The estimated heights given must be looked upon as very conjectural, although they have been derived from the best existing authorities. It is sufficient to know that the range of the altitude of the various clouds is from that of the _Nimbus_, or _thunder cloud_, 1,500 feet, to that of the _Cirrus_, 24,000 feet, the others being intermediate. The first three of the clouds above enumerated constitute what are called the _primary forms_. The remaining four are called _secondary forms_, because they arise, as their names generally indicate, out of combinations of the _primary forms_. Although, from the frequent mingling of clouds, it is not always practicable to identify them by the adopted classification, still, as there is generally a prevalence of one type of cloud over another, the observer would be able to distinguish a _"Cirrus sky,"_ or _"Cirro-cumulus sky,"_ &c. Upon some occasions the typical characters of the clouds are beautifully defined; and the contemplation of their forms, and the laws of their formation, affords infinite pleasure to the observer. The advantages of scientific knowledge are such, that whether you look downwards, to the earth, or upwards to the sky, you have still the writing of God to read. 380. _What produces the various shapes of clouds?_ 1. The state of the _atmosphere_. 2. The _electrical_ condition of the clouds. 3. The _movements_ of the atmosphere. 4. The _season of the year_. [Verse: "Behold, he withholdeth the waters, and they dry up; also he sendeth them out, and they overturn the earth."--JOB XII.] 381. _What are the dimensions of clouds?_ A single cloud has been estimated to have as many as _twenty square miles of surface_, and to be _above a mile in thickness_, while others are no larger than a _house_, or a man's _hand_. [Illustration: Fig. 7.--CUMULUS, OR PILE CLOUD.] 382. _How are clouds affected by winds?_ If _cold winds_ blow upon the clouds, the cold condenses the vapour, turning the clouds into _rain_. But if _warm dry winds_ blow upon the clouds, they _rarefy the vapour_ to a greater degree, and temporarily _disperse the clouds_. 383. _How do winds affect the shapes of clouds?_ When winds are _mild and gentle_, the clouds break into _small patches_, and rise to a considerable height. But when the winds are cold and blustering, the clouds fly low, and roll along in _heavy masses_. 384. _Why are east winds usually dry?_ Because in coming towards England they pass over vast continents of land, and comparatively little ocean. Hence they are not loaded with _vapours_. 385. _Why do west winds generally bring rain?_ Because they come across the _Atlantic_, and are heavily charged with _vapour_. 386. _Why are north winds generally cold and dry?_ Because they come from the arctic ocean, over vast areas of _ice and snow_.