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OF THE DIVING BELL.
as c to r, and 1 denote the natural density of the air at first :
the density after three strokes,
the density after n strokes.
So, if the barrel be equal to of the receiver; then c; r
384. Or, if it were required to find the number of strokes
the density after one stroke of the piston,
the density after 2 strokes,
1. r-1. c'
203 nearly. So that in 203 strokes the air
OF THE DIVING BELL & CONDENSING MACHINE.
385. On the same principles too depend the operations and effect of the Condensing Engine, by which air may be condensed to any degree instead of rarefied as in the air-pump. And, like as the air-pump rarefies the air, by extracting always one barrel of air after another; so by this other machine, the air is condensed, by throwing in or adding always one barrel of air after another; which it is evident may be done by only turning the valves of the piston and barrel, that is, making them to open the contrary way, and working the piston in the same manner; so that as they both open upward or outward in
OF THE DIVING BELL.
the air-pump or rarefier, they will both open downward or
386. And on the same principles, namely, of the compres sion and elasticity of the air, depends the use of the Diving Bell, which is a large vessel, in which a person descends to the bottom of the sea, the open end of the vessel being downward; only in this case the air is not condensed by forcing more of it into the same space, as in the condensing engine; but by compressing the same quantity of air into a less space in the bell, by increasing always the force which compresses it.
387. If a vessel of any sort be inverted into water, and pushed or let down to any depth in it; then by the pressure, of the water some of it will ascend into the vessel, but not so high as the water without, and will compress the air into less. space, according to the difference between the heights of the internal and external water; and the density and elastic force of the air will be increased in the same proportion, as its space in the vessel is diminished.
So, if the tube CE be inverted, and pushed down into wa-
it is 34+AB : 34 :: ce : CD,
that is 344AF-DE : 34 :: CE : CE-DE,
then CD will be ce; and so on. And hence, by knowing the
hence, multiplying extremes and means, 216-58x+x2=136,
388. But if the vessel be not equally wide throughout, but of any other shape, as of a bell-like form, such as is used in diving; then the altitudes will not observe the proportion above, but the spaces or bulks only will respect that proportion, namely, 34+AB: 34:: capacity CKL capacity CHI, if it be common or fresh water; and 33+ AB: 33:: capacity CKL : capacity CHI, if it be sea-water. From
which proportion, the height DE may be found, when the na ture or shape of the vessel or bell câ, is known,
OF THE BAROMETER.
389. THE BAROMETER is an instrument for measuring the pressure of the atmosphere, and elasticity of the air, at any time. It is commonly made of a glass tube, of near 3 feet long, close at one end, and filled with mercury. When the tube is full, by stopping the open end with the finger, then inverting the tube, and immersing that end with the finger into a bason of quicksilver, on removing the finger from the orifice, the fluid in the tube will descend into the bason, till what remains in the tube be, of the same weight with a column of the atmosphere, which is commonly between 28 and 31 inches of quicksilver; and leaving an entire vacuum in the upper end of the tube above the mercury. For, as the upper end of the tube is quite void of air, there is no pressure downwards but from the column of quicksilver, and therefore that will be an exact balance to the counter pressure of the whole column of atmosphere, acting on the orifice of the tube by the quicksilver in the bason. The upper 3 inches of the tube, namely, from 28 to 31 inches, have a scale attached to them, divided into inches, tenths, and hundredths, for measuring the length of the column at all times, by observing which division of the scale the top of the quicksilver is opposite to; as it ascends and descends within these limits according to the state of the atmosphere,
So that the weight of the quick-
OF THE THERMOMETER.
390. THE THERMOMETER is an instrument for measuring the temperature of the air, as to heat and cold.
It is found by experience, that all bodies expand by heat, and contract by cold; and hence the degrees of expansion become the measure of the degrees of heat. Fluids are more convenient for this purpose than solids; and quicksilver is now most commonly used for it. A very fine glass tube, having a pretty large hollow ball at the bottom, is filled about half way up with quicksilver: the whole being then heated very hot till the quicksilver rise quite to the top, the top is then hermetically sealed, so as perfectly to exclude all communication with the outward air. Then, in cooling, the quicksilver contracts, and consequently its surface descends in the tube, till it come to a certain point, correspondent to the temperature or heat of the air. And when the weather becomes warmer, the quicksilver expands, VOL. II.
and its surface rises in the tube; and
part of its bulk,
1 part of its bulk,
part of its bulk.
This division of the scale is commonly called Fabrenheit's.
and these also are the weights of a cu-