DIFFERENTIAL THERMOMETER.

The object of this thermometer is to ascertain the difference in the temperature of two currents of air, or other substances,

applied to the two bulbs or balls on the top. These bulbs are both filled with air, which also reaches a certain way down the stems; and the stems are of one continued tube. A portion of the tube is filled with a coloured liquid, and a scale is applied to one of the stems. If the object be to compare the temperature of the air with any thing else, that thing must be applied to the bulb to which the scale is not attached. If cold, the air would be condensed in the bulb, and the liquid would sink against the scale; but if warmer, the air would be expanded, and the liquid would rise. The two bulbs should be exactly equal in size and in substance of glass; and when both have the same temperature, the liquid should stand against 0 in the scale.

It is to be borne in mind, that all thermometers merely show the expansions and contractions of the substance contained in them; that the degree of heat or of cold is a matter of analogy, founded upon the assumption, that with the same degree of heat the same substance will always have the same bulk; and that, in the case of the differential thermometer, it is not the absolute heat. or cold of any of the substances that is shown, but merely the difference between the one and the other.

Every change of temperature in the atmosphere is attended with a change in the weight, if not of the whole column of air over the place, at least of the portion immediately in contact with the substance which causes the variation of temperature. If the change be an increase of heat, the air ascends, and air from all sides comes in to supply the place. Thus, in a common fire, the heated air constantly ascends the chimney, and cold air finds its way in at every chink of the room. When a room is crowded, and there is no ventilation or opening in the roof, one may observe the heated air exhaling by the top of the door, and the cool air coming in by the bottom; and, if the room be large and very warm, the current may be so strong each way, that a candle may be extinguished, by having the flame blown out of the room, at the top of the door, and into the room at the bottom. If two thermometers were, in such a case, applied to the two currents, the difference of temperature, pointed out, would be very considerable.

When heated air ascends, it gradually becomes colder, both by giving out part of its heat to the colder air, through which it passes, and by expanding in the less dense region to which it arrives; and when cold air descends, it gradually becomes warmer, both by receiving heat from the

warmer air, through which it passes, and by being condensed by the greater weight of air that presses upon it in the lower situation. In mountainous countries, the currents produced by these means are very considerable; and unless, where the valleys are very deep and confined, the air of such countries, from its continual shifting about, is more healthful than that of plains; but the changes and currents render the weather far more variable, and the storms, when they do happen, are more severe.

HYGROMETER.

Any substance which has the capacity of abstracting moisture from the air, may serve generally as a hygrometer. A porous stone, which has been long in the sea, and is saturated with salt, becomes moist in proportion as the air is damp; a lump of rock salt in the rough state does the same; the walls of houses, in damp situations, are often but too good hygrometers; and even the earth generally answers the same purpose, as it appears moist when the air in contact with it is saturated with moisture. A very good toy hygrometer is made by placing a piece of fiddle-string, or other twisted gut, within a quill or small tube, making it fast at the one end of the tube, and fastening an index to it at the other. When the gut dries, the fibres shrink, the gut untwists, and the index turns in one direction; and when the gut gets moist, the fibres swell, the gut twists, and the index moves in the other direction. This hygrometer is often made in the form of a monk with a hood, a lady with a parasol, or a man with a drinking can; the hood, the parasol, or the can, is attached to the end of the gut in such a way, as that it is moved toward the head of the figure by drought, or from it by rain, in the case of the can, and the reverse in the case of the other. A tolerable hygrometer may be made as in the following figure

upon the other end of the balance, till it stand at very dry; then expose the sponge to damp air till it can acquire no more weight by absorbing moisture, and the end of the balance will rise to very damp. Intermediate between them will be 0, or the middle of the scale, and it may be divided into degrees both ways. The small bar, A, is put against the index end of ARTS, No. 3.

G

the balance, to prevent it from being displaced. This hygrometer may be made to mark, or register, as it is called, the greatest degree of dampness or of dryness which has taken place, by fixing to the index arm a bit of very soft pencil, and placing against it a slip of very smooth paper,though, it must be confessed, that this would affect the accuracy of the instrument.

Contrivances of this sort might be multiplied without end, because any substance which alters either in bulk or in weight, by differences of moisture, may be made into a hygrometer; the best, however, are those which depend upon nearly the same principle as the differential thermometer, previously described. One of these is

This instrument consists of a bent tube, which terminates in two bulbs, A and B; these are filled with air, and there is a portion of sulphuric acid in the tube; and, in constructing it, the air in each is so managed, as that when they have the same temperature, the liquid may stand at 0 in the seale. A small bit of glass is put into the tube, and this moves along before the liquid, and serves to register the place to which it comes. In making the scale as a thermometer, the temperature by a common thermometer may be observed, and then the ball, A, being plunged into melting snow, the air in it will be condensed, and the liquid will move along the tube. The point where it becomes stationary will give the extremity of the scale; and if the temperature, as shewn by the other thermometer, be 50°, the difference between this and 32°, (the point at which the snow melts) that is 18°, will be the range of the scale; and this in the instrument is divided into 100 parts. To use this instrument, the bulb, A, is to be covered with a bit of muslin, or blotting-paper, moistened, and, in proportion as this moisture is evaporated, cold will be produced in that bulb, the air in it will be contracted, and the liquid will move towards it on ascending the scale with a rapidity and to an extent in proportion to the evaporative power of the atmosphere. The instrument will thus show the degree of dryness with very considerable accuracy.

Another ingenious instrument for the same purpose, is

ANDERSON'S ATMOMETER.

This instrument consists of a glass tube, A, B, C, D, E, F, having two bulbs upon it, B C, and E F, and terminating in the latter. It is fixed into a stand, and has a scale, G H, attached to it. A portion of pure spirit of wine is contained in the bulb at F, and the air is excluded from the whole tube. In consequence of this, the spirit, in a state of vapour, fills the remainder of the tube. To use the instrument, the ball, C, B, is covered with muslin and moistened. The cold thus produced condenses the vapour of the spirit in the bulb, and falls down in a liquid state into the tube, at A, the quantity so falling making the evaporative power of the atmosphére, and, thus, the instrument is the same in principle as Mr. Leslie's, only the vapour of spirit is more positive than common air.

MR. DANIEL'S HYGROMETER, A VERY DELICATE INSTRUMENT, BUT VERY DIFFICULT TO USE, IS SHEWN IN THE FOLLOWING FIGURE.

A and B are two thin bulbs of glass, about 1 inches in diameter, con+ nected together by a bent tube, having a bore of about one eighth of an inch. The tube is bent as in the figure, and supported on the stand, G H. G, a brass socket, at I, in which it moves freely. The ball, B, is partially filled with ether, and the air is excluded from the rest of the apparatus. A small thermometer, D, is placed in the tube with its bulb partially immersed in the ether, and a similar thermometer, IK, is attached to the stand of the instrument. When both balls are exposed to the same temperature, the instrument is so constructed, as that the degree shewn by the thermometer in the tube and that on the stand, is the same. To use the instrument, cover the bulb, A, with muslin, drop upon it a few drops of ether, and place the instrument between you and the light, but be careful not to breathe upon it. Ether evaporates with great rapidity; and, therefore, the temperature within the instrument, sinks almost instantaneously, and conti pues so to do until a cold, greater even than that which freezes water, could be

produced. In this progress there is a point to be carefully noticed: so long as the moistened bulb continues to give out, by evaporation, to the atmosphere, the evaporative power has not been determined; but the moment that the point at which that power is exhausted, is passed, the bulb begins to attract moisture back again from the atmosphere, and the moisture so attracted, falls upon the bulb in the form of a delicate cloud of dew. The difference between the thermometer in the tube and the one on the pillar, at the instant when this commences, is the measure of the evaporative power of the atmosphere. Should it be impossible, from the rapidity with which the instrument acts, to notice the moment at which the dew begins to fall on the bulb, the evaporative power may still be found, by noticing the degree at which the dew again disappears.

While the barometer denotes the absolute weight of a column of the atmosphere at the place of observation, and thus puts us in possession of a real quantity, the results obtained by the thermometer and hygrometer, even when these instruments are the most nicely constructed, are only