46 The Ottawa Naturalist. [May 



For the study of phenomena accurate observations are 

 necessary. In meteorology we have to deal not with one phen- 

 omenon but with many, each adding its quota to form the 

 aggregate which we term weather. As the instruments used for 

 observing the phenomena are fairly familiar to you I will confine 

 my remarks to a few salient points. The instrument of first 

 importance is the barometer, by means of which the pressure of 

 the atmosphere is measured ; we might call it the lead line or 

 sounding rod of the atmospheric oeean. Although there are 

 the two forms of barometer — mercurial and aneroid — yet for 

 accurate and precise work the former onlyis used. Air, in common 

 with all other forms of matter, is acted on by the attraction of 

 gravity, and consequently possesses weight. The pressure of 

 the air is a necessary consequence of its weight, and is equal, at 

 the level of the sea, to about fifteen pounds to the square inch. 



If we take a glass tube of about three and a-ha^f feet in length, 

 filled with mercury, and hermetically sealed at one end, while the 

 other end is bent like a syphon, or is inverted into a cistern ot 

 mercury, we have a means of measuring the pressure of the 

 atmosphere and its consequent fluctuations. Here in Ottawa, 

 less than 200 feet above mean sea level, we are subject on all 

 sides to a pressure of nearly 15 lbs. to the square inch to prevent 

 us from bursting, which we undoubtedly would, were that pres- 

 sure removed. 



The barometer responds to the fluctuations and pulsations 

 of the atmospheric pressure. 



Suppose now we have taken barometric readings at the 

 sam.e absolute time at various stations from Halifax to Van- 

 couver ; these readings, although correct, would not give us the 

 true measure for comparison of the various pressures, from the 

 fact that they would not be taken from the same plane— the 

 level of the sea — to which all observations must be reduced be- 

 fore comparisons can be instituted and inferences drawn. The 

 fluctuations of the atmosphere make themselves felt on our Great 



