highest clouds are supposed to consist chiefly of thin layers of snow, or ice crystals. According to one of our most eminent meteorologists, no trace of cloud has ever been observed at a greater height than seven miles.

The following heights to which the various kinds of clouds rise in the atmosphere is now generally admitted:

Cirrus cloud to 35,000 feet; cirro-stratus to 27,000; cumulus to 15,000; and the nimbus, or rain-cloud, not above 5000 feet.

Two photographic cameras, placed half a mile apart, and in telephonic communication, suffice to determine the parallax of the clouds observed, and thus to get their exact elevation. We shall say more on this subject a little later.

The quantity of water vapour in the atmosphere, at any given spot upon the Earth, varies incessantly with the pressure and temperature, the time, and the direction of the wind. The point at which it is saturated with moisture, and deposits dew, or lets rain fall, is ascertained by the difference of reading between the dry- and wet-bulb thermometers.

Supposing the dry-bulb thermometer indicated 60° Fahr., and the wet-bulb 56°: the difference, 4°, being subtracted from the wet-bulb indication, leaves 52° as the temperature at which the air would be saturated with moisture. Or, if we double this difference and subtract the sum from the dry-bulb indication, we get the same result. The greater the difference of reading between these two thermometers, hanging side by side,

the drier is the air; when the difference is very slight, 1° or 2° only, rain is about to fall.

The rain-band of the Spectroscope, first alluded to by Professor Piazzi-Smyth as being due to aqueous vapour in the Earth's atmosphere, and as being situated on the red side of the line D of Frauenhöfer, increasing in intensity, or receding from the C line, according to the nearness or quantity of rain to be expected, seems to me to be less practical, and less reliable, than the indications of the wet-bulb and dry-bulb thermometers. The use of the spectroscope for this purpose requires further investigation.

When the air of the atmosphere is saturated, at the ordinary temperature of an English summer, it contains about 1 per cent. of moisture: Carbonic acid varies from 0.03 to 0.06 per cent., and water from 0.6 to 0·9 per cent. The quantity of ammonia is to that of the carbonic acid as 3 is to 100, nearly.

Professor Macagno, in a series of interesting analyses of the air in the neighbourhood of the Observatory at Palermo, in Sicily, which stands only some 200 feet above sea-level, has shown that carbonic acid, and organic matter, increase in the air as the temperature rises; that rain invariably purifies the air; and that there is a slight diminution in the amount of oxygen during the prevalence of the sirocco.

CHAPTER XIII.

Air the only Gas that can be breathed--Important Work of the late Wilson Phipson-The Compressed-air Work of Daniel Colladon-Dr. Junod's Application of rarefied Air—Airbath Establishments-Disinfection-Special Action of given Disinfectants-Absence of Microbes in Pure Air.

THREE-QUARTERS of a century ago, the late Dr. Thomas Thomson, of Glasgow University, pointed out in his celebrated work on Chemistry,1 that air is the only gas which man and the higher animals can breathe for any length of time. Many gases are promptly fatal; others can be breathed only for a very short time.

This shows the immense importance of good ventilation in crowded edifices. Here I may be permitted to refer to the splendid results obtained by my late brother, Wilson W. Phipson, M. Inst.C.E., of London, to whose marvellous energy and indefatigable labours— ending, alas! in premature death-most of the largest public buildings in Great Britain owe their superiority in this respect, notably the Royal Albert Hall, the Banks and Clubs of London, the Universities and Medical Schools of Glasgow, Edinburgh, Liverpool,

1 Thomson, A System of Chemistry (in four vols.), London, 1820.

F

Birmingham, etc., all works obtained in public competition. His method is described in an excellent paper read to the Institution of Civil Engineers, and published in their Journal.1 The application of air in motion to chemical industries, such as getting rid of acid vapours in gold refining, and chemical laboratories, etc., was also carried out most successfully by him for Baron Rothschild, and for the professors at the Glasgow and Edinburgh Universities, as well as in many other

cases.

The great Swiss engineer, Daniel Colladon, who has recently departed from among us at 91 years of age, has also left an imperishable name, by his talented application of compressed air for the boring, and ventilation, of tunnels and mines. His blowing machines, worked by hydraulic pressure, by which the Mont Cenis and St. Gothard tunnels through the Alps were bored, were already under consideration as early as 1852.

The air-gun is one of the oldest applications of compressed air (1650); and the briquet à air, by which a lighted tinder is produced, by sudden compression, as I have already mentioned, dates back about two centuries.

Great economy would be realized if compressed air could be used in place of steam for locomotion. Some attempts have been made in this direction; among others, one by the French inventor, M. Julienne, about

1 Wilson W. Phipson, Journ. of the Inst. of Civil Engineers, London, 1878-79. For a notice of his career, see the same Journal

the year 1852 or 1853, when the Swiss engineer just named first thought of his; and, also, by means of the hydraulic press. But neither compressed air, nor heated air (machines of Ericsson, etc.), nor electricity, have, up to the present time, been made to compete successfully with steam. In the case of such machines as that of Julienne, steam or some other motive power must be used to work the hydraulic press.

Air itself is the motor power in the windmill; and an ingenious apparatus of this description, known as the "Rollaston wind motor," is at present being experimented with in the neighbourhood of London, with the view of applying it to electric lighting by wind power.

With regard to the use of compressed or rarefied air as a therapeutic agent, Dr. Junod, a Swiss physician, whom I met in Paris in 1856, had invented in the early part of the century a metallic boot, coming up beyond the knee, where it was fixed by an air-tight bandage. A small tube near the foot, or ankle of the boot, allowed the air to be extracted by means of a small air-pump. This apparatus was used in cerebral congestion, etc., with apparently good results. The last occasion that came under my notice was in the case of the celebrated chemist, Regnault, after his fall through the skylight at the Porcelain works at Sèvres, of which he was then director. By the production of a vacuum in the boot, Dr. Junod hoped to withdraw the blood from the congested parts (head, lungs, abdomen) into the leg, and so relieve the congestion. The apparatus was, in fact, an extension of the dry cupping-glass.