air can hold as water-vapour depends on its t ture. Therefore, if the temperature be lowered vapour will be condensed on any solid surfa which it comes in contact until the amount le maximum which can be held at the new temp The temperature at which any portion of air con to deposit vapour in the above manner is ter dew-point." Also the Also the pressure exerted by vapour is a measure of the amount of this vapour, and is usually but not happily tern tension of aqueous vapour. The proportion

the actual vapour-tension at any particula perature and the maximum vapour-tension at that temperature is called the "relative hun Hence the amount of aqueous vapour wh exist in any portion of the atmosphere diminish rapidly with decrease of temperature, so t have, roughly speaking, only to ascend abou feet (1830 m.) to get above one half the amount of vapour contained in the atmo while at about 25,000 feet (7620 m.) the amount of vapour is very trifling.

The distribution of water-vapour at the surface of course depends greatly on the res geographical position of water and land s and of damp and dry surfaces generally. is therefore most abundant over the oceans a great lakes and the land adjacent to the water gives off vapour in proportion to i temperature on the one hand, and to the c for additional vapour in the air on the

(which depends on the temperature of the air), the amount of vapour over a sea surface in the tropics. is greatly in excess of the vapour at the poles. And, considering the distribution of temperature over the globe, it is obvious that a very small proportion indeed of the inter-tropical vapour can ever pass into the temperate zones during winter.

7. We can now easily picture to ourselves some of the simplest of those conditions under which the vapour present in the atmosphere is condensed into particles of water or congealed into those of ice. These processes must evidently take place whenever a layer of moist air has its temperature sufficiently lowered. This result may be attained when the dust particles in the air lose heat by radiation into space, or when the surface of the earth itself loses so much heat by radiation that the vapour immediately above it becomes mist or fog, or when fog or cloud itself radiates heat into space and has the temperature of its upper surface lowered. Again, a similar result will be effected when a warm moist. wind moves over a relatively cold surface, such as the ocean surface in the summer of the temperate zone, or when such a wind travels into higher latitudes, and therefore has its temperature diminished, or into lower latitudes within the tropics. whenever the temperature near the tropic is greater than that near the equator. Also when the temperature of a moist current is diminished by simple expansion, as where a damp wind has to cross a range of mountains, producing the cloud-caps familiar

to all observers. Also where a cold wind inter with a current of warmer air with a temperatu the dew-point.

8. Remembering the law that condensatic duces heat, and expansion absorbs heat or p cold, we are now in a position to understan the effect of the presence of aqueous vapour in an "inversion movement" taking taking place atmosphere in "unstable equilibrium" (§ 5) imagine that from some cause or other, w this be difference of pressures or rise of wateritself, or accidents in the earth's surface, or causes, a current of air is rising in a more vertical direction through layers of air in wh tensions of aqueous vapour approach their m Here at some level above the earth's surfa pendent on distribution of temperature and pr tension of vapour, a great amount of vapo be simultaneously condensed, forming the portion, often level, of a body of cloud. mediately this condensation occurs the air in which it takes place are heated, expand rising, further augment the ascending curren temperature again falls, and condensation takes in layers of air above, and so the process go Now it is evident as we rise higher and highe colder layers of air the heat given out by densation will become less and less, because lavers contain less water-vapour. Also as w higher and higher we shall have reached lay air which are at such great altitude that th

not much affected by radiation from the earth, and that therefore are in a state of " stable equilibrium." Our ascending current will therefore have lost the accelerating effects of condensation, and will have become much colder and denser than the air on all sides of it. It will therefore flow outwards and downwards, and cooling the air in its descent, condensation will take place in a downward direction, water particles pouring in a stream down the exterior surface of the cloud until the current is finally checked at some lower level by the increasing heating effects of this condensation. And it will be fairly evident that such a process may take place, although in a much less degree, in an atmosphere in "stable equilibrium." The cloud in this case will not attain nearly such great vertical proportions. Thus in

winter and on cool summer days we are not likely to see those great masses of cloud which produce heavy local thunderstorms. Fig. 1 is a rough representation of a cloud formed in the manner above described, the length of the arrows intending to represent roughly the velocities of the currents.

9. We thus see how great are the effects produced by the presence of aqueous vapour in "inversion movements," and how truly this presence may be said to make mountains out of molehills. The following data may give us some idea of the work done by condensation of vapour :

"For one-tenth of an inch of rain precipitated from the lowest mile of the atmosphere we should have a rise of temperature of this lowest mile

amounting to 6° Fahrenheit (3.3° C.), quite enough to produce a powerful ascending current. The ascending current expanding and cooling produces further condensation. The heat developed over one square foot of the earth's surface under these conditions is equivalent to the work done at the rate of one horse-power continued for twelve minutes. Over one square mile this would be ten million horse

FIG. 1.-INVERSION MOVEMENT PRODUCING A CLOUD.

power for half an hour. A fall of one-tenth of an inch of rain over the whole of Great Britain gives heat equivalent to the work of one billion of horses for half an hour!"

Statements like the above appeal vividly to the imagination, but it is unfortunate that the work done by the conversion of vapour into cloud without rain cannot be definitely measured. If it could, we might then be able to "cap" the above with a row of figures which would dwarf it into insignificance.