In general the lowest stratum of air about the earth contains the greatest quantity of aqueous vapour ; and hence it might be expected that more rain should fall on low level plains than in elevated countries. The contrary, however, is the fact : and thii may be accounted for by the variety of currents among mountains, and by clouds resting frequently on the summits of hills without descending to the plains. While the average annual depth of rain at Keswick is inches, in the interior of the country and on the sea-coast it is but 25 inches : and while the average depth on the St. Bernard is inches, that at Paris is 20 inches only. Yet, from the observations of Dr. lieberden, Mr. Luke Howard, and M. Arago, it appears that the depth of rain on the level of the ground is greater than at the top of a building. The first of these philosophers found that the annual depth at the top of Westminster Abbey was 12.099 inches, while at a lower level, on the top of a house in the neighbourhood, it was 18-139 inches; and on the ground, in the garden of the house, it was inches. M. Arago observed, from observations during twelve years, that on the terrace of the Observatory at Paris the annual depth was centi metres (19'88 inches), while in the court of that building, which is 28 metres (30 yards) lower, the annual depth was 50'371 centimetres 22'21 inches).
On this subject we are now able to add some precise informa tion derived from modern research. la 1833 was communicated to the British Association for the advancement of Science, a ' Report of Experiments on the Quantities of Rain Falling at different Elevations above the surface of the ground at York, undertaken at the request of the Association:. by William Gray, jun., and John (now Professor) Phillips ; to which remarks ou their results were added by the latter ; and the whole published in the Third Report' of the Association, pp. 401-412. These experiments were made at three stations; the summit of York Minster, at the height of 241 feet 10i inches above the river Ouse, which is nearly level with high water in the Humber; the roof of the Yorkshire Museum, at the height of 72 feet 8 inches, and at the distance of 1100 feet from the Minster; and in the Museum grounds, at the height of 20. feet, and distance of 130 feet from the Museum ; the second and third stations were nearly equidistant from the Minster. These points embraced in a remarkable manner the desired conditions of gradation of altitude, openings of sky, and contiguity of position. The results for twelve months, exclusive of snow, were, in the order of altitudes, as follows inches, On these results Mr. Phillips remarks, after arranging them in relation to mean temperature and the season of tho that the diminution of the quantity of rain received at eights above the ground, as compared with that received on the ground, is very accurately represented by a simple formula involving one constant, namely, the square root of the height of the station above the ground, and one variable coefficient. lie then examines Dr. Ifeberden's results at Westminster, noticed above, and finds that it is probable that they obey the same constant relation to height as those of York, but the values of its variable co-efficient are very different, a subject which he next proceeds to investigate, with reference also to Arago's observations at Paris. He finally obtains a remarkable and continued accordance betweeu the co-efficient fixed by observation and those derived by two methods from a very simple view of the condition of the air as to heat and moisture, which appears to him decisive of the question as to the general cause of the variation of the quantity of diminution of rain at any one height above the ground.
The observations strictly warrant the conclusion that the ratio of diminution at different heights is constant throughout the whole year. Professor Phillips, therefore, offers the hypothesis, as a matter of very probable inference, "That the whole difference in the quantity of rain, at different heights above the surface of the neighbouring ground, is caused by the continual augmentation of each drop of rain from the commencement to the end of its descent, as it traverses successively the humid state of air at a temperature so much lower thnn that of the surrounding medium as to cause the deposition of moisture upon its surface. This hypothesis," he remarks, "takes account of the length of descent, because in passing through more air more moisture would be gathered ; it agrees with the fact that the augmentation for given lengths of descent is greater in the most humid seasons of the year ; it accounts to us for the greater absolute size of rain-drops in the hottest months and near the ground, as compared with those in the winter and on mountains; finally, it is almost an inevitable conse quence from what is known of the gradation of temperature in the atmosphere, that some effect of this kind must necessarily take place." Mr. Howard has observed that, in this country, when the moon has south declination there falls but a moderate quantity of rain, and that the quantity increases till she has attained the greatest northern declination ; and on some such results of observation the popular opinion that there is a connection between the alternations of rain and fair weather and the changes of the moon may be founded. The vexed question of the influence of the moon upon the weather appears to have received at length at least an approximative solution. Hum boldt, in his ' Personal Narrative,' mentions as a phenomenon well known to the pilots and seamen of Spanish America, the tendency to disappearance of clouds uuder the full moon. This meteorological fact has also been independently observed by Sir John F. W. Herschel, who was the first to recognise its importance in relation both to the radia tion of heat from the moon and to the phenomena of the earth's atmosphere. The reviser of the present article has constantly observed, since Sir J. Herschel called attention to the subject, that when the full moon, or the moou so nearly full as to appear round, rises in a cloudy sky, she soon clears of cloud a circle of sky around her, which becomes larger and larger, often 'until the entire sky is freed of cloud. Referring to the induction of Arago, from a com parison of rain registered as having fallen during a long period, that a slight preponderance in respect of quantity falls near the new moon over that which falls near the full moon, Sir J. Herschel remarks, (' Outlines of Astronomy,' 5th edit., 432 and note), " This would be a natural and necessary consequence of a preponderance of a cloudless sky about the full, and forms, therefore, part and parcel of the same meteorological fact." [HAIL; 3IETEOROLOOY ; 3IIST ; SNOW.]