14. It will be seen, therefore, that the presen of water-vapour aids the condition of "unstab equilibrium," mentioned in § 5, for its rapid increas in amount with decrease of altitude will, by absorp tion of heat rays, unduly augment the increase o temperature of the air with decrease of elevation.

15. We now come to another important ingredient in our atmosphere. This ingredient, which is in reality an impurity, has hardly received the attention which it deserves. There is no doubt that dust particles in the air not only bear an important relation to organic life on the earth, but also have a very great influence in altering the conditions of the atmosphere above. Researches on the subject have been principally carried out by Professor Aitken, who has devised apparatus by which the number of dust particles in a given portion of air can be counted. He finds that this number depends on the position and altitude of the locality, the direction of the wind, the season of the year, and the hour of the day or night; further, that they are much more sensitive than the air around them to the heating and cooling effects of radiation, so that the temperature of the dust particles, if the temperature is increasing or decreasing, is always respectively greater or less than the temperature of the air.

There are further obscure seasonal variations dependent on velocity of wind and on organic processes taking place on the earth. Aitken found the mean number of dust particles at Kingairloch in the west of Scotland to be about 26,240 per cubic inch (1600

per c.c.), while in London the mean amounted to

inch (100,000 per c.c.), Looking at the horary

about 1,640,000 per cubic and in Paris rather more. curves of the variations in the quantity of dust on Ben Nevis, we see that all the hours from midnight till 10 A.M. present numbers which are below the diurnal mean, the minimum being about dawn. The curve then rises with great rapidity till 4 P.M., the first part of the rise being somewhat the greatest. It then begins to fall, and continues to do so until 7 P.M., when a considerable check takes place, and the fall is not continued till nearly midnight.

16. Now in all regions of the globe, with but few exceptions, there are diurnal oscillations of atmospheric pressure at the earth's surface. These variations are related to the condensation and evaporation of moisture in the atmosphere, and therefore must be noted in these pages. The exact relationship is not yet fully understood, and forms at subject of essential interest which, when thoroughly worked out, will undoubtedly throw much light on some of the more obscure phenomena of cloudformation. The diurnal phases may be thus enumerated:

1. A nocturnal minimum of pressure occurs at early dawn.

2. This is followed by a morning maximum, generally occurring between 8 a.M. and 9 A.M.

3. Pressure then declines to the afternoon minimum, occurring between 2 P.M. and 4 P.M.

C

4. This is succeeded by an evening maximum, generally occurring between 8 P.M. and 11 P.M.

5. Pressure then decreases gradually to the nocturnal minimum.

These phenomena are much more marked in low than in high latitudes, and near the equator they are so pronounced and so slightly interfered with by the passage of temporary atmospheric disturbances that the hour of the day can frequently be read off by a glance at the barometer.

17. Now Buchan's explanation of these diurnal variations of pressure as caused indirectly by the action of dust particles seems the most plausible. During the night the dust particles lose heat rapidly by radiation into space. They lower the temperature of the surrounding air below the dew-point, and consequently decrease the vapour-tension. This decrease is greatest just before dawn, because then the work of radiation has reached its climax; thus the pressure at the earth's surface is lowest at this time. After the sun has risen vapour-tension is again increased by the evaporation of moisture from the surface of the dust particles; the latter also get superheated by the sun's rays, and help to increase the temperature and the elastic force of the air itself. This double increase is sufficient to affect the barometric column producing the morning maximum at the hour when the lower strata of air are increasing in temperature with the greatest rapidity; for the tendency to lift the whole atmosphere without affecting the pressure at the earth's surface is not

sufficient to overcome the inertia and viscosity of the higher layers, especially as those layers have much fewer dust particles, and are not similarly affected.

Now, as insolation upon the surface of land and water increases, temperature and evaporation also increase, and form strong ascending currents which overcome the resistance of the higher layers mentioned above. Thus pressure again decreases to the afternoon minimum. These descending currents are broken up over land surfaces, and are partially. compensated for over each locality by descending currents on all sides of them. It should be observed that dust particles being more numerous and of larger size in the lower strata of the atmosphere than in the higher, the whole operation which results in the afternoon minimum is confined to the lower and middle layers.

After the afternoon minimum caused by the superheating of the lower atmosphere has passed away from the longitudes affected by it, barometric pressure at the earth's surface commences to rise until the evening maximum is attained, which is usually the case after sunset. This maximum may be attributed partly to the overflow of the atmosphere still rising in the west, and partly to descensional currents established in the rear of the upward current.

18. These fluctuations of pressure are, at least over land surfaces, intimately connected with certain definite cloud-formation, especially where there are no very strong horizontal currents. These will be treated of when we come to speak of the cloud-forms

of the globe. It will, however, be seen here that the morning and evening maxima will generally coincide with the least amount of cloud, because in the first the increase of vapour-tension is not rapid enough to enable the resistance of the upper strata to be overcome by an ascensional current, and because in the second case descending currents exist which cause evaporation of water particles. The afternoon minimum, however, frequently coincides, over those land surfaces which are not extremely arid, with the maximum formation of "Inversion" clouds, and therefore of precipitation.

It may also be mentioned that wherever there are, owing to geographical position, upper currents of considerable velocity, unaffected by the heating of the lower layers of air, the rapid nocturnal cooling of the middle and upper strata tends to produce high night cloud of Inclination and Interfret types. In such cases we frequently observe the greatest amount of cloud and also of precipitation in the hours of the night. This is particularly the case upon and in the neighbourhood of high mountains, the nocturnal radiation from which tends to lower the temperature of the surrounding atmosphere.