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3. Besides foretelling, changes in the weather, many plants . close and open their petals. at certain hours of the day. .
Linnæus has enumerated' forty-six flowers which possess this kind of sensibility; he divides them into three classes. (1) Meteoric flowers, which less accurately observe the hour of folding, but are expanded sooner or later according to the cloudiness, moisture, or pressure of the atmosphere. (2) Tropical flowers, that open in the morning and close before evening every day; but the hour of expanding becomes earlier or later as the length of the day increases or decreases. (3) Equinoctial flowers, which open at a certain and exact hour of the day, and for the most part close at another determinate hour.-- Darwin). The most common of these, in our own country, are thus prettily enumerated by Mrs. Charlotte Smith :
In ev'ry copse aud sheltered dell,
How pass the hours and seasons by..
„Will mark the periods as they pass,
And bind with flowers his silent glass.
Soft flowing o'er their tranquil bed:
Nymphea rests her lovely head.
She rises from her humid nest;
And sees reflected in the stream
Declines in Ocean's surge to lave, .
**She slumbers on the rocking wave. ? in :;*:s See Hieracium's various tribe,
• Of plumy seed and radiate flowers, . i. . . ;,.'The course of Time their blooms describe, floor. And wake or sleep appointed hours.
- Broad o'er its imbricated cup ..,;; The Goutsbeard spreads its golden rays,..
But shuts its cautious petals up, .
Retreating from the noontide blaze.
The Bethlem-star her face unveils,
And shades it from the vesper gales.
The humble Arenaria creeps;
But soou within its calyx sleeps.
With young Aurora's rosy hue,
But shut their plaits against the dew.
The hour, when, as the dial true,
Lifts her soft eyes, serenely blue.
Gatherest thy fringed mantle round
When nightdrops bathe the turfy ground;
The garish noontide's blazing light;
Gives all her sweetness to the night.
That in our path untrodden lie,
How fast their winged moments fly.
§ iv. Prognostics from Philosophical Instruments.
.1.-The Barometer. The barometer is an instrument to measure the weight or pressure of the atmosphere, and is so well known, that it is unnecessary here minutely to describe it. Suffice it to say, that the mercury in the glass tube is supported by the pressure of the air upon the mercury in the bason, in which the lower and open end of the tube is immersed; and the space in the tube above the mercury is a vacuum. When therefore the pressure of the air is increased, the mercury must rise in the tube ; and when the pressure is diminished, the mercury must fall. Upon hte level of the surface of the earth, the limits of the height of the mercury in the tube above the surface of the mercury in the bason, is from 28 to 31 inches; a graduated scale is therefore placed against the tube from 28 to 31 inches, in order to ascertain the height of the mercury in the tube. But those barometers which are made to measure the heights of mountains, are graduated much lower; because, as you ascend in the atmosphere, the mercury falls. When the mercury stands at the altitude of 30 inches, the pressure of the air upon every square inch of the earth's sur. face is about 15 lbs. avoirdupois. : At any other altitude of the mercury, the pressure will be in proportion to the altitude. Hence, if we take the surface of a middle-sized man to be 14} square feet, when the air is lightest, its pressure on him is .13.2 tons, and when heaviest it is 14.3 tons; the difference of which is 2464 lbs. This difference of pressures must greatly affect us in respect to our animal functions, and therefore in respect to our health ; more especially when the change is sudden. The pressure of the air upon the whole surface of the earth is about 7767029793563429 tons..
. : : The following rules are given for judging of the weather by Mr. Patrick, and are esteemed the best which we have :
1. The rising of the mercury presages, in general, fair weather';' and its falling, foul weather ; as rain, snow, high winds, and storms.
2. In viery hot weather, the falling of the mercury indicates thunder. . . i 3: In winter, the rising indicates frost : and, in frosty'weather, if the mercury fall three or four divisions, there will follow a thaw. But in a continued frost, if the mercury risé, it will snow. s
4. When foul weather happens soon after the fall· ing 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 so continues for two or three days before the foul weather is quite over, then expect a continuance of fair weather to follow.
6. In fair weather, when the mercury falls much and low, and thus continues, for two or three days before the rain comes, then expect a great deal of wet, probably high winds.
7. The unsettled motion of the mercury denotes certain and changeable weather.".
The words on the plate are not strictly to be adhered to, though they will, in general, agree, for the height of the mercury does not so much indicate the weather, as its motion up and down; to know, therefore, whether the mercury is actually rising or falling, the following rules should be observed. .
1. If the surface of the mercury be convex, it is rising.
2. If the surface of the mercury be concave, it is
3. If the middle of the mercury be plain, it is neither rising nor falling ; for mercury put into a glass tube, will naturally have the parts adjacent to the tube convex.
4. As the mercury will adhere a little to the tube, before you note its height, it is proper to shake 'the barometer a little, by giving it a gentle tap with the knuckle.
The barometer never fails to show the true cause of the alterations of weather, and we are thereby prepared to expect them; but it may sometimes happen that the column of mercury will not alter its altitude agreeably to the foregoing rules : for when the atmosphere is charged with more aqueous matter than it can dissolve (for the atmosphere is known to be a dissolvent medium), the sur
plus will form clouds, and these produce showers of rain when the mercury stands very high ; and for the contrary reason, there may be sometimes no rain when the mercury is very low. Hence it follows, that we are generally satisfied by the barometer what weather we may at all times probably expect, though sometimes the contrary may happen, and a general monitor (to any wise man) is better than none at all. ; .
2.-The Thermometer. A thermometer is an instrument to measure different degrees of heat. It is a small glass tube with a bulb at the bottom, having the bulb and part of the tube filled with mercury, or spirits of wine. The tube is closed at the top, and the part not occupied by the fluid is a vacuum. Against the tube there is a scale to measure the expansion of the fluid under different temperatures ; for fluids expand by heat, and contract by cold. An increase of temperature will therefore make the fluid rise in the tube, and a decrease of temperature will make it fall. The thermometer now in use is that which is constructed by Fa. hrenheit. On this scale, the fluid stands at 32 'when it just begins to freeze, and at 212 when put into boiling water; at temperate it stands at 55; at summer heat, at 76 ; at blood heat, at 98. If the scale be continued to 600, it gives the heat of boiling mercury; and if it be continued downwards to 39 below 0, it gives a degree of cold which will freeze mercury'. By means of the barometer and thermometer, the al. titude of a mountain may be found to a great degree of accuracy.
3.-The Rain-gauge, or Pluviometer. The rain-gauge is an instrument to show the quantity of rain which falls upon the earth at any
* Though Reaumur's scale is seldom used, it may be proper to observe that its freezing point is 0, and boiling water at 80 deg.