1906-1907.] 503 



o'clock Mr. T. E. Farrington read a paper on " Clouds and 

 their Evolution," with limelight illustrations, during which he 

 said that the chief difficulties in the study of clouds arise from. 

 a somewhat confused condition of the nomenclature, the 

 mutability of the clouds themselves, and from the complexity 

 of the operations of the natural laws which affect them. 

 Clouds are not results of chance, but of unchanging physical 

 laws. Representations of clouds are unavoidably inadequate, 

 because their ever-acdng light cannot be transferred to a 

 picture. Cloud is properly defined as a visible collection of 

 particles floating in the atmosphere. Production of clouds 

 depends on the evaporation of water by the sun's heat, and its 

 condensation when its acquired heat is withdrawn ; and also 

 upon the coolmg effects of expansion of moist air. The 

 presence of dust also' has much to do with it. A miniature 

 cloud may be seen when the steam escaping from the funnel 

 of a locomotive is cooled by the air and by its expansion, and 

 condensed into minute water spherules, called by Tyndall 

 "water dust." In 1803 Luke Howard classified clouds under 

 three primary types — Cirrus, Cumulus, and Stratus — which types 

 still form the basis of cloud classification. The present official 

 arrangement recognised sixteen kinds of cloud, but recently Mr. 

 Claydon has described thirty-two varieties. Cumulus, or the 

 heap-cloud, is formed by successive condensation in the upper 

 portions of moist ascensional currents of air. If such a current 

 is of small area or is soon checked, the cloud will be small, but 

 if the area is larger and the current vigorous. Cumulus of great 

 extent and grandeur will be evolved. Cumulo-nimbus, or 

 thunder-cloud, is a development from Cumulus. When Cumulo- 

 nimbus is not formed. Cumulus may degrade into Fracto-cumu- 

 lus, or into an evening cloud, sometimes called the fall-cloud, 

 which evaporates as the twilight advances. Common Stratus, 

 which lies in horizontal sheets, is produced either by a very 

 gentle ascensional current, or by a downward movement of 

 cold air. Fog and Stratus are closely allied, and one form of 

 Stratus is really lifted fog. Cirrus in its many forms consists 

 of spicules of ice, but there are intermediate clouds between 

 Cirrus and Cumulus termed " alto-clouds," the distinguishing 

 feature of which is that they consist of water-spherules un- 



