204 
Also a portable barometer,, for moving 
from place to place without injury ; and for 
measuring the heights of mountains, by ob- 
serving the difference of the altitude of the 
mercury at the bottom and top of the moun- 
tain. 
A thermometer should always be attached 
to the barometer, as a necessary appendage ; 
and by the side of it a scale of correction, 
to shew how much to add or subtract from 
the height of the mercury in the barometer 
for the degree of temperature ; for it is evi- 
dent that the mercury in the tube will be 
affected by heat and cold in the same man- 
ner as the thermometer, and on that account 
it will not shew the true weight of tiie atmo- 
sphere. This correction is, therefore, very 
necessary. 
Ever since it was observed that a change 
of weather generally accompanied or followed 
a variation in the height of the barometer, 
it has been used as a prognostic of the wea- 
ther. A great variety of observations have 
been made by different people, relative to 
the effect which certain changes of weather 
have upon this instrument; and thence they 
have derived a system of rules, to enable any 
one to know what change will happen in the 
weather, by knowing the alteration that has 
taken place in the height of the mercury. 
Before we proceed to mention the rules, 
which are the result of the long observation 
and experience of philosophers, it is neces- 
sary to observe, that they are by no means 
so certain, and so much to be depended 
upon, as many people suppose.'. So nume- 
rous are the causes that affect the state of the 
atmosphere, with which we are but little ac- 
quainted, that no single instrument can point 
out with precision the alterations likely to 
happen. Besides the barometer, there are 
several other instruments used for meteoro- 
logical purposes, such as the thermometer, 
hydrometer, wind-gage, rain-gage, electro- 
meter, &c. with which we shall become ac- 
quainted in their turn. To behest enabled 
to prognosticate the change of weather, ac- 
curate observations ought to be made with 
all these instruments, aided by considerable 
experience and the knowledge of natural 
philosophy and chemistry ; and even then, 
it requires more science than we are pos- 
sessed of, to predict with certainty the alter- 
ations ot the weather. See Meteorology. 
As the barometer, however, is the most 
useful ot these, and as it undoubtedly affords 
us considerable assistance, we shall lay down 
such directions as are most approved of for 
this purpose. 
1. i he 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 win- 
ter, the rising presages frost ; and in frosty 
weather, if the mercury falls three or four di- 
visions, there will certainly follow a thaw. 
But in a continued frost, if the mercury rises, 
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 and high, and continues so for two or 
ihree days before the foul weather is quite 
over, then expect a continuance of fair wea- 
BAROMETER. 
flier to follow. 6. In fair weather, when the 
mercury fads much and low, and thus con- 
tinues for two or three days before the rain 
comes, then expect a great deal of wet, and 
probably high winds. 7. The unsettled mo- 
tion of the mercury denotes uncertain and 
changeable weather. 8. You are not so 
strictly to observe the words engraved on the 
plates (though in general it will agree with 
them), as the mercury’s rising and falling; 
for it it stands at much rain, and then rise up 
to changeable, it presages fair weather; 
though not to continue so long as if the mer- 
cury had risen higher ; and, on the con- 
trary, if the mercury stood at fair, and falls 
to changeable, it presages foul weather ; 
though not so much of it as if it had sunk 
lower. 
f rom these observations it appears, that it 
is not so much the height of the mercury in 
the tube that indicates the weather, as the 
motion of it up and down ; wherefore, in 
order to form a right judgment of what wea- 
ther is to be expected, we ought to know 
whether the mercury is actually rising or 
falling ; to which end, the following rules 
are of use : 
1 • If the surface of the mercury is convex, 
standing higher in the middle of the tube 
than at the sides, it is generally a sign that 
the mercury is then rising. 2. ' If the surface 
is concave, it is then sinking. And, 3. if it 
is level, the mercury is stationary, or rather, 
il it is a little convex ; for mercury, being 
put into a glass tube, especially a small one, 
will naturally have its surface a little con- 
vex, because the particles of mercury attract 
one another more -forcibly than they are at- 
tracted by glass. If the glass is small, shake 
the tube ; and then if air is grown heavier, the 
mercury will rise about half the tenth of an 
inch higher than it stood before ; if it is 
grown lighter, it will sink as much. This 
proceeds from the mercury’s sticking to the 
sides of the tube, which prevents the free 
motion of it until it is disengaged by the 
shock ; and therefore, when an observation 
is to he made by such a tube, it ought al- 
ways to be shaken first ; for sometimes the 
mercury will not vary of its own accord, 
until the weather it ought to have indicated 
is present. 
Here we must observe, that the above- 
mentioned phenomena are peculiar to places 
lying a considerable distance from the equa- 
tor ; for in the torrid zone, the mercury in 
the barometer seldom either rises or falls 
much. In Jamaica, it is observed by sir 
\\ illiam Beeston, that the mercury in the 
morning constantly stood at one degree 
below changeable, and at noon sunk to one 
degree above rain ; so that the whole scale 
of variation there was only three-tenths of 
an inch. At St. Helena, too, where Dr. 
Ilalley made his observations, lie found 
the mercury to remain almost stationary, 
whatever weather happened. Of these phe- 
nomena, their causes, and why the baro- 
meter indicates an approaching change of 
weather, the doctor gives us the following 
account: 
] . In calm weather, when the air is in- 
clined to rain, the mercury is commonly low. 
2. In serene, good, and settled weather, the 
mercury is generally high. 3. Upon very 
violent winds, though they are not accompa- 
nied with rain, the mercury sinks lowest of 
all, with relation to the point of the compass 
the wind blows upon. 4. The greatest heights- 
of the mercury are found upon easterly or 
north-easterly winds. 5. In calm frosty 
weather, the mercury generally stands high. 
6. After very great storms of wind, when- 
the mercury has been very low, it generally 
rises again very fast. 7. The more northerly 
places have greater alterations of the baro- 
meter than the more southerly. 8. Within 
the tropics, and near them, according to the 
accounts we have had from others, and the 
observations made at St. Helena, the changes 
of the weather made very little or no vari- 
ation in the height of the mercury. 
Hence it is thought that the principal cause 
of the rise and fall of the mercury is from the 
variable winds which are found in the tempe- 
rate zone, and whose great inconstancy in 
England is notorious. 
A second cause is,, the uncertain exha- 
lation and precipitation of the vapours lodg- 
ing in the air, whereby it is at one time much 
more crowded than at another, and conse- 
quently heavier ; but this latter depends in 
a great measure upon the former. Now, 
from these principles we may explain the 
several phenomena of the barometer, taking 
them in the same order as they are laict 
down. Thus, 
1. The mercury being low, indicates rain ; 
because the air being light, the vapours are 
no longer supported by it, being become spe- 
cifically heavier than the medium in which 
they floated ; so that they descend towards 
the earth, and, in their fall, meeting with 
other aqueous particles, they incorporate 
together, and form little drops of rain : but 
the mercury’s being at one time lower than 
another, is the effect of two contrary winds 
blowing from the place where the barometer 
stands ; whereby the air of that place is car- 
ried both way s from it, and consequently the 
incumbent cylinder of air is diminished, and 
accordingly the mercury sinks : as, for in- 
stance, if in the German Ocean it should blow 
a gale of westerly wind, and at the same time 
an easterly wind in the Irish Sea ; or if in 
f ranee, itshould blow a northerly wind, and in 
Scotland a southerly ; it must be granted, that 
that part of the atmosphere impendent over 
England would be exhausted and attenuated, 
and the mercury would subside, and the va- 
pours which before floated in these parts of 
tlie air of equal gravity with themselves, 
would sink to the earth. 
2. The greater height of the barometer is 
occasioned by two contrary winds blowing 
towards the place of observation, by which 
the air of other places is brought thither and 
accumulated ; so that the incumbent cylinder 
of air being increased both in height and 
weight, the mercury pressed by it must needs 
stand high, as long as the winds continue so 
to blow; and then the air being specifically, 
heavier, the vapours are kept suspended, so 
that they have no inclination to precipitate, 
and fail down in drops; which is the reason 
of the serene good weather which attends the 
greater heights of the mercury. 
3. The mercury sinks the 'lowest of all by 
the very rapid motion of the air in storms of 
wind. For the tract or region of the earth’s 
surface, in which the winds rage, not extend- 
ing all round the globe, that stagnant air 
which is left behind, as likewise that on the 
sides, cannot come in. so fast as to supply the 
