1893.] Mr. Shelf ord Bidwell on Fog, Clouds and Lightning. 97 



WEEKLY EVENING MEETING, 

 Friday, May 5, 1893. 



Sir Frederick Bramwell, Bart. D.C.L. LL.D. F.R.S. Honorary 

 Secretary and Vice-President, in the Chair. 



Shelford Bidwell, Esq. M.A. LL.B. F.R.S. M.B.I. 



Fogs, Clouds and Lightning. 



The air, as every one knows, is composed almost entirely of the two 

 gases, oxygen and nitrogen. It also contains small quantities of 

 other substances, of which the chief are carbonic acid gas and water 

 vapour, and it is the latter of these constituents, water vapour, or 

 " steam " as it is sometimes called, that will principally concern us 

 this evening. 



The quantity of invisible water vapour which the air can at any 

 time take up depends upon the temperature ; the higher the tem- 

 perature of the air the more water it can contain. The proportion, 

 however, never exceeds a few grains' weight of water to a cubic foot 

 of air. Air at any temperature, containing as much water as it can 

 possibly hold, is said to be " saturated," while the temperature at 

 which air containing a certain proportion of water becomes saturated 

 is called the " dew point." 



The water vapour contained in the atmosphere plays a very im- 

 portant part in many natural phenomena. Among other things, it 

 is the origin of clouds and of fogs. If a body of air containing water 

 in the form of invisible vapour is quickly cooled to a temperature 

 below its dew point, a portion of the vapour becomes condensed into 

 a number of minute liquid particles of water, forming a visible mist, 

 which, when it is suspended in the upper regions of the air, is called 

 a cloud, and when it rests upon the surface of the earth is only too 

 familiarly known as a fog. 



The cooling of water-laden air may be brought about in various 

 ways, resulting in the formation of clouds of several distinct char- 

 acters. [Photographic examples of cumulus, stratus and cirrus 

 clouds were exhibited upon the screen.] For experimental purposes 

 a small body of air may be most conveniently cooled by allowing it 

 to expand. I have here a flask of air which can be connected with 

 the partially exhausted receiver of an air-pump. Inside the flask is 

 an electrical thermometer or thermo-junction, the indications of 

 which can be rendered evident to all present by the movement of a 

 spot of light upon a scale attached to the wall. A deflection of the 



Vol. XIV. (No. 87.) h 



