USE OF BAROMETER. 23 



possible amount at that temperature." When the air is colder than 

 the temperature of the dew-point, the surplus amount of vapor is 

 condensed by the amount of difference of pressure, and the result 

 is dew, frost, fog, etc. The first deposition is dew ; then, according 

 as the radiating body is acted upon with greater or less amount of 

 temperature, we have frost ; then fog, which becomes clouds of dif- 

 ferent kinds, forms, and shapes ; these in turn condense and pre- 

 cipitate rain, which pressure still forms into a spray called mist ; and 

 this, crystallizing, gives us, first hail, then sleet, and finally snow in 

 all its perfection of microscopic crystals. 



Before closing my remarks on the barometer a few quotations 

 from the Sailing Directions will serv^e to give you a good idea of 

 the use of this instrument, or rather of the use it may be if carefully 

 and properly studied. "The barometer has a range of from 29 to 

 30.5 inches in the Gulf and River St. Lawrence during the navigable 

 season, and its changes accompany those of the winds and weather 

 with a considerable degree of constancy. The fluctuations of the 

 barometric column are much greater and more frequent there than 

 in lower latitudes, and sudden alterations, which in other climates 

 would be alarming, may occur there without being followed by any 

 corresponding change either in the wind or in the weather." The 

 most practical part is still to follow, and expresses the pith of all 

 that can be said thus concisely : "But the navigator should not be 

 inattentive to these minor changes, as a constant attention to the 

 instrument can alone enable him to appreciate those decisive indi- 

 cations of the mercury which seldom or never prove deceptive." 



Quoting further from the same source, of the fogs it says : "they 

 may occur at any time during the open or navigable season, but 

 are most frequent in the early part of summer ; they are rare, and 

 never of long continuance during westerly winds, but seldom fail 

 to accompany an easterly wind of any strength or duration. The 

 above general observation is subject, however, to restriction, ac- 

 cording to locality or season. Thus winds between the south and 

 west, which are usually clear weather winds above Anticosti, are fre- 

 quently accompanied with fog in the eastern parts of the gulf. 



