

 tricity is to fix a metal clip at the end of a fishing-rod. Place 



in this some lighted amadou, or a sponge dipped in spirit and 

 lighted, and let a wire connected with it ran along the rod. If 

 now it be made to project out of an upper window and carefully 

 insulated, the smoke or flame will collect tha electricity, and it 

 may then be examined. 



During a fog the air is usually Tory highly charged, and 

 though all insulators most become damp and therefore imper- 

 fect, great quantities of electricity are frequently collected. 

 The whole theory of fogs is as yet Tory imperfectly understood, 

 but it seems highly probable that electricity is an important 

 agent in their production, and some hare even gone so far as to 

 believe that as our acquaintance with their electrical phenomena 

 increases, we may even be able to disperse or prevent them. 

 Though this idea appears highly extravagant, it in still clear 

 that this is an important field for scientific inquiry. 



Tho investigation of the causes of atmospheric electricity has 

 engaged the attention of many scientific men, and many dif- 

 ferent hypotheses have been started. One of the principal 

 sources appears to bo the evaporation of water from the surface 

 of the earth. Tho production of electricity in this way is shown 

 by placing a heated platinum capsule on a condensing electro- 

 scope, and placing a few drops of water in it. As soon as it 

 has evaporated, and the collecting plate is raised, tho leaves 

 will diverge slightly. If, however, distilled water be employed, 

 no effect will be produced. 



Clouds arc in general charged with electricity, but in storm 

 clouds tho charge is much more intense, and if two such clouds 

 oppositely charged approach ono another the electricity darts 

 from one to the other in the form of lightning. This kind of 

 lightning is by far tho most common, and is not dangerous. 



At times, however, the clouds are highly charged with similar 

 electricity, and the electric fluid then darts to the earth, often 

 striking any lofty or pointed object that happens to bo near. 

 A violent shock is sometimes produced by what is called the 

 return stroke ; this is produced by the action of induction, 

 When a highly-charged cloud is i.ear any object, the hitter 

 becomes charged by induction with the opposite electricity. 

 As soon, however, as the cloud is discharged this inductive 

 action suddenly ceases, and the result is a shock. A similar 

 effect is felt on standing near the conductor of a very large 

 machine while sparks are being drawn from it. If we watch 

 the motions of the clouds during a thunder-storm, we shall see 

 their attractions and repulsions giving signs of electric action. 

 We have seen that pointed and lofty objects attract electricity, 

 and thus we shall understand the action of lightning conductors. 

 To lofty buildings there is usually affixed a metallic rod, ending 

 in a point, or series of points, projecting some way above the 

 highest part of the building. The lower end of this is connected 

 with a plate of metal buried in the ground, or placed in a reser- 

 voir of water. If then a lightning-cloud passes over the 

 building the fluid usually passes off silently by this, and is 

 conveyed to the ground. 



It is found in practice that a conductor protects all around it 

 within a radius of about double its own height ; for instance, if 

 it projects twelve feet above the roof of a building it will 

 protect all around a space included within a radius of twenty- 

 four feet. 



Sometimes a lateral shock is experienced, that is, a portion of 

 the fluid darts off to a good conductor near by. To avoid this, 

 all pieces of metal near tho conductor should be placed in 

 metallic connection with it. On account of this kind of dis- 

 charge it is very dangerous to stand under a tree during u 

 storm, as the fluid may strike the tree, and then leave that to 

 enter the body of the person standing under it. It is always 

 safer then to stand at a little distance from it ; disregard of 

 this has cost the lives of many who, in seeking shelter from the 

 rain, have been struck. The neighbourhood of water should 

 also bo avoided. 



Tho Aurora Borealis, or Northern Lights, is another very 

 beautiful manifestation of aerial electricity. It is apparently 

 caused by its presence in the upper and highly rarefied regions 

 of the atmosphere. Faint imitations of it hove been produced 

 by passing tho current through exhausted tubes, and recent 

 experiments have shown that tho presence of certain vapours 

 greatly modifies the results, and produces a more striking 

 resemblance to the grand natural phenomena. In polar regions 

 tho aurora is seen much more frequently than in our own 

 latitude, and is much more brilliant. 



