LIGHTNING CONDUCTORS.] 



METEOROLOGY. 



1153 



inside of the jar. The hand is represented in the act 

 of holding a glass rod, arouud which one end of the wire 

 is coiled, and the extremity of the wire is finished off 

 with a ball. All these arrangements, the electrician 

 will perceive, are necessary for giving effect to the 

 efficient discharge of the Leyden jar through the 

 conducting wire. The chief point for observation, 

 however, is the bend of the wire, by means of which 

 one part is caused very closely to approach another 

 part, as represented at a 6. Now, it is possible by 

 choosing a wire sufficiently small, and causing the two 

 bauds to pass sufficiently near, to determine the passage 

 of an electrical spark from a to 6, instead of proceeding 

 through the entire length of the wire, from the internal 

 to the external coating of the jar. This is what electri- 

 cians call the lateral discharge, and it requires to be stu- 

 diously guarded against in the construction and arrange- 

 ment of lightning-conductors. 



Api'lication of the foregoing Deductiom. Perhaps the 

 deductions already arrived at, will suffice for practical 

 guidance in the matter of lightning-rods, though these 

 deductions by no means exhaust the science of the sub- 

 ject. The fact may be considered as proved, that all 

 bodies, even the most dangerous and inflammable all 

 edifices, all living beings, may be shielded from the evil 



quences of lightning, by the safeguard of lightning- 

 conductors. The conductors, however, must present a 

 sufficient external area ; and the point has been made 

 out by numerous trials, that a copper rod, a square inch 

 in sectional diameter, will convey away the utmost fury 

 >t highly-charged thunder-cloud ever proved to 

 exist. Copper is one of the best electrical conductors 

 amongst metals ; but, by providing a sufficiently increas- 

 ing sectional area to compensate for inferior conducting 

 power, any metal may be matte to perform the function 

 of copper. Whatever be the conductor, its upper extre- 

 mity should project considerably above the edifice to be 

 protected ; and if pointed theoretically, all the better ; 

 though, practically, the bluntest termination could be 

 only as a point by comparison with the enormous DIM* 

 of a tlinndi-r-cloud. Far from preventing contact be- 

 tween the building to be protected and the conductor, as 

 is sometimes done by the interposition of glass or earthen- 

 ware guards, a lightning-conductor cannot be brought 

 into too intimate metallic connection with every part of 

 the edifice to be protected. The conductor should 

 branch and ramify over the surface of the building, and 

 should be brought into contact with every important 

 system of metal line work, such as the iron pipe which 

 f n .|ticiitly runs down the side of a wall ; finally, the con- 

 ductor should, at its lower extremity, be brought into 



ict as efficiently as possible with some good electric 

 conductor, such as the system of gas or water-pipes 

 which run underneath most houses, especially under the 

 streets of most civilised towns. As regards the number 

 of conductors necessary to be supplied to one building, 

 that will depend on the shape of the building, whether it 

 be composed of many elevations, or whether, like a 

 column, it has only one. Perhaps the best practical 

 testimony on this point is gleaned from the fact, that in 

 a ship having three masts, one of which only was pro- 

 tected by a lightning-conductor, the unprotected masts 

 have been shattered by the effects of a thunder-storm, 

 while the other remained untouched. If a column be 

 surmounted by a metallic statue, it is worse than useless 

 to disfigure the head of the statue by a projecting me- 

 tallic spike as the beginning of a lightning-conductor ; 

 nothing more is requisite, in this case, than to provide 

 sufficient metallic conduction for the electricity down- 

 ward* into the ground. Lightning-conductors, it should 

 be remembered, do not, as they are commonly said to do, 

 ( electricity. They no more attract electricity, 

 than a gutter attracts water. They merely open a 

 rluiimbl for electricity to pass through. Before the 

 demonstrations of Sir William Snow Harris had taken 

 effect, marine lightning-conductors were something more 



nuis than li^litniii^ itself: consist! ly of 



i-t, which were only elevated aloft after the thunder- 

 storm had come oil. Marino lightniug-coudu^tois are 

 VOL. i. 



now fixtures on the masts ; they are made of copper, 

 running band-like down the mast, and imbedded in the 

 latter in such a manner, that, whether the masts be ele- 

 vated or lowered, perfect metallic contact, between any 

 two pair of masts, remains uninterrupted. 



RELATIVE PREVALENCE or THUNDER-STORMS. The 

 phenomena of thunder and lightning are nowhere so 

 violent or so frequent as in the so-called region of calms ; 

 but they are very prevalent throughout the torrid zone, 

 more especially during the rainy season. Thunder and 

 lightning are almost always absent in the polar regions ; 

 and even at a spot no farther north than Bergen, in 

 Norway, the annual number of thunder-storms does not 

 average more thau six. As the rule, thunder-storms 

 chiefly occur in hot weather, winter thunder-storms 

 being comparatively rare. Iceland and the western 

 coast of North America, however, are remarkable for the 

 predominance of thunder-storms in winter. In Sweden, 

 winter thunder-storms are almost unknown ; thus fur- 

 nishing another example of the circumstance already 

 noted, that the Scandinavian range of mountains effects 

 a remarkable difference between the general climate of 

 Sweden and Norway, though the two, geographically 

 considered, are so close together. 



AEROLITES SHOOTING STARS METEORIC STONES.* 

 The beautiful phenomenon of shooting stars is common 

 enough ; but at certain periods it is peculiarly remark- 

 able, the whole sky being filled with these fleeting me- 

 teors. The beginning of August and the beginning of 

 November are noticeable for their connection with shoot- 

 ing stars ; more especially have they been recorded be- 

 tween the 9th and 14th of August. The bouquet of 

 shooting stars, observed at this period in North America, 

 has been sometimes called the Shower of St. Law- 



len '<. 



For our knowledge respecting the periodicity of the 

 phenomenon of falling stars, we are indebted to Quet- 

 elet, Besenberg, and others ; but Musohenbroek, so long 

 book as 1762, first directed attention to the so-called 

 Shvtcer of St. Lawrence. In addition to the August and 

 November phenomena of the kind under consideration, 

 other periods have been noticed for instance, in April, 

 and from the 6th to the 12th of December ; but these 

 bouquet* of shooting stars, whicli thus occur, are less con- 

 siderable and less regular thau the former. Besides 

 these showers, the periodicity of which is well attested, 

 single meteors of this kind are frequently noticed, and 

 they occur at all seasons ; therefore, whatever may be 

 the cause of shooting stars, it must be regarded as con- 

 tinuously operating. 



Sometimes the luminous meteor attains a largo magni- 

 tude ; observers then speak of it as a fire-ball. The 

 identity of shooting stars and fire-balls is now well estab- 

 lish^!, though formerly they were treated of as dis- 

 tinct. Fire-balls are sometimes seen alone, but more 

 frequently in connection with shooting stars. Their 

 light and their bulk are frequently so considerable, that 

 they can be seen in bro.-wl daylight. Their velocity 

 through the heavens, or rather through the upper layers 

 of our atmosphere, is various ; but it generally exceeds 

 that of the earth. As regards the nature of shooting 

 stars, we have only theory and analogy for our guidance ; 

 but our knowledge of fire-balls is far more accurate, and 

 there seems no reason to doubt that they quite illustrate 

 the nature of falling stars. Numerous fire-balls dart 

 through our atmosphere, become luminous, and disap- 

 pear, no one knows wliither. Others, though passing 

 near to our atmosphere, fail to enter it, and therefore 

 are not rendered visible. A third division not only come 

 within our atmosphere, shine, and burst with a loud 

 report, but they fall ; yet, falling either into the sea or 

 upon an unfrequented spot of land, the locality of their 

 fall remains unknown. Whilst a fourth division of fire- 

 balls may be seen to fall, be dug out, and examined. 



Masses of this kind are termed aerolites, and their 

 connection with fire-balls has been placed beyond all 

 duubt. Fire-balls have been seen to fall, and aerolites 



Further remarks oa thia subject will be fuund hi Chapter V., at the 

 end of thi* tectiou. 



