CHAMBERS'S INFORMATION FOR THE PEOPLE. 



the body, though contained in tubes, transmit 

 pressure through all their ramifications, as the 

 \vater in the box B does through its mass ; and 

 the direct pressure of the atmosphere on any spot 

 of the outer surface, tending to squeeze in the 

 sides of the tubes containing the fluids, is met by 

 an equal pressure on the part of the fluids, tending 

 to force them out ; which outward pressure itself 

 is caused by the atmosphere pressing on other 

 p:irts of the system. 



We are as little sensible, in ordinary circum- 

 stances, of this outward pressure of the fluids on 

 the vessels of the body, as we are of the compress- 

 ing force of the atmosphere. But it makes itself 

 felt whenever the external pressure is removed j 

 from any part, as in the familiar act of sucking j 

 the finger, or more strikingly in the operation of 

 cupping. A small bell-shaped glass, in which the 

 air has been rarefied by burning a slip of paper, 

 or by other means, is suddenly applied to a flat 

 surface of the body. The small portion of air 

 tinder the glass, as it cools, presses with dimin- 

 ished force on the surface of the skin ; and the 

 fluids within, released from the usual check, swell 

 out the vessels and skin, and if a wound has been 

 made with a lancet, the blood is forced out. 



When the usual pressure of the atmosphere is 

 much diminished over the whole body by ascend- 

 ing great heights, uneasy sensations are often felt, 

 occasioned most likely by air contained in the 

 liquids and in cavities of the solid parts of the 

 body, seeking to expand. 



A boy's sucker lifting a stone affords a good 

 illustration of the operation of atmospheric pres- 

 sure. Correctly speaking, the sucker does not lift 

 the stone ; it only lifts the weight of a column 

 of the atmosphere from off the stone, and thus 

 allows the remaining atmospheric pressure to push 

 it up. 



The bird-cage fountain and the ink-bottle of 

 the same make, are other illustrations. The 

 same cause sustains the liquid in such a vessel 

 that sustains the column of mercury in fig. 26, and 

 keeps it from flowing out at 

 C the atmospheric pressure, 

 namely, on the surface at C, 

 while the top at A is close, like 

 the top of the tube in fig. 26. 



The ink is introduced into a 

 bottle of this kind by inclining 

 it over, and pouring in gradu- 

 ally at C. When the ink in the 

 tube CB sinks down by use to 

 the level of the horizontal com- 

 munication, a bubble of air gets in, and rising to j 

 the top at A, fills part of the space, and forces 

 down a portion of ink into C. 



On the principle now explained, if the lid of a 

 tea-pot were quite close, so as to exclude the 

 pressure of the atmosphere from the top of the 

 liquid within, the liquid could not be poured out. 

 It is for this reason that in drawing liquor from an 

 opening in the lower part of a cask, there must be 

 a vent-peg at the top to admit air. 



BUOYANCY IN AERIFORM FLUIDS. 



The fundamental law in this is exactly the same 

 as in liquids. A body in air displaces its own bulk 



Fig- 35- 



of the air, and is borne up with a force equal to 

 the weight of the air so displaced. If the body's 

 own weight is less than this, it must rise up, as a 

 piece of cork does in water ; if greater, the body 

 only loses a portion of its weight, as a stone ia 

 water. 



In weighing bodies in air, the fact that the air 

 deprives them of part of their weight, does not 

 interfere with the accuracy of the process so long 

 as the substance is nearly of the same density as 

 the weight against which it is balanced, because 

 then both are equally affected. But where there 

 is great difference of density, it leads to sensible 

 error. A pound of feathers as it is sometimes 

 paradoxically stated is heavier than a pound of 

 lead that is, it contains more matter. If a quan- 

 tity of feathers occupying a cubic foot of space, 

 were balanced in a vacuum against a piece of 

 metal, the balance would be destroyed on taking 

 them into the air. The feathers would lose above 

 an ounce the weight of a cubic foot of air the 

 metal would lose only a few grains. 



BALLOONS. 



Solid bodies cannot of themselves float in air, 

 being all heavier than an equal bulk of it ; but by 

 inclosing air rarefied by heat, or a light gas, sue a 

 as hydrogen, in a bag, it may be made to buoy up 

 more or less solid matter along with it. Such a 

 contrivance is a balloon. A soap-bubble blown 

 from hot water with warm air, is the simplest form 

 of balloon, but soon loses its buoyancy by cooling ; 

 if blown with hydrogen, it continues ascending till 

 it bursts. A body floating in air rises till it 

 arrives at a stratum of the atmosphere whose 

 density is the same as its own. 



A balloon is a bag made of varnished silk, and 

 surrounded by a net-work, from which a car is 

 suspended. Balloons were originally filled with 

 heated air, and had a fire suspended in the open- 

 ing below, to keep up the temperature. So far as 

 lightness is concerned, hydrogen would be the 

 best gas for the purpose ; but, from its cheapness, 

 common coal-gas is generally employed, whose 

 density is about half that of air. 



The air contained in a room fifty feet long T 

 thirty wide, and twenty high, weighs upwards of a 

 ton. A balloon of this capacity (30,000 cubic 

 feet) would thus displace a ton of air, while the 

 gas contained in it weighed only half a ton ; it 

 would therefore ascend with a force equal to the 

 difference, or half a ton, after deducting, of course, 

 the weight of the silk, the car, and other parts 

 of the apparatus. 



The most remarkable recent balloon ascents 

 have been made by Mr Glaisher for the pur- 

 pose of meteorological observations. On one 

 occasion he ascended to a height of about 

 40,000 feet (7^ miles), the barometer standing at 

 7 inches. The balloon contained 90,000 cubic 

 feet of gas, and carried a load of 600 Ibs. As it 

 is impossible to direct the horizontal motion of 

 balloons, they are almost useless as a mode of 

 conveyance. During the siege of Paris in 1870, 

 a few persons made their escape in balloons from 

 the city to the provinces, but there could, of 

 course, be no return. 



