under very different circumstances. For instance, cumulus may be formed by the uptake of a cyclone, by the evaporation of a fine day, and by the collision between winds, such as land and sea breezes.

In applying any of these names, it must be borne in mind that the varieties of cloud forms are really infinite and that no two are ever exactly alike; also that almost every form can be found gradually merging into some other. All that can be done is to give certain typical kinds, and the observer must assign any particular cloud to the type which he thinks the form most nearly approaches.

On the above principles, 90 per cent. of all clouds in every part of the world may be sufficiently described by the following

ten terms:

HIGH LEVEL CLOUDS OVER 20,000 FEET.

1. Cirrus (contracted abbreviation cir.)-A wisp of hairy, fibrous, or feathery clouds, usually thin and white. Figure No. 1 in the accompanying photographic plate represents a typical example of white cirrus wisps floating far above the dark rocky cumulus which occupies the lower part of the picture. This was taken in the south of England. The forms of cirrus are innumerable and are capable of classification for specialists, but the only variety which need be remarked by general observers is the long linear or stripe form. These stripes are sometimes composed of fibrous bundles which lie in the same direction as the stripe itself, as in figure 3 e.d., while at other times the stripe is made up of a series of short bars, or ribs as it were, as in figure 3 b.c. It is important to observe the direction in which these stripes lie, as will be explained later on.

Sometimes the top of a cumulus, such is shown at the bottom of No. 1 in the photographic plate, is combed out into a hairy

*The nomenclature used here is that proposed by M. M. Hildebrandsson and Abercromby for international use. The photographic illustrations were all taken by Mr. Abercromby.

structure. This formation is known as " false cirrus," and must not be mistaken for true high level cirrus. A typical example is given in the accompanying engraving (fig. 1), from a photograph by the Author of a thunderstorm in Borneo.

2. Cirro-Stratus (contracted Cir-str.).-A thin, high sheet of cloud, which instead of being uniform like pure stratus, is composed of fibres of cirrus in any complexity. Sometimes the fibres or cirrus interlace and give this cloud a reticulated appearance like a woven cloth. Besides thin fibres, cirro-stratus is usually associated with a sort of general white formless veiling of the sky, due to the presence of ice dust at a high level, and the sun and moon usually appear surrounded by halos when they shine through this form of cloud.

It is almost impossible to photograph such a thin, hazy cloud as cirro-stratus, but No. 2 of the photo plate is a very heavy reticulated example from the south of England. The reticulated portion is at a much higher level that the dark uniform stratus at the top and sides of the picture.

Cirro-stratus, in some form, is the invariable precursor of cyclones in every part of the world.

3. Cirro-Cumulus (contracted Cir-K).-A high, thin cloud, usually in flocks of small detached fleecy structure. The word is purely conventional, for this cloud has neither the fibrous texture of cirrus, nor the rocky structure of cumulus; nevertheless the word is almost universally used for this form of cloud, and so long as we know what form the name denotes, it does not much matter whether the term is logical. The upper part of No. 3 of the photographic plate is a good specimen of cirrocumulus from the Falkland Islands.

Sometimes the flocks of cloudlets are in detached round masses, without the characteristic fleecy structure, and the whole appearance is then commonly known as "mackerel sky." This may also be reported as cirro-cumulus, as both forms are closely allied.

MIDDLE LEVEL CLOUDS. 12,000-20,000 FEET.

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4. Strato-Cirrus (contracted Str-Cir).-A cloud identical in general structure with cirro-stratus, only denser and at a lower level. It is impossible to reproduce by photography the difference of density which practically enables an observer to distinguish between high and middle level forms of this cloud structure, as the relative scale of light and shade is lost in the camera; but generally in practice, if the cloud is very thin it may be assumed to be floating at a higher level than a denser specimen of similar structure. No absolute rule can, however, be given; but the observer must look at all the surrounding clouds, and form the best estimate he can. He will often be greatly assisted by remembering how the clouds have become formed, whether by the gradual subsidence of the highest forms, or by the ascent of the lower clouds. No. 4 of the photo. plate

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is an example of strato-cirrus from the Indian ocean, about 2° S. of the Equator. The thin flat layer of cloud is drawn out at the edges into fibres, which shows that the cloud is a compound of cirrus and stratus; but as the cloud is moderately dense and comparatively low, we know that it ought to be called strato-cirrus.

The varieties of strato-cirrus are as numerous as those of cirro-stratus, and, like the latter, it often assumes a reticulated structure.

5. Cumulo-Cirrus (Contracted K-Cir).-A low, dense form or cirro-cumulus which is formed at a middle level, and therefore, by the principles before explained, must be called cumulocirrus. No. 5 of the photo. plate is an example from about 7° N. latitude, in the Atlantic Doldrums. The lower, dark uniform patches of cloud below, are detached masses of stratus, while at a higher level the dense layer of characteristically fleecy cloud can be readily distinguished.

The fleecy structure of cirro-cumulus might appear to be absolutely distinct from the fibrous character of cirro-stratus, but in nature the two forms turn from one into the other, as a glance at No. 2 of this plate will readily show.

It may seem at first sight that this distinction of cirro-stratus and cirro-cumulus from strato-cirrus and cumulo-cirrus is a useless refinement, but the necessity for doing so will be readily understood from the following considerations:-One of the most important objects of these observations is the determination of the direction in which clouds move at different heights. For instance, it has been discovered that the low cumulus clouds in front of a cyclone drift from S., the middle level strato-cirrus from S.W., and the highest cirro-stratus from about W. Sometimes the middle strato-cirrus is replaced by cumulo-cirrus, but if an observer mistook this middle cloud form for the high cirrocumulus, we should infer a very erroneous conception of the nature of the circulation of the air round a cyclone. We should

think that the middle level currents came from the S.W. instead of from the W.

Low LEVEL CLOUDS. UNDER 12,000 FEET.

6. Strato-Cumulus (contracted Str-K).-A cloud which is too irregular to be called stratus, but not rocky enough to be called cuinulus. The general form can be readily gathered from an inspection of No. 6 in the photo. plate, which was taken near Teneriffe. Observe how the flat, irregular masses of clouds get thinner and thinner as they are seen more and more edge on towards the horizon, till at last they look like a series of parallel bars. This, of course, is an effect of perspective, and must not be mistaken for the parallelism of cirrus stripes to the horizon in one particular direction. The peculiarity of these stratocumulus bars is that they are always parallel to the horizon which ever way you turn, whereas cirrus stripes-being really and not apparently in lines-appear to converge on two points of the horizon exactly opposite one another. This will be fully explained in a future paragraph.

8. Cumulus (contracted K.).—A rocky-topped cloud, usually with flat base. No. 7 of the photo. plate is an excellent example of the simplest form, taken at sea, about 100 miles east of Santos in Brazil.

8. Cumulo-Nimbus (contracted K.-nim.).—Sometimes rain falls from cumulus-topped cloud. In this case, the base is a confused mass of gloom, and the whole is called cumulo-nimbus. The example, No. 8 of the photo. plate represents a fine cloud of this type hanging over the city of Rio Janeiro.

9. Nimbus (contracted Nim.)-Rain often falls from clouds whose form at a distance cannot be observed, and sometimes from a cloud like flat strato-cumulus. In all cases, except when the cumulus form is noted, rain clouds must be reported as nimbus. The value of the distinction between nimbus and cumulo-nimbus is that cumulo-nimbus is practically characteris