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362 THE PHOTOGRAPHIC NEWS. [JUNE 5, 1885 The word meteorology is derived from two Greek words (uerwpa, Aoyos), signifying the science of the things above the earth, and in its widest sense it includes all pheno mena of the sky, both atmospherical and astronomical. It has been found convenient, however, to restrict the term to those atmospherical conditions only upon which depend climate and weather, although it is impossible to exclude astronomical phenomena entirely, since, as will be seen hereafter, it is the sun which is the chief cause of all atmospheric changes, and the great regulator of every meteorological phenomenon. The motions of the earth, also, claim to be considered, producing as they do the daily and annual variations of such phenomena as depend upon the relative positions of the earth and sun. It will be necessary here, however, to assume that our readers possess sufficient knowledge of such elementary astronomical notions as the movements of the earth, the discussion of which would carry us too far from the scope of the present article. Since, therefore, the atmosphere is to be the main ob ject of our enquiries, it will be well at the outset to have clearly before our minds its nature and composition. The atmosphere, which literally means vapour sphere, is a gaseous envelope, surrounding and probably conforming to the figure of the earth. Its density progressively diminishes above the earth’s surface, and it has been esti mated that three-fourths of the whole atmosphere encircling the earth lies within four miles of the surface. Its total height has been variously estimated, but it can be shown that at a distance of seven miles from the earth it is so attenuated that respiration is scarcely possible, at forty miles it is no longer capable of refracting the sun’s rays, at one hundred miles it no longer retards meteorites sufficiently to cause incandescence, and at two hundred miles it may be said to be practically absent. Thus, on a sphere of 2 in. diameter the atmospheric envelope would not exceed one-twentieth of an inch in thickness. W e must not, however, regard the upper limits of the atmosphere as a sharply-defined surface, for gases have no surface. It appears rather that its height is constantly changing, more atmosphere being accumulated over any locality at one time than at another time, so that the upper limits of the aerial ocean are ill-defined, graduating imperceptibly into space, and constantly being agitated by waves and currents. Respecting the composition of our atmosphere, it has been proved by many hundreds of careful analyses to con sist essentially of two gases, oxygen and nitrogen, mixed together (not in chemical combination) in the proportion of 20-9 parts by volume of the former to 791 parts of the latter. This proportion, moreover, is not exactly constant. Thus, Dr. Angus Smith has shown that while, near the sea-shore or on the Scotch moors and mountains, the oxygen amounted to 20‘99 per cent., in towns, especially in foggy weather, it may sink as low as 20-82, in crowded rooms as low as 20-28, and in mines as low as 20-26. In tropical countries, from some unknown causes, the pro portion of oxygen appears to sink exceptionally low, diminishing, in one of the cases observed, to 20-4 per cent, even over the ocean. As regards its variation in altitude, Frankland finds that the composition remains practically constant up to heights of 14,000 feet above sea-ievel. Now although these differences in the proportion of oxygen may seem small and unimportant, they are specially worthy of being noticed by the photographer, since even small variations in the composition of the air may exert considerable influence upon the absorption and actinism of light. Besides, a difference of one-tenth per cent, amounts to a considerable quantity when we consider the whole volume of the atmosphere. But if the variations in the amount of oxygen are small, this is not the case with the other atmospheric constituents, of which the most abundant are water vapour and carbonic acid. The former of these plays such an important part in all meteorological phenomena, that its consideration will be deferred to subsequent chapters. It will be sufficient to mention here that the average quantity of water vapour in the air of England is about 1} per cent., and that it usually varies between 4 and 16 parts per 1,000. Carbonic acid—more properly called carbon di-oxide—is generally present in the propertion of 4 parts in 10,000 ; but this amount increases rapidly in the air of towns and of badly-ventilated rooms. Over the sea its amount is somewhat less, but at high elevations it has been found to be generally greater. The meteorological importance of carbonic acid consists chiefly in its effect in absorbing the sun’s rays. Amongst the minor constituents of the atmosphere, a most important part, from a photographic point of view, appears to be played by ozone. This substance, as is well known, is a more active condition of oxygen, and its powerful action as an oxidising agent may possibly exert an important influence upon length of exposure ; for Abney has shown that solarization is impossible unless an oxidis ing agent be present, and that it takes place more quickly in ozone than in ordinary air. In addition to this, ozone, as will be shown hereafter, has a great effect upon the absorption of light in the atmosphere, and its presence has more than once been hinted at as a producer of atmo spheric effect in a photograph. Unfortunately, it is very hard to determine accurately the amount of ozone present in the air. Zeuger's experiments gave results corres ponding to about -001 milligrams of ozone in a hundred litres of air ; but it is doubtful whether this amount can be taken as representing the general average. Smyth and other chemists maintain that its amount is constant; but the experiments of Houzeau and Reiset point to an increase in the proportion of ozone at high elevations, and also with the prevalence of south-westerly winds. Its amount depends also, according to Neuman, upon the electrical condition of the air ; and it is believed to be largely pro duced during the spontaneous evaporation of water. Ozone is absent from the air of towns, being reduced to the condition of ordinary oxygen by the sulphurous acid and organic matter present in such localities. Even here, however, it probably exists at high elevations. Besides the above-mentioned substances, the atmo sphere, especially near towns, usually contains very appre ciable quantities of sulphuric and sulphurous acids, nitric acid, sulphuretted hydrogen, and ammonia. Ammonia, indeed, has been stated to be a normal constituent of the atmo sphere at high elevations, although this point requires further confirmation. Dust-particles and smoke are most important elements in the production and persistence of fogs; and as an illus tration of the commercial importance of these atmospheric impurities to the professional photographer, the following fact may be noticed. In the year 1880 it was found that Kew enjoyed 150 hours more sunshine than Greenwich, the difference being due entirely to London smoke, which was wafted towards the east by a prevalence of westerly winds. Perhaps, also, the air is never entirely free from organic germs ; and even these become of considerable interest to photographers when it is remembered that mildew, one of the great enemies of carbon prints probably originates, in them. Pasteur’s researches show that germs of plant and animal life are always present in air, and that these bodeis are the propagators of fermentation and putre faction. (To be continued). TRIP WITH THE CAMERA INTO HAMPSHIRE. BY CHARLES ALLAN FERNELEY. In the village of Froyle, Hampshire, may be found several beautiful studies worthy the attention of both the artist and photographer. The village is divided into two parts —viz., Upper and Lower Froyle. In it are very many