I IO

Of Pneymatia.

and I have heard that lightning slriking upon the mariner’s compasi,will sometimes turn it round ; and osten make it stand the contrarywayj or with the north pole towards the south.

Much of the fame kind with lightning, are those explostons, calledfulminating or ßre-damps , which sometimes happen in mines; and areoccasioned by sulphureous and nitrous, or rather oleaginous particles,rising from the mine, and mixing with the air, where they will takesire by the lights which the workmen are obliged to make use of.The fire being kindled will run from one part of the mine to another,like a train of gunpowder, as the combustible matter happens to lie.And as the elastici ty of the air is increased by heat, that in the minewill consequently swell very much, and so, for want of room, willexplode with a greater or less degree of force, according to the densityof the combustible vapours. It is sometimes so strong, as to blow upthe mine; and at other times so weak, that when it has taken fire atthe flame of a candle, it is easily blown out.

Air that will take fire at the flame of a candle may be producedthus. Having exhausted a receiver of the air-pump, let the air runinto it through the flame of the oil of turpentine; then removethe cover of the receiver, and holding a candle to that air, it willtake fire, and burn quicker or flower, according to the denfity of theoleaginous vapour.

Barthqaakes. When such combustible matter, as is above-mentioned, kindles inthe bowels of the earth, where there is little or no vent, it producescarthquakes , and violent storms or hurricanes of wind when it breaksfbrth into the air.

An artificial earthquake may be made thus. Take io or 15 poundsof sulphur, and as much of the filings of iron, and knead them withcommon water into the consistence of a paste : this being buried inthe ground, will, in 8 or 10 hours time, burst out in flames, andcause the earth to tremble ali around to a confiderable distance.

From this experiment we have a very natural account of the fire ofmount /Etna, Vesuvius, and other volcano’s, they being probably seton fire at first by the mixture of such metalline and sulphureousparticles.

pute XIV. . The air-pump being in estect the fame as the water-pump, whoeverFi s- understands the one, will be at no loss to understand the other.

The air-pump. Having put a wet leather on the piate LL of the air-pump, place* the glafs receiver M upon the leather, so that the hole i in the piate

may be within the glafs. Then, turning the handle F backwardand forward, the air will be pumped out of the receiver; which will2 ' then