320 H. F. Blanford — On the Physical Explanation of the [No. 4, 



places, and at one and the same place at different times of the year. These 

 variations affect the hour at which the pressure attains its maximum and 

 minimum values, the absolute amplitude of the oscillations, and lastly, 

 their relative amplitude. It is this phenomenon — the variation in the 

 relative amplitude of the day and night oscillations — the probable physical 

 explanation of which I have now to bring to notice. 



It was observed by Arago, apparently some years prior to 1811, that 

 in Europe " the proximity of the sea has the effect of diminishing the 

 amplitude of the interval during which the diurnal fall lasts, viz., that 

 which occurs between 9 A. m. and 3 P. M. ;" and considering the whole 

 phenomenon as made up of a single and double oscillation, it may easily be 

 shewn that this interval is determined mainly by the relative amplitude of 

 these two elements. Tbe latest notice on the subject is given in the follow- 

 ing extract from Mr. Buchan's Memoir, a copy of the first part of which, 

 (for which I am indebted to the author,) has reached me only within the 

 last week.* In summing up the characteristics of the midday fall of 

 pressure, he says : — " Whatever be the cause or causes on which the diurnal 

 oscillations of the barometer depend, the influence of the relative distribu- 

 tion of land and water in determining the absolute amount of the oscillation 

 in particular localities as well as over extended regions, is very great. 

 From the facts detailed, (in Mr. Buchan's paper), it will be seen that 

 this influence gives a strong local colouring to the results, particularly 

 along the coasts ; and that the same influence is extensively felt over the 

 Channel, the Mediterranean, the Atlantic, and other sheets of water on the 

 one hand ; and on the other over the inland portions of Great Britain, 

 Europe, and the other continents;" and, further on, he adds : — " While, as 

 has been pointed out, numerous illustrations can be adduced, shewing a 

 larger oscillation over the same region with a high temperature and a dry 

 atmosphere than with a low temperature and a moist atmosphere ; the small 

 summer oscillation on the coasts of the Mediterranean and those of the 

 Atlantic adjoining, is in direct opposition to the idea that any such conclu- 

 sion is general. For over those parts of the Mediterranean and Atlantic, 

 the temperature is hottest in summer, and the air is driest — so dry, indeed, 

 that no rain, or next to none, falls ; and yet there, the amplitude of the 

 oscillation now contracts to its annual minimum. On the western coasts of 

 the Atlantic, from the Bahamas northwards to Newfoundland, the tem- 

 perature is at the annual maximum, but the air is not dry, being liberally 

 supplied with moisture, and the rainfall is generous. But with these very 

 different meteorological conditions, there occurs, equally as in Southern 

 Em-ope, a diminished oscillation during the summer months in the islands 

 and near the coasts of North America ; and, in the south of Europe, the 

 * Written in the beginning of March., 1876. 



