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Reading the Barometer (Following manual)

The following manual of the barometer has been compiled by Rear-Admiral Fitzroy, and published by the Board of Trade. It has been slightly altered to suit the climate of the United States.

Familiar as the practical use of weather-glasses is, at sea as well as on land, only those who have long watched their indications and compared them carefully, are really able to conclude more than that the rising glass usually fortells less wind or rain, a falling barometer more rain or wind, or both; a high one fine weather, and a low one the contrary. But useful as these general conclusions are in most cases, they are sometimes erroneous, and then remarks may be rather hastily made, tending to discourage the inexperienced.

By attention to the following observation (the results of many years' practice, and many persons' experience), any one not accustomed to use a barometer may do so without difficulty. The barometer shows whether the air is getting lighter or heavier, or is remaining in the same state. The quicksilver falls as the air becomes lighter, rises as it becomes heavier, and remains at rest in the glass tube while the air is unchanged in weight.

Air presses upon everything within about 40 miles of the world's surface, like a much lighter ocean, at the bottom of which we live, not feeling its weight because our bodies are full of air, but feeling its currents, the winds. Towards any place from which the air has been drawn by suction, air presses with a force or weight of nearly 15 lbs. on a square inch of surface. Such a pressure holds the limpit to the rock when, by contracting itself, the fish has made a place without air under its shell. Another familiar instance is, that of the fly, which walks on the ceiling with feet that stick. The barometer tube, emptied of air and filled with pure mercury, is turned down into a cup or cistern containing the same fluid, which feeling the weight of air, is so pressed by it as to balance a column of about 30 inches (more or less) in the tube, where no air presses on the top of the column. emptied of air, filled with water, the open end If a long pipe, closed at one end only, were kept in water, and the pipe held upright, the water would rise in it more than 30 feet. In this way this effect is shown by any well with a suckingwater barometers have been made. A proof of pump, up which, as is commonly known, the water will rise nearly 30 feet by what is called suction, which is, in fact, the pressure of air towards an empty place.

The words on scales of barometers should not the rising or falling of the mercury, for if it stand be so much regarded for weather indications as at "changeable," and then rise towards "fair," it presages a change of wind or weather, though not so great as if the mercury had risen higher; and, on the contrary, if the mercury stand above not to so great a degree as if it had stood lower; "fair," and then fall, it presages a change, though besides which, the direction and force of the wind point at which the mercury may stand that we are not in any way noticed. It is not from the are alone to form a judgment of the state of the weather, but from its rising or falling, and from the movements of immediately preceding days, as well as hours, keeping in mind effects of change of direction and dryness or moisture, as well as alteration of force or strength of wind. latitudes, the quicksilver ranges, or rises and falls, In this part of the world, towards the higher nearly three inches-namely, between about thirty inches and ninetenths (30.9), and less than twenty-eight inches (28.0) on extraordinary occasions; and a half (30.5) to about twenty-nine inches. but the usual range is from about thirty inches Near the Line, or in equatorial places, the range is but a few tenths, except in storms, when it sometimes falls to twenty-seven inches. into 10 parts each, or hundredths of an inch. The

The sliding scale (Vernier) divides the tenths number of divisions on the Vernier exceeds that in an equal space of the fixed scale by one.

No air is within the tube, none can get in. But By a thermometer the weight of air is not shown. the bulb of the tube, is full of mercury which contracts by cold and swells by heat, according to which effect the thread of metal in the small tube is drawn down or pushed up so many degrees, and thus shows the temperature. the bulb, wetted enough to keep it damp by a If a thermometer have a piece of linen round show less heat than a dry one, in proportion to thread or wick dipping into a cup of water, it will the dryness of the air and quickness of drying.

In very damp weather, with or before rain, fog, nearly alike. or dew, a wet and dry bulb thermometer will be the readiest and surest method is the comparison For ascertaining the dryness or moisture of air, of two thermometers, one dry, the other just moistened and kept so. much as the state of the air admits, the moist (or Cooled by evaporation as wet) bulb thermometer shows a temperature nearly equal to that of the other one, when the atmosphere is extremely damp or moist; but lower at other times in proportion to the dryness of air and consequent evaporation-as far as 12° or 15° in this climate, 20° or even more elsewhere. From 4º to 8º of difference is usual in England, and about 7º is considered healthy for inhabited rooms.

The wet and dry bulb thermometer on the same frame, the water being supplied by a bird fountain, constitutes August's or Mason's hygrometer, The thermometer fixed to a barometer intended to be used only as a weather-glass, shows the temperature of air about it, nearly, but does not show I the temperature of mercury within, exactly. It