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W<?«tlur itud ^,^aiM-(Bl}nm^, 



Our subject ia " Weather and Weather-Glasses." Let us try to 

 ascertain what this word "weather" really means, and what it is to under- 

 stand the science of the weather, so as to be able to forecast its condition 

 at a given time and place. Noav those of us who are in the habit of 

 putting oiu" thoughts into precise language, will not think it a difficult 

 matter to say what the word "weather" usually means. In a general way 

 it may be said to signify "the state of the atmosphere;" i.e., the state of 

 the atmosphere in resj^ect to weight, dryness, electricity, &c., &c. To 

 understand and ascertain the ever-varying weight of the atoms of air and 

 vapour; the electrical condition of the air; the manner in which its move- 

 ments are caused by heat, electricity, magnetism, hills, moimtains, seas, 

 deserts, the earth's daily motion, planetary influence, &c. — this is to under- 

 stand the science of the weather : this is what Meteorology teaches. "We 

 can already see the immense proportions of the subject, and perhaps also 

 something of the extreme difficulty that attends its investigation. At first 

 sight the difficulties seem to be insiu-moun table; and it is only by patient, 

 laborious, and continued observation of details that we are able to discover 

 tlie real principles of the science. 



We propose firstly to take a rapid review of the physical properties of 

 the atmosphere, and the principal causes affecting the state of the weather : 

 secondly, to describe the principles on which the weather-glass is con- 

 structed ; lastly, to give a few simple rules for reducing and correcting the 

 readings of the instriunent, together with some hints for interpreting its 

 indications. 



1. Physical properties of the atmosphere. 



The air has weight. It is found that 100 cubic inches of air Aveigh 

 about 30 grains; that the pressure of the whole atmosphere is about 151bs. 

 on every square inch, and that the entire weight is 11,44:9,000,000 

 hundreds of millions of pounds. The air is also elastic, i.e., contracts 

 when pressure is applied, and recovers its volume when it is removed. If 

 two cubic feet of air be compressed into the sj^ace of one cubic foot, the 

 elastic force is doubled. As each stratum of air is compressed only by the 

 Aveight of air above it, it is plain that the portions nearest the earth are 

 heaviest, or most dense; and the higher we go towards the upper limit of 

 the atmosphere, the less the pressure becomes. About three-fourths of all 



