of formation and propagation of low-pressure systems ; (2. ) Imperfect knowledge of the relations of the formation of cloud and aqueous precipitation to barometric fluctuations; (3.) A want of information with reference to the merely mechanical effects of ascending, descending, and horizontal currents of air on the barometric pressure ; in other words, we do not know how far the barometric pressure is an indication of the mass of air in the column vertically over it, when that column is traversed by air-currents; (4.) An almost total absence of really good wind observations ; and (5.) Deficient information in nearly everything that respects aqueous vapour its relation to radiant heat, both solar and terrestrial ; its mode of diffusion vertically and horizontally in the free atmosphere, especially from an evaporating surface ; the influence which its condensation into cloud and rain exerts on aerial currents, in regard to all which more satisfactory methods of observing this vital element, and discussing the results of observation, are greatly to be desired. There are here large important fields of inquiry awaiting experimental and observational physicists. The law of the dilatation of gases, known as the "Law of Boyle" or "Law of Mariotte," is this: The volume occupied by a gas is in inverse ratio to the pressure under which it exists, if the temperature remains the same; or the density of a gas is proportioned to its pressure. Consequently, air under a pressure equal to that of two atmospheres will occupy only half the volume it occupied under the pressure of one atmosphere ; under the pressure of three atmospheres, one- third of that volume, &c. By doubling the pressure we double the elasticity. If, however, the temperature be increased, and the air occupy the same space, the pressure will be increased ; but if the pressure is to remain the same, the air must occupy a larger space. From Regnault s experiments, it is concluded that the co-efficient which denotes increase of elasticity for 1 Fahr. of air whose volume is constant equals 002036 ; and that the co efficient which denotes increase of volume for 1 Fahr. of air whose elasticity is constant equals 002039. Those portions of the atmosphere in contact with the earth are pressed upon by all the air above them. The air at the top of a mountain is pressed upon by all the air above it, while all the portion below it, or lying between the top of the mountain and the surface of the sea, exerts no pressure whatever upon it. Thus the pressure of the atmosphere constantly diminishes with the height. If, then, the pressure of the atmosphere at two heights be observed, and if at the same time the mean temperature and humidity of the whole stratum of air lying between the two levels were known, the difference in height between the two places could be calculated. For the development of this principle, see BAROMETRIC MEASUREMENTS OF HEIGHTS. The air thus diminishing in density as we ascend, if it consists of ultimate atoms, as is no doubt the case, it follows that the limit of the atmosphere will be reached at the height where the force of gravity downwards upon a single particle is equal to the resisting force arising from the repulsive force of the particles. It was long supposed, from the results of observations on the refraction of light, that the height of the atmosphere did not exceed 45 miles ; but from the observations of luminous meteors, whose true character as cosmical bodies was established a few years ago, it is inferred that the height of the atmosphere is at least 120 miles, and that, in an extremely attenuated form, it may even reach 200 miles. Though there are considerable differences in the specific gravities of the four constituent gases of the atmosphere, viz., oxygen, nitrogen, carbonic acid gas, and aqueous vapour, there is yet no tendency to separation among them, owing to the law of diffusion obtaining among elastic fluids mixed together. While the proportion of these gases is in a general sense constant, there are, however, consistent differences in the amounts of oxygen and nitrogen in the air of unwholesome places, as first shown by Eegnault. Tlio following figures, showing the volume per cent, of oxygen, rest on the authority of Dr Angus Smith, who has given much attention to this subject : Sea-shore of Scot land and Atlantic (lat. 43 5 K, long. 17 12 W.), 20 99 ; tops of Scottish hills, 20 -98 ; in sitting-room feeling close but not excessively so, 20 S9 ; backs of houses and closets, 20 70 ; under shafts in metalliferous mines, 20 424 ; when candles go out, 18 50 ; when it is very difficult to remain in the air many minutes, 1 7 20. The variations in the amounts of carbonic acid in different situations are great ; thus in the London parks it is 0301; on the Thames, 0343; whero fields begin, 0369 ; in London streets in summer, 0380 ; during fogs in Manchester, 0679 ; in workshops it rises to 3000, and in the worst parts of theatres to 3200 ; and the largest amount, found in Cornwall mines, is 2 5 000. Great differences have been observed by Dr A. Smith between country rain and town rain : country rain is neutral; town rain, on the other hand, is acid, and cor rodes metals and even stones and bricks, destroying mortar rapidly, and readily spoiling many colours. Much infor mation has been obtained regarding impurities, in the air of towns and other places by examining the rain collected in different places. The air freest from impurities is that collected at the sea-coast and at considerable heights. Again, ammonia is found to diminish, while nitric acid increases, in ascending to, at least, habitable heights. As regards organic matter in the air, it corresponds to a con siderable extent with the density of the population. As might have been supposed from the higher temperature, more nitric acid is contained in rain collected on the Continent than in the British Islands. This inquiry, which is only yet in its infancy, will doubtless continue to be vigorously prosecuted, particularly since we may hope thereby to arrive at the means of authoritatively defining the safe limits of the density of population, and the extent to which manufactures may be carried on within a given area. The influence of atmospheric impurities on the public health has received a good deal of attention. The relation of weather to mortality is a very important inquiry, and though a good deal has been known regarding the question for some time, yet it has only recently been systematically inquired into by Dr Arthur Mitchell and Mr Buchan, the results of the investigation which deals with the mortality of London being published in the Journal of the Scottish Meteorological Society (New Series, N"os. 43 to 46). Considering the weather of the year as made up of several distinct climates differing from each other according to temperature and moisture and their relations to each other, it may be divided into six distinct climates, characterised respectively by cold, cold with dry- ness, dryness with heat, heat, heat with moisture, and cold with moisture. Each of these six periods has a peculiar influence in increasing or diminishing the mortality, and each has its own group of diseases which rise to the maxi mum, or fall to the minimum mortality, or are subject to a rapid increase or a rapid decrease. The mortality from all causes and at all ages shows a large excess above the average from the middle of November to the middle of April, from which it falls to the minimum in the end of May ; it then slowly rises, and on the third week of July suddenly shoots up almost as high as the winter maximum of the year, at which it remains till the second week of August, falling thence as rapidly as it rose to a secondary minimum in October. Regarding the summer excess, which is so abrupt in its rise and fall, it is almost altogether
