REPORT ON ATMOSPHERIC CIRCULATION. 17 



amount of aqueous vapour present in the atmosphere ; but it is here particularly 

 insisted on that the barometric oscillations themselves are independent of any changes 

 of temperature of the floor on which the atmosphere rests. We shall, then, consider 

 the phenomena chiefly, as the results of observation present them to us, as existing 

 over the free ocean, and therefore cleared of all complications arising from the diurnal 

 heating of the surface. 



Physicists are divided in opinion as to whether the aqueous vapour of the air, 

 while in the purely gaseous state, is or is not as diathermanous as is the dry air of the 

 atmosphere, no decisive experiment having yet been made to prove the relation of 

 purely gaseous vapour to radiant heat. But it is quite different as regards the water 

 suspended in the atmosphere in the liquid, and in the solid form in minutely divided 

 states, and as regards the particles of dust which recent research has shown to be every- 

 where present in the atmosphere. It is from Mr. John Aitken's ingenious experiments 

 and researches that an insight may be obtained as to the relations of the dust particles 

 to the aqueous vapour of the atmosphere. 



Mr. Aitken showed, in his paper on Dust, Fogs, and Clouds, 1 that a solid nucleus 

 is necessary for the condensation of water-vapour in the formation of fogs and clouds, 

 and in subsequent communications to the Eoyal Society of Edinburgh he has shown 

 that even the purest air that can lie obtained contains an enormous number of fine 

 dust particles. The purest air examined, which was obtained at Ben Nevis Observatory, 

 contained 2100 dust particles per cubic inch; in Edinburgh, on a fine clear day, the 

 number was 738,000 ; whilst in air taken from near the ceiling of a hall about the close 

 of a meeting, the dust particles to the cubic inch were 57,400.000. 



Let us now look at the phenomena of the diurnal oscillation as found in the Pacific 

 near the equator, and in the midst of the largest water surface of the globe. Plate I. 

 fig. 7 shows the hourly variations of pressure from observations by the Challenger, 

 September 1 to 12, 1875, in mean lat. 1° 10' S. and long. 150° 4'W., the mean pressure 

 for these days having been 29 "928 inches. The most remarkable feature of the curve is 

 the amplitude of the range from the morning maximum to the afternoon minimum, and 

 the rapidity of the fall in the four hours from 10 a.m. to 2 p.m., amounting to 0'087 

 inch. This and the other features of the curve are substantially the same for all 

 positions on the open sea for at least 12° on each side of the equator. In higher 

 latitudes, over land, in anticyclonic regions, and in particular geographical situations, 

 the curves become more or less modified. They all agree in showing the double maxima 

 and minima, except in a few restricted regions of high latitudes already referred to. 



If the temperature of the whole of the earth's atmosphere were raised, atmospheric 

 pressure would be diminished, inasmuch as the mass of the earth's atmosphere would 

 thereby be removed to a greater distance from the earth's centre of gravity. But quite 



1 Trans. Roy. Soc. Ediv., vol. xxx. pp. 337-368. 

 (PHTS. CHEJr. CHALL. EXP. — PART V. — 1889.) 



