as a conductor. Bed is the safest place, as blankets are non-conductors. Cellars are not the safest by any means. Lightning may, and it frequently does, strike the house and descend to the basement. If the air be very full of electricity, and a flash be near, a person running away may conduct the lightning to himself by creating a vacuum into which the flash may dart.

Arago classified lightning into three kinds zig-zag, globular, and sheet. The first we call forked lightning, and frequently this kind branches out at the end, so that although there may be only one flash, it may strike out in two or three directions at the same time. This may be accounted for by the unequal power of the air strata to conduct the electricity. The forked flashes are of very great length, extending frequently for miles, and the bifurcations also are often miles apart. The duration of the flash is less than the thousandth part of a second; so instantaneous is it that no motion can be perceived even in a most rapidly-moving wheel, as proved by Professor Wheatstone. We sometimes fancy that the flash lasts longer, but the impression received by the eye quite accounts for the apparently prolonged sight of the lightning.

Sheet lightning, the faint flashes frequently seen upon the horizon, are quite harmless. Sheet lightning is that which is seen reflected behind clouds or from far-distant storms. It is sometimes very beautiful. Ball, or globular lightning, is dangerous, and globes of fire have been seer. to descend, and striking the ground, bound onwards for some distance. The descent of these forms of electric discharge has given rise to the popular notion of "thunderbolts." The "Mariner's Lights," or St. Elmo's fire, is frequently observed in ships. It is usually regarded as a fortunate occurrence. It was noticed by Columbus. One voyager describes the phenomena as follows:-"The sky was suddenly covered with thick clouds. . . . There were more than thirty of St. Elmo's fires on the ship. One of them occupied the vane of the mainmast. I sent a sailor to fetch it. When he was aloft

he heard a noise like that which is made when moist gunpowder is burned. I ordered him to take off the vane. He had scarcely executed this order, when the fire quitted it and placed itself at the apex of the mainmast, whence it could not possibly be removed."

There have been occasions when the manes and tails of horses, and even the ears of human beings, have shown a phosphorescent light which

emitted a hissing noise. Alpine travellers have noticed similar phenomena; and Professor Forbes, when crossing the Theodule Pass into Italy, heard

Fig. 290-Thunderstorm.

the hissing sound in his alpenstock. The tips of rocks and grass points were all hissing too. The party were in the midst of an electric cloud. When the Professor turned the point of his alpenstock upwards, a vivid flash was emitted, but no thunder followed. They descended as quickly as possible from such a dangerous neighbourhood.

It is observable that the properties of lightning and of the electric spark are identical-the faint crackle of the latter being magnified into the loud rolling of the thunder. The disturbance of the atmosphere is the cause of the loud reverberations, and echoes produced from clouds tend to intensify and prolong the peal. The sound rises and falls, and varies accordingly as the cloud is near or far. A smart sharp report or rattle denotes the nearness of the lightning, while the gradual swelling and subsidence, followed, mayhap, by an increasing volume of sound, which in

its turn dies away, tells us that the danger is not imminent. The cause of this loud rolling, unless it proceeds from echoes from different clouds, has not been satisfactorily explained. Sound travels less quickly than light, and therefore we only hear the thunder some seconds after we have perceived the flash. It is therefore conceivable that we may hear the last reverberations and its echoes first, and the sound of the first disturbance with its echoes last of all. Thus there will be distinct sounds. Firstly, the actual noise we call thunder from the air strata nearest to us; secondly, the echoes of that disturbance from the clouds, of course fainter; then we hear the sound caused by the tearing asunder of the air particles farthest off, and again the echoes of that disturbance. This theory will, we think, account for the swelling peals of thunder, and the successive loud and fainter reverberations. At any rate, in the absence of any other theory, we submit it for consideration. The sound of thunder is seldom or never heard at a distance greater than fifteen miles.

Lightning conductors are such every-day objects that no description is necessary; but the reason the lightning runs along it harmlessly is because the galvanized iron rod is the best conductor in the immediate neighbourhood. Where there is not a good conductor lightning will accept the next best, and so on, any conductor being better than none. The point of the rod cannot contain any electricity, there being no room for it, and the "fluid," as it is termed, runs down to the ground, to terminate, when possible, in water or charcoal. A great deal of electricity is no doubt carried away from the air by the numerous conductors without any spark passing. Until Sir W. Snow Harris brought forward his lightning conductors for ships, the loss was great at sea. But now we rarely hear of any vessel being disabled by lightning. We owe to Franklin the idea of the lightning conductor.

According to observations made by Mr. Crosse, the following statement shows the tendency of the atmosphere, in certain conditions, to thunderstorms. We may accept the deduction of M. Peltier that grey and slatecolour clouds are charged with negative, and white, rose-colour, and orange clouds with positive electricity. The order of arrangement in the following table places the most likely source of thunderstorms first, and the least likely source at the end, with regular rotation of intermediate probabilities intervening :

1. Regular thunder clouds.
2. Driving fog with small rain.
3. Fall of snow, or hailstorm.
4. Smart shower on a hot day.

5. Smart shower on a cold day.
6. Hot weather after wet days.
7. Wet weather after dry days.

8. Clear frosty weather. 9. Clear warm weather. 10. Cloudy days.

II.

"Mackerel" sky.

12. Sultry weather and hazy clouds.
13. Cold damp night.

14. Cold, dry north-east winds.

We have thus briefly touched upon some of the atmospherical phenomena directly attributable to electricity. In our articles upon Meteorology we will consider the aurora and many other interesting facts concerning the atmosphere, and the effects of sound, heat, and light upon the air.

CHAPTER XXIII.

AERONAUTICS.

PRESSURE OF AIR IN BODIES-EARLY ATTEMPTS TO FLY IN THE AIRDISCOVERY OF HYDROGEN-THE MONTGOLFIER BALLOONS-FIRST EXPERIMENTS IN PARIS-NOTED ASCENTS.

IN the first part of this volume we entered into the circumstances of air pressure, and in the Chemistry section we shall be told about the atmosphere and its constituents. We know that the air around us is composed principally of two gases, oxygen and nitrogen, with aqueous vapour and some carbonic acid. An enormous quantity of carbonic acid is produced every day, and were it not for the action of vegetation the amount produced would speedily set all animal life at rest. But our friends, the plants, decompose the carbonic acid by assimilating the carbon and setting free the oxygen which animals consume. Thus our atmosphere keeps its balance, so to speak. Nothing is lost in nature.

We have illustrated the pressure of the atmosphere by the Magdeburg hemispheres, and we know that the higher we ascend the pressure is lessened. The weight of the atmosphere is 15 lbs. to the square inch at sea level. This we have seen in the barometer. Now pressure is equal. Any body immersed in a liquid suffers pressure, and we remember Archimedes and the crown. It displaced a certain amount of water when immersed. A body in air displaces it just the same. Therefore when any body is heavier than the air, it will fall just as a stone will fall in water. If it be of equal weight, it will remain balanced in the air, if lighter it will rise, till it attains a height where the weight of the atmosphere and its own are equal; there it will remain till the conditions are altered. Now we will readily understand why balloons float in the air, and why clouds ascend and descend in the atmosphere.

In the following pages we propose to consider the question of ballooning, and the possibility of flying. We all have been anxious concerning the unfortunate balloonist who was lost in the Channel, so some details concerning the science generally, with the experiences of skilled aeronauts, will guide us in our selection of material. We will first give a history of the efforts made by the ancients to fly, and this ambition to soar above the earth has not yet died out.