REMARKS ON THE BAROMETER. 679 



2. The amount, taken by itself, of vapour, moisture, wet, rain, or snow, 

 in the wind (direction and strength remaining the same), seems to cause 

 a change amounting, in an extreme case, to about half an inch. 



3. The strength or force alone of wind from any quarter (moisture and 

 direction being unchanged) is preceded or foretold by a fall or rise, accord- 

 ing as the strength will be greater or less, ranging in extreme cases to 

 more than two inches. ^ 



Hence, supposing the three causes to act together — in extreme cases — 

 the mercury might range from about thirty-one (30-8) inches to near 

 twenty-seven (27"1) inches, which has happened occasionally. 



Generally, however, as the three act much less strongly, and are less in 

 accord, ordinary varieties of weather (the wind varying between S.W. and 

 N.W., with more or less cloudiness or rain) occur much more frequently 

 than extreme changes. 



Another general rule requires attention ; which is, that the wind usually 

 veers, shifts, or goes round with the sun (right-handed in the northern 

 regions, left-handed in the southern parts of the world) ; and that, when 

 it does not do so, or backs, more wind or bad weather may be expected 

 instead of improvement. 



In a Barometer the mercury begins to rise occasionally before the con- 

 clusion of a gale, sometimes even at its commencement, as the equilibrium 

 of the atmosphere begins to be restored. Although the mercury falls lowest 

 before high winds, it frequently sinks considerably before heavy rain. The 

 Barometer falls, but not alivays, on the approach of thunder and lightning, 

 or when the atmosphere is highly charged with electricity.* Before and 

 during the earlier part of serene and settled weather, the mercury com- 

 monly stands high, and is stationary. 



Instances of fine weather, with a low glass, occur exceptionally, but 

 they are always preludes to a duration of wind or rain, if not both. 



After very warm and calm weather, rain or a storm is likely to occur ; 

 or at any time when the atmosphere has been heated much above the 

 usual temperature of the season. 



Allowance should invariably be made for the previous state of the column 

 during some days, as well as hours, because its indications may be affected 

 by remote causes, or by changes close at hand. Some of these changes 

 may occur at a greater or less distance, influencing neighbouring regions, 

 but not visible to each observer whose Barometer feels their effect. 



There may be heavy rains or violent winds beyond the horizon and the 

 view of an observer, by which his instruments may be affected considerably, 

 though no particular change of weather occurs in his immediate vicinity. 



It may be repeated, that the longer a change of wind or weather is fore- 

 told by the Barometer before it takes place, the longer the presa; ed 

 weather will last ; and, conversely, the shorter the warning, the less time, 



• Thunder clouds sometimes rise and spread against the wind (lower current). It is 

 probable that there is a meeting, if not a conflict of air currents, electrically different, 

 whenever lightning is seen. Their meeting, when the new one advances from the polar 

 region, does not affect the Barometer, except in oscillation, which is very remarkable 

 at the time. 



