Page:Small-boat sailing; an explanation of the management of small yachts, half-decked and open sailing-boats of various rigs; sailing on sea and on river; cruising, etc (IA smallboatsailing01knig).pdf/235

 equilibrium. When the glass falls to twenty-nine inches there is a certainty of bad weather.

It is not possible to arrive at a reliable forecast with the barometer alone. In order to foretell the nature of the change of weather indicated by a moving glass, that is, in order to ascertain from which quarter the wind may be expected to come, whether it will be accompanied by rain or snow, and so forth, one must take into consideration, in conjunction with the barometer readings, the existing meteorological conditions—the temperature of the air, the amount of moisture in the air, and the direction and force of the wind blowing at the time. It is not necessary to provide oneself with an instrument for testing the amount of moisture in the air, and a hygrometer seldom forms part of the equipment of the small yacht, as good forecasts can be made without its assistance; but a thermometer must always be carried at sea to supplement the information given by one's aneroid.

In our latitudes the thermometer rises with winds from south round by south-east to east, and falls with winds from north round by north-west to west.

Air charged with vapour is lighter than dry air; and heated air, being more rarified, is lighter than cold air. It therefore follows that, other conditions remaining unchanged, either an increase of moisture or an increase of temperature will cause the barometer to fall; while drier weather, or a fall in the