METEOROLOGY. 



ATETEOROLOGY explains the laws which 

 1V1 regulate weather, seasons, and climates. 

 It involves particularly the consideration of the 

 atmosphere its pressure; its moisture; the altera- 

 tions made upon it by heat and cold ; and its 

 electrical condition. It is by an accurate know- 

 ledge of these that we are enabled to understand 

 the nature and causes of the incessant movements 

 and changes going on in the air around us, and 

 of the ever-varying appearances of the sky over 

 our head ; including the phenomena of winds and 

 storms, of rain, dew, hail, snow, mists, and clouds, 

 and their connections with one another, and with 

 seasons and places. 



THE ATMOSPHERE. 

 Its Composition General Structure Density Pressure. 



The atmosphere is a vast ocean of invisible 

 gaseous matter, enveloping the terraqueous globe, 

 and extending to a considerable height. Although 

 it cannot be seen by the eye, it is yet felt to be an 

 inert, material mass, which resists bodies in their 

 motion through it, and when set in motion itself, 

 possesses momentum or impetus, like a flying ball 

 or a running stream. Another property of the 

 atmosphere, which proves its genuine materiality, 

 is its weight or pressure. It presses upon the 

 earth, exactly as the sea does, under the influence 

 of gravitation. At the sea-level, the average pres- 

 sure is I4 pounds on every square inch ; it is 

 nearly the same as the pressure of a lake of 

 water 33 feet deep, or a lake of mercury 2^ feet 

 deep. The construction of the barometer, or 

 instrument for constantly measuring this weight, 

 and shewing the degrees of its fluctuation from 

 day to day, is explained under PNEUMATICS. 



The fluid of which the atmosphere consists, 

 is found to be not a single substance, but a 

 mixture of several substances, totally distinct in 

 their properties, and serving quite different 

 offices in the economy of nature. The two chief 

 ingredients nitrogen and oxygen which make 

 up more than fgths of the whole mass, are as 

 different in their character as water and alcohol. 

 The proportions of these two are very nearly 

 77 of nitrogen to 23 of oxygen by weight. The 

 nitrogen is therefore the principal element in 

 point of quantity ; but the oxygen performs the 

 greatest variety of functions : it is the supporter 

 of life, and the indispensable agent in combustion, 

 in putrefaction, and in many other natural pro- 

 cesses. 



Deferring in the meantime the consideration of 

 the other ingredients of the atmosphere, we have 

 to study in the first place the mode of mixture of 

 these two gases, and the general structure of the 

 mass they compose. Although the mechanism 

 and constitution of a gas are not apparent to our 

 senses, yet we can infer with certainty that it is 

 made up of atoms or molecules which keep one 

 another at a distance by a repulsive force. From 

 the fact that any gas can be compressed into a 

 very small fraction of its ordinary bulk, we are 

 sure that its particles are usually at a great distance 

 3 



from each other in comparison with their size. 

 Now, suppose two flasks connected by a tube 

 having a stop-cock, by which communication can 

 be cut off, and suppose the one filled with nitrogen, 

 and the other empty ; on turning the stop-cock, 

 part of the nitrogen will rush into the empty flask 

 until it is equally spread over the whole space ; 

 for there is no stable balance of the repulsion 

 if a particle is nearer its neighbour on one side 

 than on the other. This uniform diffusion takes 

 place almost instantaneously when the one flask 

 is empty. But now, suppose both filled, the one 

 with nitrogen, and the other with oxygen ; when 

 the communication is opened, each gas spreads 

 itself uniformly through the whole space of both 

 flasks, precisely as the nitrogen alone did, only in 

 this case time is required. The gases mutually 

 interpenetrate, and the molecules of the one retard 

 the progress of those of the other, but do not 

 otherwise affect the ultimate result. Each gas is 

 independent, and distributes itself, and exerts its 

 pressure precisely as if the other were not there. 

 It is owing to this important principle that the 

 proportion of oxygen to nitrogen is the same in 

 all parts of the world and at all heights. The same 

 law of uniform diffusion, and independence of the 

 presence of other gases, holds with regard to the 

 carbonic acid gas and the aqueous vapour existing 

 in the atmosphere, although, in the case of aqueous 

 vapour, its constant liability to condensation 

 prevents its ever attaining to a state of actual 

 uniformity. 



Taking the entire mass of the atmosphere, con- 

 sidered as an ocean of gaseous or elastic fluid, its 

 density must diminish as we ascend from the 

 earth. The rate at which the density of the air 

 diminishes upward, and the estimation of its entire 

 height, are complicated by the decrease of tem- 

 perature that is found to take place as we ascend 

 (see page 41). It may be stated here, in a general 

 way, that one-half of its material mass lies within 

 three miles from the earth, and three fourths of it 

 within less than six miles. From the observation 

 of luminous meteors, it is inferred that it is at 

 least one hundred miles high, and that, in an 

 extremely attenuated form, it may even reach two 

 hundred miles. 



MOVEMENTS OF THE ATMOSPHERE. 



All the movements and changes to which the 

 air is subject, originate, some way or other, in the 

 unequal distribution of heat 



If two adjoining spots of ground are of unequal 

 temperature, and communicate an unequal tem- 

 perature to the columns of air lying upon them, 

 the column which is most heated will be expanded, 

 so as to overtop the other, and will also be made 

 rarer or lighter. Two effects will arise from this : 

 a lateral or horizontal movement of the air from 

 the cold to the warm column will take place below, 

 according to the general law of hydrostatics, that 

 a heavy fluid buoys up a light one. But if we 

 consider the condition of the two columns above y 

 or at their upper ends, we will find that the 



