The thermometer fixed to a barometer intended to be used only as a weather-glass shows the temperature of air about it nearly--but does not show the temperature of mercury within exactly. It does so however near enough for ordinary practical purposes--provided that no sun, nor fire, nor lamp heat is allowed to act on the instrument partially. The mercury in the cistern and tube being affected by cold or heat, makes it advisable to consider this when endeavouring to foretell coming weather by the length of the column. * * * Briefly, the barometer shows weight or pressure of the air; the thermometer--heat and cold, or temperature; and the wet thermometer, compared with a dry one, the degree of moisture or dampness.[10] It should be remembered that the state of the air _foretells_, rather than shows present weather (an invaluable fact too often overlooked); that the longer the time between the signs and the change foretold by them, the longer such altered weather will last; and, on the contrary, the less the time between a warning and a change, the shorter will be the continuance of such foretold weather. * * * If the barometer has been about its ordinary height, say near thirty inches, at the sea level,[11] and is steady, or rising--while the thermometer falls, and dampness becomes less--North-westerly, Northerly, or North-easterly wind--or less wind--may be expected. On the contrary--if a fall takes place, with a rising thermometer and increased dampness, wind and rain may be expected from the South-eastward, Southward, or South-westward. A fall, with a low thermometer, foretells snow. Exceptions to these rules occur when a North-easterly wind, with wet (rain or snow) is impending, before which the barometer often rises (on account of the _direction_ of the coming wind alone), and deceives persons who, from that sign only, expect fair weather. When the barometer is rather below its ordinary height, say, below twenty-nine inches and nine-tenths (at the sea level _only_), a rise foretells less wind, or a change in its direction towards the Northward,--or less wet; but when the mercury[12] has been low, say near 29 inches--the first rising usually precedes, and foretells, strong wind--(at times heavy squalls)--from the North-westward--Northward--or North-eastward--_after_ which violence a rising glass foretells improving weather--if the thermometer falls. But, if the warmth continue, probably the wind will back (shift against the sun's course), and more Southerly, or South-westerly wind will follow. "Backing" is a bad sign, with any wind. The most dangerous shifts of wind, and the heaviest Northerly[13] gales happen after the mercury first rises from a very low point.