Page:The American Cyclopædia (1879) Volume XIII.djvu/649

 PNEUMATICS PNEUMONIA 629 gnault has confirmed these results, and has even shown a deviation from the law in the case of a confined hody of pure air. He obtained, for instance, a 10 and 20 fold density of air by applying respectively 9'9 and 19*7 atmos- pheres of pressure; of carbonic acid, by 9*2 and 16-7 atmospheres; of hydrogen, by 10-05 and 20-26. It follows that Mariotte's law is to be accepted as but approximately true, the variations being different for different gases ; but the deviation in the case of non-perma- nent gases, such for example as carbonic acid gas, decreases as the temperature is raised, and at the boiling point of water it is much less than at ordinary temperatures. The conclu- sion of modern physicists is, that there is for each gas a certain normal temperature at which it exactly conforms to Mariotte's law, while above and below this temperature it varies in opposite directions. This deviation, however, especially in air, is so slight, that for ordinary determinations of the volume of gases, and in the use of air in manometers, or pressure gauges, it may be overlooked. (See MANOME- TER.) The earth's atmosphere being subject to compression by its own weight, it results that at heights in it increasing in an arithmetical ratio the density and tension diminish in a geometrical ratio. All the relations expressed in Mariotte's law and its consequences are con- veniently exhibited in a table like the follow- ing ; and by including the last column, that of heights, the of height being the sea level, and the height 1 denoting that experimentally found as 2-705 miles, all the" relations in the first four columns become represented as they exist theoretically, and very nearly actually, in our atmosphere : iu: Densities. Tensions. Volumes. Heights in the air. i 1 1 1

X X X 2 1 i % y* X K 4 8 16 2 3 4 iV. 1024 10 &c. &C. &C. &c. &c. In the atmosphere, however, other causes of slight deviation from the relation of density to height exist. Among these are: 1, that the earth's attraction diminishes somewhat, about 7^ part for each mile near the earth, at points taken in ascending through the atmosphere; 2, that the attractions of the sun and moon at some times and places conspire with, at others oppose, the action of the earth ; 3, variations due to changing temperatures; 4, admixture of vapors, &c., in the lower atmosphere. The general effect is a slightly more rapid diminu- tion of density than that above given, with increase of altitude. Aero-dynamic problems, or those investigating the flow and delivery of gases through orifices, in tubes, and in currents, and the consequences of the impact and mo- mentum of moving air, are too intricate to be presented fully except in special treatises on the subject. Torricelli's principle for liquids, that the velocity of discharge from an orifice is that which the body of liquid would acquire in falling freely from the height of its surface to the centre of the orifice, applies quite as strictly to gases as to liquids. A heavy body, in falling through one foot, acquires a velocity of 8 ft. a second ; and the velocities of discharge being as the square roots of the depths, and the height of the surface of a homogeneous atmosphere above the sea level being 27,720 ft., it follows that, at the latter level, the velo- city with which air should jet into a vacuum through an opening not too small will equal nearly the product 84/27,720=1,332 ft. Ex- periments show that the actual velocity, as in the case of water, is somewhat less ; that for orifices in a thin w r all it is about 65 per cent, of that named; for short cylindrical spouts, 93 ; and for conical, narrowing outward, *94. These facts correspond in a degree with the results in spouting liquids, and show that, as well as in these, the " contracted vein " exists in the efflux of gases. The movement of gases through pipes is also subject to retardation similar to that affecting the delivery of liquids ; and roughness of the inside of tubes, sharp angles, inequalities of size, &c., here also in- crease the amount of retardation. This re- tarded flow has proved, unexpectedly, a chief difficulty in the way of using the pneumatic power transmitter proposed by Papin in sub- stance a hydrostatic press containing air, with a long pipe connecting the two pistons, so as to allow of action at distant points. So, in the case of a blowing tube constructed in Wales to catch the air impelled by a waterfall, and con- vey it to a -distance, in order to feed the blast of a foundery, the time estimated for the deliv- ery of the air being six seconds, it was found that the jet of air did not arrive until after the lapse of ten minutes, and it was then but feeble. The remarkable retardation of gases in tubes must be due in a considerable degree to adhe- sion of the gases to the solid surfaces, a prin- ciple well known ; and Kobison has also sup- posed much of it due to an undulation arising from this and other causes in the transmitted air. It is well ascertained that, besides vary- ing in the force of horizontal movement, pro- ducing gusts, winds also undulate vertically, as do water waves. Moreover, winds are retarded by obvious causes near the surface of the earth, just as a stream of water flows slowly at its bottom ; and thus they are always less violent in cities than in the country. Similar influ- ences must modify their flow at the sides and above, and especially where winds flow in unlike directions along an aerial plane dividing them. PNEUMONIA, Peripnenmonia, or Lung Fever, in- flammation of the proper substance of the lungs. Pneumonia is one of the most fre- quent forms of inflammation, and is common to all ages. It prevails more frequently in