porting the inftrument fafely from one place to another. The guage confifts of a finall stem of ivory, arifing from float of the fame fub. ftance; a circular divifion is cut round this ftem; the ftem paffes through a fhort-cylinder of ivory, which is cut open in front; on this front two fmall divifions are cut; at the bottom of this cylinder is a male fcrew, to fit the female fcrew of the ciftern; the upper part of the guage is protected by a tube of glafs perforated at top. TO USE THIS BAROMETER. 1. The barometer being fixed in a perpendi cular position, unfcrew the fcrew at bottom as far as it will go without forcing it. 2. Take out the ivory fcrew at the top of the ciftern, and place it between the fcroles on the upper part of the frame. 3. Screw the guage into the place from whence the ivory fcrew was taken. 4. Screw up that fcrew which is at the bottom of the frame, until the line on the float exactly coincides with the two lines on the front of the ivory cylinder. 5. Strike the barometer gently with the knuckles, and then fo fet the lower edge of the front index to the convex furface of the mercury that it may be at the fame time in a line with the edge of the index behind the tube; and the nonius will then give the true height of the mercurial column, from the furface of the mercury in the cistern. 6. The preceding rule for fetting the guage muft be complied with previous to every obferva, tion. 7. If the barometer is to be tranfported from one place to another, the guage must be removed, and the folid ivory fcrew inferted in it's place; after which, the mercury in the tube may be forced gently up to the top thereof, by the fcrew at the bottom of the frame. OF THE SCALE OF CORRECTION, This fcale is placed close to that of the thermometer; but on the right-hand fide, the zero, or o degree of this fcale, correfponds to the 55th degree of the thermometer. 1. If the barometer is at 30 inches, and the thermometer at 55 degrees, no correction is neceffary. 2. But if the thermometer be under 55, and the barometer at 30 inches, you must add to the height of the barometer as many of the 100ths of inches as are on the scale of correction opposite to the degree of the thermometer. 3. If the thermometer be above 55, and the barometer at 30 inches, you must subtract as many Tooths as are indicated by the given degree of the thermometer on the scale of correction. baro 4. The fcale applied to the thermometer anfwers for the general range of meteorological obfervations; but if the height of the barometer be very far distant from 30 inches, it will be neceffary to make use of the rule of three, in order to obtain the true correction: for instance, let meter be at 26 inches, which we will ca correction indicated by the thermometer, correction: then as 30: P:c: x; or is to be added to the whenever the thermomet to be fubtract PC 30 of the ba der 55 degre c the True ch fer OF THE THERMOMETER. No inftrument is of more importance for making difcoveries in metcorology than the thermometer, as it points out the temperature or degrce of heat of the air and other bodies. Heat and cold are perceptions, the ideas of which we acquire by the fenfes. Our fenfations are however inadequate meafures of heat and cold, for they depend not only on the fubftance which excite them, but on the actual ftate of our bodies at that time: we cannot, therefore, conclude the exact identity or fimilarity of the caufe, from the famenefs of the fenfations, unlefs we can be affured that our bodies are in the fame ftate: if they be not, the fame objects will produce very different fenfations. Thus if the hand be plunged into lukewarm water, this water will appear cold if the hand be warm; but if the hand be cold, the water will appear to be warm; though in both cafes it poffeffes the fame temperature. Our fenfes are, therefore, both imperfect and deceitful meafures of heat; and we cannot afcertain, by their means only, the ftate of the furrounding bodies with refpect to heat or cold. This occationed philofophers to feek for fome method by which they might determine the temperature of bodies with more certainty. This they found in the property of fire to dilate and expand all bodies, whether folid or fluid; and of cold, to contract or condense them. This expanfion and cortraction is confidered as a measure infinitely more certain of the degrees of heat and cold, than the fenfes. It would appear from this expansion, that fire, when it is agitated by that motion which we call beat, always acted as if it wanted more room, and this in fuch a wonderful manner, as if every parti cle cle of space in which it exifts were a radiant point or center, from whence it spreads forcibly outwards in every direction; and confequently when fire, thus acting, is admitted into the pores of bodies, their parts must be ftretched out, and their dimenfions every way increased, according to the degree of fire by which they are acted on. Some idea of the force of this expanfion may be gained by confidering how vaft a weight may be fufpended from a bar of iron, or brass, in a vertical pofition, without feparating the parts of the metal, or overcoming the force with which they cohere. Now this, fire eafily executes, fo far relaxing the texture of brass and iron, that their parts will fall afunder with nothing but the force of gravity. Thermometers are inftruments which meafure the degree of beat by the expansion of bodies. Fluids are thofe generally ufed, because they are dilated more readily than folids; and quickfilver is preferred to other fluids, because it's expanfibility is not affected by the different circumftances in which it is placed; it does not foil the tube like many other fluids, and at the fame time affords an extensive scale of divifions. A thermometer is a tube of glass, the end of which is blown into a ball or cylinder; the ball, and part of the tube, is filled with mercury. The fluid in the ball dilates by the heat, and contracts with the cold, which occafions the fluid in the tube to rife and fall; and the fmaller the bore of the tube is in proportion to the ball, the more vifible will be the rife of the fluid by a fmall expanfion. We may, there ment as a convenient heat and cold, which which the tube is affixel But it is not fufficie fur t; it must be confider this intruof the changes of n by the fcale to found a meaalways fpeak ing the fame language, and awaking the fame ideas in the mind, in all places, and at all times. To this end it is neceffary, 1. That this meafure should begin from a known and determinate point. And, 2. That another point, equally certain as the first, but at fome diftance from it, be fixed upon. And, 3. That the space between them be divided into a certain number of parts, which in all inftruments will have a conftant proportion. It has been fully proved, that the temperature of freezing water, or melting ice, is conftantly the fame in all places, and at all times. The fame may be faid of boiling water, under a given preffure of the atmosphere. If, therefore, the ball of a ther mometer be plunged into melting ice, and afterwards into boiling water, and left in each till it acquires their temperature, and marks are made at the refpective heights at which the mercury ftands in each, two fixed points will be obtained. To be more particular: When ice is at the melting temperature, whatever be the beat you apply to it, it does not become botter; a thermometer in the middle of the mafs, continually ftands at the thawing point as long as any of the ice remains about it, fo that the fame caufe, which in other circumftances would produce heat, here only produces liquifaction. Hence it is, that melting ice, or freezing water, is fo well adapted for giving one of the fixed points of a thermometer. The quantity of fire abforbed by ice in melting, is such as would increafe the temperature of the water about 140 degrees: converfely, water may be cooled 18 degrees below the freezing point, without freezing: congelation cannot take place till a certain portion of the combined or latent fire is difengaged; when any part does congeal, the fire let loofe, raifes the thermometer to |