most corrosive and deadly poisons. Thus, of all the combinations of these two gases, atmospheric air is the only one fit for sustaining life! How easily could the destruction of the globe be effected, were the Creator to change the proportions of these fluids!"*

One of the most remarkable discoveries of late years, and, as we shall probably find out hereafter, one of the most important in a sanatory point of view, is ozone,† which we now know to be oxygen existing in an allotropic form. This gas is found to be susceptible of becoming odorous, corrosive, and irritating when breathed; its chemical action is susceptible of being exalted and modified, so that whilst ordinary oxygen gas neither bleaches nor corrodes silver, nor decomposes iodide of potassium, the allotropic or second form will accomplish all these results, and many more too numerous to mention here." Ozone is insoluble both in water and in solutions either of acids or alkalies, and a temperature of 270° F. entirely destroys it.

Atmospheric air may be ozonized by placing a clean stick of phosphorus, moistened with water, in a closed vessel; but at the expiration of an hour or so the phosphorus must be removed, or a combination of the ozone with it will take place, and the ozone will gradually disappear. Ozone may also be obtained by passing a succession of electric sparks through atmospheric air, or dry oxygen, when a peculiar odour will result, resmbling in some slight degree that of weak chlorine. Andrews and Tait remark that in order to obtain a maximum effect, the discharge must be made silently. These same observers are disposed to question whether ozone is an allotropic form of oxygen, thinking rather that the true hypothesis is that the oxygen is decomposed into constituents, of which ozone may be the most important.§ This theory, necessitating another theory, that oxygen is not an element, is not received with much favour. Schönbein has recently followed up his original discovery by conjecturing the existence of another and opposite form of oxygen, which he terms "antozone."||

A good exemplification of the great pressure of the atmosphere is afforded by the following well-known experiment. Take a hollow glass cylinder, and over one end tie a piece of wet bladder, and place the other on the plate of an air-pump, taking care that

Introd. to Met.

P. 13.

tow, "I smell;" Faraday, Proc. Roy. Inst. vol. i. p. 94, 1851, also vol. i. P. 338, 1853; Andrews, Phil. Trans. vol cxlvi. pp. 1–13. 1856.

Phil. Trans. vol. cl. p. 124. 1860.

Ibid. pp. 113, 128, and elsewhere.

Phil. Mag. vol. xvi. p. 178. 1858; Faraday, Proc. Roy. Inst. vol. iii.

p. 70. 1859.

all interstices are air-tight. Exhaust the air-pump with the cylinder, and it will be seen that the surface of the bladder, which before was flat, will gradually become concave; and if the exhaustion be carried on to an extreme, the bladder will crack with a loud report; or should it still remain whole, a slight touch with some solid body will cause it to rupture; the whole forming a proof of the powerful pressure of the external air.

Otto von Guericke's Magdeburg Hemispheres also illustrate the pressure of the atmosphere in nearly the same way. He prepared two strong hemispheres of brass, which fitted closely at the bases; by exhausting them of the air they contained, they were found to adhere so strongly as to require the united efforts of thirty horses to separate them.

The atmosphere surrounding our globe resembles, probably, in general figure, the earth itself—that is to say, its exterior outline is that of an oblate spheroid.

The atmosphere has weight. 100 cubic inches of dry air, at a temperature of 60° F. and pressure of 30 inches, weigh, according to

Biot and Arago

Dumas and Boussingault.

Regnault

Prout

It is probable that the first result is the most accurate, as it exactly corresponds with the density deduced from an analysis of a mixture of oxygen and nitrogen, in the proportions in which they occur in the atmosphere.*

The atmosphere also exerts pressure At the level of the sea, the average pressure is equal to 14.73 lbs. on the square inch. This would be equivalent to the pressure exerted by a lake of water 33 feet deep, or a lake of mercury 2 feet deep. If we assume the superficial area of the globe to be 790,116,426,647,756,800 square inches, the absolute weight of the atmosphere amounts to no less than 11,456,688,186,392,473,600 lbs. avoirdupois. Even if this be converted into tons, we get the amazing result of 5,114,592,940,353,782.85 tons, equivalent to the weight of a sphere of lead 60 miles in diameter.

An experimental acquaintance with the atmosphere, however slight, teaches us one very important fact-namely that it is not of uniform density; that the density varies with the temperature and with the elevation of the place of observation above the sea level, diminishing as the elevation increases.

In this way the atmosphere becomes rarer and lighter, and at the same time colder as we ascend, until it finally ceases alto

* Miller, Elem. of Chem. vol. ii. p. 28. ·

gether. At what height it actually terminates we do not know, but it is certain that it reaches to at least 45 miles from the earth's surface, for at this height it exercises a sensible influence on the refraction of celestial objects. Its own elasticity would carry it away to an infinite distance, were it not for the influence of gravitation, which tends to keep the particles within certain limits, which probably do not extend beyond 60 or 80 miles from the earth's surface.

A curious effect due to the diminution of the atmospheric pressure is the fall of the boiling point of water. For general purposes it is sufficient to say that water boils at 212° F. Strictly speaking, however, this is only true when the barometric column of mercury stands at exactly 30° inches. The following table will conveniently point out the variations which occur.

A difference of 1° in the boiling point of water, corresponds to a change of altitude amounting to 530 feet. A knowledge of this fact is often of importance to travellers ascending mountains. If a tourist is provided with a thermometer and a kettle of water, and a fire, he can directly determine the elevation of his station above the sea level and the mercurial pressure. Thus Humbold, when on one of the summits of the Andes, noted that the water boiled at 175° F. whence we can infer that he was at an altitude of nearly 20,000 feet.

Mr. Darwin, of" Origin-of-species" notoriety, mentions the following amusing anecdote of an occurrence which happened to his party while crossing the Andes in 1835, due to the phenomenon we have just been considering. The altitude was so great, and the boiling point so low, that "our potatoes, after remaining some hours in the boiling water, were nearly as hard as ever. The pot was left on the fire all night, and next morning it was boiled again, but yet the potatoes were not cooked. I found out this by over-hearing my two companions discussing the cause; they had come to the simple conclusion that the potatoes were bewitched, or that the pot, which was a new one, did not choose to boil them." *

Another effect of a high altitude, that is to say, highly Voyage of the Beagle.

attenuated air, is the weakening of the intensity of sound consequent thereon. An American writer who, some years ago passed

a night on the summit of Mont Blanc, states that a heavy thunderstorm occurred in the valley; that the lightning was distinctly seen, but that no sound reached the ear; whilst, on the other hand, at Chamouni in the valley below the peals were loud and numerous.*

The atmosphere in a limited extent is clear, transparent, and colourless; but viewed in the aggregate its general tint is deep blue, one extreme being pale blue, nearly white; the other, indigo blue, verging on black. These variations are due to the presence of a greater or less quantity of opaque vapour, the particles of which reflect chiefly blue or bluish rays. these particles are of a maximum abundance, as towards the horizon, the colour of the sky is faintest; when of a maximum as in the zenith, the colour is deepest. If the earth had no atmosphere at all, the sky would be intensely black.

When

It will not be difficult to understand that viewed from the sunmits of lofty mountains the intensity of the blue of the sky greatly exceeds that observed at the lower surface of the earth. The consequence then is, that astronomical observations can be carried on with far greater facility and satisfaction at high altitudes than at low ones. Thus Moorcroft, at Zinchin, in Chinese Tartary, 16,136 feet above the level of the sea, was able to see stars of the 5th magnitude in the day time, with the aid of a small telescope of only 30 inches focus,† and more recently the similar results of Professor C. P. Smyth's expedition to Teneriffe are well known. Not to enlarge upon this subject, I may simply mention, that that observer was able to detect in his telescope stars four magnitudes smaller than he could see with the same instrument at Edinburgh.‡

Many years ago M. de Saussure constructed an instrument which he called a cyanometer, for measuring the intensities of sky tints; the results of some of his observations§ in and near the Alpine chain are as follows:- .

They confirm what I have said above.

* Grant, in Silliman's Journal, vol. xlvi. p. 293. 1844.

+ Travels in the Himalayan Provinces, 2 vols. 8vo. Lond. 1841. Phil. Trans. vol. cxlviii p. 477. 1858.

§ Voyages dans les Alpes. 4to. Geneva, 1786. § 2084.

CORRESPONDENCE.

N.B. We do not hold ourselves answerable for any opinions expressed by our correspondents.

FINDING CELESTIAL OBJECTS WITHOUT CIRCLES.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-There are two methods by which Mr. Coulcher can find celestial objects without circles, and without putting you to the troublesome task of indicating celestial places by verbal descriptions.

First. If he has a celestial globe or atlas of any kind, he can easily note thereon, by the lines of right ascension and declination, the places of any object for any day (presuming, of course, he knows the right ascension and declination of his object); and upon comparing this noted place with the nearest stars upon the globe or map, he can easily, upon turning his telescope to the corresponding point of the heavens, sweep in his object.

Second. If he knows the stars well, and has a "star-catalogue," he can, by running his eye over the places in the star-catalogue, pick out one or more stars of which the right ascensions and declinations are tolerably near to that of his object. Then he may either construct a rough chart of the district for himself, or, by taking the difference of right ascension and declination between his object and any known star, fix the point in the heavens his object occupies with reference to that star.

If his globe, or atlas, or star-catalogue, be of ancient date, and the stars' places hence refer to a distant epoch, he will have to make allowance for precession, by applying to the place of his object the precession proper for a star in about the same part of the heavens, taking care to apply the correction with an opposite sign to that given in the catalogue, if the stars' places on his globe, or in his catalogue, are given for an epoch which has passed. I am, Sir, yours faithfully,

J. C.

THE VARIABLE STAR IN CORONA BOREALIS.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir, So interesting have been the phenomena attending the extraordinary increase in brightness of the small star in the Bonn Catalogue known as (T) Coronæ, that an enumeration of its successive independent discoveries can hardly fail to be acceptable.

The first discoverer was Mr. Bermingham, of Tuam, in Ireland, who saw it about midnight on the 12th of May, and described it then as of the 2nd magnitude.

The next evening, May 13, it was independently discovered by Herr Schmidt, the director of the Observatory at Athens, who saw it about 9h. local time, as soon as the clouds broke, and calls it a little fainter than a Coronæ; and by M. Courbe-Caisse at Rochefort.

On the night of May 14 it was detected the other side of the Atlantic, by Mr. S. C. Chandler, assistant to Mr. Gould, on the United States Coast Survey. He states that in magnitude it was between 8 and y Herculisnearer to y.