sure 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 143 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 77670297935.63429 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 very hot weather, the falling of the mercury indicates thunder.

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 rise, it will 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 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 falling.

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 Fahrenheit. 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 altitude 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.

place where you may wish to make observations. It consists of a funnel communicating with a cylindrical tube at its bottom, into which the rain is conveyed by the funnel. The depth of the water in the cylinder is measured by a rule fixed to a float, the rule passing through the centre of the funnel. The divisions on the rule show the number of cubic inches of water that have fallen on a surface equal to the area of the top of the funnel. The funnel is so contrived as to prevent the water from evaporating. To use the rain-gauge, so much water must first be into the cylinder as will raise the float, so that O on the rule may exactly coincide with the aperture of the funnel. The gauge should be firmly fixed in a place, where, whatever winds blow, the fall of the rain may not be intercepted by any obstacles. By this instrument, the mean annual depths of rain in inches is determined.

4.The Hygrometer.

put

The hygrometer is an instrument to measure the moisture and dryness of the air, and is formed of substances which will expand or contract upon any alteration of moisture. Wood expands by moisture, and contracts by dryness; on the contrary, cord, catgut, &c. contract by moisture, and expand. by dryness. The weather-houses are very good hygrome ters for common purposes. The contraction of the string, by moisture in the atmosphere, forces the male figure out of the door at the approach of bad weather; and, as this gradually becomes dry, the string resumes its natural length, and forces the female out of the door at the approach of good weather.

The only hygrometrical instrument I have used,' says Mr. Dalton, is a piece of whip-cord, about six yards long, fastened to a nail at one end, and thrown over a small pulley; in this manner it has been kept stretched, by a weight of two or three ounces, since September 1787. It is in a room without a fire, and

where the air has a moderate circulation: the scale is divided into tenths of an inch, and begins at no de-. termined point; the greater the number of the scale, the longer is the string and the drier the air. This. string has varied in length above 13 inches, or of its whole length.'

§ v. Miscellaneous Prognostics.

1. Lord Bacon's Prognostics from the Seasons.

1. If the wainscot or walls that used to give out moisture be drier than usual, in the beginning of winter, or the eaves of houses drop more slowly than ordinary, it portends a hard and frosty winter; for it shows an inclination in the air to dry weather, which, in winter, is always joined with frost.

2. Generally, a moist and cool summer portends a hard winter.

3. A hot and dry summer and autumn, especially if the heat and drought extend far into September, portend an open beginning of winter, and cold to succeed towards the latter part of the winter and beginning of spring.

4. A warm and open winter portends a hot and dry summer, for the vapours disperse into the winter showers; whereas cold and frost keep them in, and convey them to the late spring and following

summer.

5. Birds that change countries at certain seasons, if they come early, show the temper of the weather, according to the country whence they came: as, in the winter, woodcocks, field-fares, snipes, &c. if they come early, show a cold winter; and the cuckoos, if they come early, show a hot summer to follow.

6. A serene autumn denotes a windy winter; a windy winter, a rainy spring; a rainy spring, a serene summer; a serene summer, a windy autumn: so that