?') 336 ON THE BAROMETER. {^ 
meter : — 1 . A reduction for the temperature of the mercury to reduce the reading to the freezing point. 
To show the necessity of this : suppose the mercury is at a temperature of 75°, the barometer will indi- 
cate a pressure a tenth of an inch too high. Most instruments have a thermometer attached to them 
for this purpose, yet, from it we only attain an approximation, for it merely gives the heat of the apart- 
ment in which the instrument is placed, and not that of the mercury ; to give the latter, the bulb of the 
thermometer should be plunged into the quicksilver ; for ordinary purposes, however, the former is 
sufficiently near. A barometer ought to be placed in a room not subject to much variation of tempera- 
ture ; one cool, and without fire in the morning, and with a blazing hearth in the evening, should be 
avoided, and in like manner should one cool as a cellar (being in shade) at one part of the day, but with 
a hot sun shining in at the windows at another period. An outer wall must also be avoided. 
2. Another correction, that for capillarity, it is necessary to explain. The tubes of barometers be- 
low a certain diameter (a size greatly beyond that of most instruments) are influenced by capillary 
action ; ;'. e. the action in the tube is greater than in the cistern of the barometer, and the more so, the 
smaller the diameter of the tube. Beyond a diameter of six-tenths of an inch, this influence all but 
ceases. For ordinary purposes, gently rapping the barometer two or three times before observing its 
height, will sufficiently correct it for capillarity. 
3. A third correction is of great importance, viz., that for the depth of mercury in the cistern of 
the barometer ; and this correction, of course, cannot be applied to the wheel barometers or those 
having leathern bags. When the barometer rises or falls, the depth of mercury in the cistern becomes 
less or greater; so that at one time we may be measuring our barometer, not from the surface of the 
mercury in the barometer, but a few tenths of an inch below or above the surface. In order to over- 
come this error, the scale of inches is made to move by rack-work, and a rod attached to the scale 
extends to the cistern, the lower end of the rod terminating in a point. This rod is moved by rack- 
work (of course, bringing the scale of inches with it) higher or lower, as the case may be, until the 
point just touches the surface mercury, and by this means we have the exact length of the mercurial 
colu.nn. 
There is another barometer which has been invented within the last two or three years, by M. Vidi 
of Paris, upon a new principle, and is, from its action, called the Aneroid Barometer. The atmosphere 
presses upon a metallic box, from which the air has been exhausted, and which has then been herme- 
tically sealed. This instrument has not been brought to a state sufficiently correct for scientific pur- 
poses. However, for ordinary use, it is accurate enough ; indeed, much more so than any other form 
of the barometer, excepting the standard one; and being very portable, and not costly, it is an excellent 
instrument for gardeners. 
Too much confidence must not be placed on the predictions of a barometer, for the indications of fair, 
rain, or change, engraved on the instrument, are merely placed there in a popular sense. Strictly 
speaking, the barometer merely indicates the weight of the atmosphere, >'. e., the weight conjointly of 
the dry and wet air, or, in other words, the combined pressure of the aqueous and gaseous atmospheres. 
If the barometer falls rapidly, most persons look with confidence to the approach of rain. Now, if we 
consider for a moment, this fall can but little indicate it : for in England, as an example, the range of 
pressure is 3 inches, from 28 to 31 inches, and the mean pressure about 29j inches ; of this, 29 inches 
is the pressure of the gaseous or dry air, whilst only half an inch is that of the aqueous or wet air, or 
that portion from which we derive moisture. The barometer alone is but a poor guide ; but, taken to- 
gether with the dry and wet bulb thermometer, it becomes a much better prognosticator. Supposing, 
for instance, the barometer is descending, the air containing almost as much moisture as it can sustain 
without rain at a certain temperature ; if the temperature, as indicated by the bulb thermometer, is 
descending, we may with confidence look out for rain, for we then know that, unless the moisture in 
the atmosphere decreases, the temperature of the air will soon have descended below the point where 
it can no longer hold the same amount of water in suspension, and rain must fall 
In an age like the present, when science is being studied by all classes of the community, the gar- 
dener and the agriculturist should not be behindhand in meteorology ; and it is hoped that ere long all 
well educated men in these branches will take daily observations on the weather, and in such a manner 
that their observations may not only be of the greatest use to themselves, but also to the succeeding 
generations. 
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