Page:Passages from the Life of a Philosopher.djvu/231

Rh stretching wet parchment over a large tin funnel. On directing the point of the funnel at a candle placed a few feet distant, and giving a smart blow upon the parchment, it is observed that the candle is immediately extinguished.

This arises from what is called an air shot. In fact, the air in the tubular part is projected bodily forward, and so blows out the candle. The statements about persons being killed by cannon balls passing close to but not touching them, if true, are probably the results of air shots.

Wishing to trace the motions of such air shots, I added two small tubes towards the large end of the tin funnel, in order that I might fill it with smoke, and thus trace more distinctly the progress of the ball of air.

To my great delight the first blow produced a beautiful coronet of smoke, exactly resembling, on a small scale, the explosions from cannon or the still more attractive ones from Vesuvius.

If phosphoretted hydrogen or any other gas, which takes fire in air, were thus projected upwards, a very singular kind of fire-work would be produced.

It is possible in dark nights or in fogs that by such means signals might be made to communicate news or to warn vessels of danger.

Vesuvius was then in a state of moderate activity. It had a huge cone of ashes on its summit, surrounding an extensive crater of great depth. In one corner of this was a smaller crater, quite on a diminutive scale, which from time to time ejected red-hot fragments of lava occasionally to the height of from a thousand to fifteen hundred feet above the summit of the mountain.

I had taken apartments in the Chiaja, just opposite the volcano, in order that I might watch it with a telescope. In fact,