280 



THE POPULAR EDUCATOR. 



METEOROLOGY. IV. 



LIGHTNING ST. ELMO'S FIRE AURORA BOREALIS OZONE 

 RAINBOW. 



CLOUDS are usually charged with electricity, sometimes posi- 

 tively and sometimes negatively. When two clouds charged 

 with different electricities meet one another, the fluid will dart 

 from one to the other in the form of lightning. If a cloud 

 highly charged be suddenly condensed into vapour, the electri- 

 city in it will be set free in a similar way, and a thunder-storm 

 will be produced. These are most common in tropical regions, 

 and, in general, the air is noticed to be peculiarly heavy and 

 close before their occurrence. 



Flashes of lightning most commonly pass from one cloud to 

 another, or else into the air. It is comparatively seldom that 

 they strike the earth. Four distinct classes of lightning have 

 been observed, viz., forked lightning, sheet lightning, summer 

 lightning, and ball or globular lightning. 



The electric fluid always selects the path which offers the 

 least resistance to its passage, and hence, when the flash is 



after that produced at the nearer end. The continued peal is 

 thus accounted for. 



By observing the interval between the occurrence of the flash 

 and the commencement of the thunder, the distance of the 

 storm can be easily estimated. A little over a mile should be 

 allowed for every five seconds. The duration of the peal is fre- 

 quently increased by reverberations and echoes from the clouda 

 or mountains near by. 



The mode of protecting lofty buildings from injury, by the 

 erection of lightning-conductors, has been explained in our 

 Lessons on Electricity. 



The remarkable appearance known as the Mariner's Lights, 

 or St. Elmo's Fire, which is frequently observed during storms 

 at sea, is another of the electrical meteors. A bright, flame-like 

 light is seen at the top of the masts, and sometimes at the ends 

 of the spars. This flame often points towards an approaching- 

 cloud, increasing in length as the cloud passes over it ; at times 

 it has been seen as much as two or three feet long. 



The appearance is easily accounted for, and a good illustration 

 of the same effect is seen by holding a sharp point near the con- 



Fig. 12. THE AURORA BOREALIS OR NORTHERN LIGHTS. 



powerful, it is frequently seen to assume a zigzag appearance, 

 or even to divide into branches, and is then said to be forked. 

 The sheet lightning is the most common form ; in it the flashes 

 are usually shorter and more rapid. Frequently they appear 

 to be within the cloud, and light up its whole surface. The 

 thunder which follows this is usually low and distant, and falls 

 on the ear in a long and muffled peal. Summer lightning is 

 closely allied to this, and is frequently seen at intervals during 

 the long summer evenings, illuminating the horizon with re- 

 peated flashes, unaccompanied by thunder. It is probably caused 

 in general by the reflection of a distant storm, and in some 

 places is regarded as a precursor of unsettled weather. 



The last form of lightning is by far the most dangerous, but 

 fortunately it is very rare. The lightning in these cases appears 

 'to assume a globular form, and moves much more slowly than 

 usual. Sometimes the ball comes in contact with some object, 

 ..and explodes with a loud report, at the same time doing con- 

 siderable damage to everything near. 



Thunder is the noise which is heard after a flash of lightning. 

 Ib is caused by the concussion of the air, as it rushes in and fills 

 the vacuum caused by the passage of the electricity. Flashes of 

 lightning frequently extend several miles in length, and are quite 

 instantaneous ; the thunder is therefore produced at the same 

 moment at every part of its path. The sound vibrations, how- 

 ever, only travel about 1,100 feet a second, and therefore the 

 sound from the further end will reach the ear several seconds 



ductor of an electrical machine. The electricity from the cloud, 

 instead of passing off in the form of a flash of lightning, is in 

 this way silently carried off by induction. In mountainous 

 regions travellers have occasionally noticed a somewhat similar 

 appearance at the end of their sticks or umbrellas, and a faint 

 hissing sound usually accompanies it. The air in this case is 

 highly electrical, and the pointed ends of the sticks have served 

 to attract the fluid and discharge it. 



Perhaps the most beautiful of all the luminous meteors is the 

 Aurora Borealis, or Northern Lights (Fig. 12). This is not 

 vinfrequently seen in our own latitudes, but as we travel into 

 polar regions it appears in greatly increased splendour, and 

 much more frequently. Its form varies very greatly. When 

 most perfectly developed, an arch of light appears to cross the 

 sky a little way above the horizon, and from this quivering 

 streamers dart upwards continually towards the zenith, giving rise 

 to the name of " the merry dancers," by which the phenomenon 

 is sometimes known. Frequently several auroral arches are 

 seen at once, and the effect is then very grand. At other times 

 the streamers appear to shoot up from behind distant hills. It 

 has been known also to assume the form of a huge curtain 

 suspended in massive folds, which reflect various colours. 

 When seen in England the aurora is almost always far less 

 distinct and perfect than any of these : at times its ruddy glow 

 has been mistaken for that of a distant fire. 



Various hypotheses have been started to account for these 



