118 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



standard altitude, and the difference between the greatest 

 and least altitudes is called the limit or scale of variation. 



The mercury in the barometer tube will subside, till the 

 column be equivalent to the weight of the external air upon 

 the surface of the mercury in the basin, and it is therefore 

 a criterion to measure that weight, and chiefly directed to 

 that purpose. In this country the standard altitude fluctu- 

 ates between twenty-nine and thirty and a half inches. 



Rules for observing the Barometer, 1. The rising of the 

 mercury presages, in general, fair weather ; and its falling, 

 foul weather ; as rain, snow, high winds, and storms. 2. In 

 very hot weather, the falling of the mercury foretells thunder. 

 3. In winter, the rising presages frost ; and in frosty 

 weather, if the mercury fall three or four divisions, a thaw 

 will certainly follow. But in a continued frost, if the mer- 

 cury rise, it will certainly snow. 4. When foul weather 

 happens soon after the falling of the mercury, expect but 

 little of it ; and, on the contrary, expect but little fair 

 weather, when it proves fair shortly after the mercury has 

 risen. 5. In foul weather, when the mercury rises much 

 arid high, and so contimies for two or three days before foul 

 weather is quite over, then expect a continuance of fair 

 weather to follow. 6. In fair weather, when the mercury 

 falls much and low, and thus continues for two or three 

 days before the rain comes, then expect a great deal of wet, 

 and probably high winds. 7. The unsettled motion of the 

 mercury denotes uncertain and changeable weather. 8. You 

 are not so strictly to observe the words engraved on the 

 barometer plate, as the rising or falling of the mercury, for 

 should it stand at much rain, and then rise to changeable, it 

 presages or foretells fair weather, although its continuance 

 is not to be depended on so much as if the mercury were 

 higher. But when it stands at fair, and sinks to changeable, 

 it indicates bad weather, though not so much in proportion 

 as if it hfid sunk lower. Therefore, in order to pass a right 

 judgment of the weather, the point where the mercury 

 stands is not so much to be regarded, but it should be par- 

 ticularly considered whether the mercury is actually in a 

 rising or falling state. For its minutest alterations should 

 be accurately attended to for the purpose of forming a right 

 judgment respecting the weather expected to succeed. 



When we would observe the barometer with accuracy, 



