iSo] BAR , it generally indicates the rising of this fluid metal ; 2. If its surface be concave, is is men sink- ing; 3. If it appear level, it is sta- tionary; 4. If, after shaking the tube of a smnll glass, the mercury rises about half a tenth of an inch higher than it stood before, it is a proof that the air has become heavier; but if it sinks as much, it follows that the atmosphere is lighter. Hence, in making obser- vations on the weather, such a glass should always be previously shaken; because the metal which adheres to the sides of the tube, prevents its free motion, till disengaged by a slight agitation of the instrument. These phenomena are peculiar to places situated at a distance from the equator, and therefore deserve to be attended to in our climate : on the contrary, at St. Helena, they would be of little or no ser- vice; for, according to the accurate observations of Dr. Halley, made in that island, the mercury remain- ed stationary in all weathers. This judicious naturalist has furnished us with the following curious ac- count of these phenomena, and their causes; of which we shall give a concise abstract : 1. In calm weather, when there is a prospect, of rain, the mercury is commonly low ; 2. In serene and settled wea- ther it is generally high ; 3. On the approach of tempestuous winds, though unaccompanied with rain, the mercury sinks lower than on any other occasion; 4. The greatest height of the mercury is observable during the prevalence of easterly or north-easterly winds ; 5. In calm, frosty weather, the quicksilver ge- nerally stands high j 6. After very great tempests, when the mercury has been very low, it generally again quickly ascends ; ",'. The ba- BAR romcter in the northern regions ex- hibits greater variations than in those lying more towards th South; 8. Within and near the tropics, there is little or no alteration in the mercury, as before observed. Hence Dr. Halley is of opinion, that the principal cause of the rise and the fall of the quicksilver must be at- tributed partly to the variable winds in the temperate zone, the great inconstancy of which, in Britain, is well-known: and partly to the un- certain exhalation and precipitation of the vapours floating in the at- mosphere, which is at one time more saturated than at another, and consequently heavier; though the precipitation of aqueous vapours chiefly depends on the previous de- gree of evaporation. Our plan does not permit us to accompany these propositions with the explanations given by Dr. Hal- ley; and, as the theoretical part has been strongly contested by other philosophers, though not re- futed, till the late Dr. Black, of Edinburgh, endeavoured to prove the fallacy of all preceding theo- ries, we shall conclude this sub- ject with a summary outline of his doctrine. According to Dr. Black, 1. Va- pour is formed by an intimate union between fire and water, by which the fire, or heat, is so to- tally enveloped, and its action so entirely suspended by watery par- , that it loses its properties of giving light and heat, and conse- quently is in a latent siaie; 2. If the atmosphere is affected by any unusual degree of heat, it becomes incapable of supporting a column of mercury so long as before ; for which reason that in the barome- ter sinks. — From these preliminary axioms it follows, that as vapour i»
