sarily precede science. The wants and cravings of our animal constitution must be satisfied; the comforts, and some of the luxuries, of life, must exist. Something must be given to the vanity of show, and more to the pride of power: the round of baser pleasures must have been tried, and found insufficient, before intellectual ones can gain a footing; and when they have obtained it, the delights of poetry and its sister arts still take precedence of contemplative enjoyments, and the severer pursuits of thought; and when these in time begin to charm from their novelty, and sciences begin to arise, they will at first be those of pure speculation. The mind delights to escape from the trammels which had bound it to earth, and luxuriates in its newly-found powers. Hence, the abstractions of geometry-the properties of numbers-the movements of the celestial spheres-whatever is abstruse, remote, and extramundane-become the first objects of infant science. Applications come late: the arts continue slowly progressive, but their realm remains separated from that of science by a wide gulf, which can only be passed by a powerful spring. They form their own language and their own conventions, which none but artists can understand. The whole tendency of empirical art is to bury itself in technicalities, and to place its pride in particular short cuts and mysteries known only to adepts; to surprise and astonish by results, but conceal processes. The character of science is the direct contrary. It delights to lay itself open to inquiry; and is not satisfied with its conclusions, till it can make the road to them broad and beaten and in its applications it preserves the same character; its whole aim being to strip away all technical mystery, to illuminate every dark recess, with a view to improve them on rational principles. It would seem that a union of two qualities almost opposite to each other—a going forth of the thoughts in two directions, and a sudden transfer of ideas from a remote station in one to an equally distant one in the other-is required to start the first idea of applying science. Among the Greeks, this point was attained by Archimedes, but attained too late, on the eve of that great eclipse of science which was destined to continue for nearly eighteen centuries, till Galileo in Italy, and Bacon in England, at once dispelled the darkness: the one, by his inventions and discoveries;

the other, by the irresistible force of his arguments and eloquence. Herschel.

THE THERMOMETER.

THE perfection of science depends upon accurate measurement and instrumental precision: there can be no advance of a single step without these essential means. The unassisted senses are by no means sufficient to determine quantities with anything approaching to the necessary degree of exactness. The hand, or the application of muscular force, may indeed inform us that one body differs very greatly from another in gravity; but without the balance to determine weights which would be wholly inappreciable in this way, the science of chymistry could not have existed. So, our sensations may inform us that one body is hotter or colder than another; but without some means of measuring differences of temperature, which would wholly fail to affect our senses, our knowledge of heat would have been small indeed. It is by observing that expansion or enlargement of volume is always produced by the same causes which affect us with the sensation of heat, that we come to regard expansion as the indication of heat; and as this is an effect which can be ascertained with the utmost precision, we adopt its measure as that of the cause which produces it.

If a certain quantity of air, or of a liquid, or solid, undergo an augmentation of volume, when exposed to a certain source of heat which we can determine, and when we expose it to another source of heat that expansion is doubled, there is reason to infer that the intensity of the second source is double that of the first: and this is the principle of that useful instrument the thermometer.

The honour of the first invention is generally ascribed to Sanctorio, an Italian physician, about the year 1590; but the same contrivance probably suggested itself in an independent manner to Cornelius Drebel, about the year 1610. One thing, however, is certain, viz., that it dates from about the beginning of the seventeenth century. The thermometer of Sanctorio consisted of a hollow glass

globe, attached to a long stem, open at the opposite extremity; a portion of air was expelled from it by the expansive force of heat, and the end of the tube was then immersed in a coloured liquid. As the included air cooled and returned to its former volume, the liquid rose in the tube, from the superior elasticity of the external air pressing upon the external surface of the liquid. A scale of equal parts was applied to the stem, by which the expansion of the included air from heat, or its contraction from cold, could be measured, by the movement of the column of liquid. It was liable, however, to the objection of being acted upon, not only by the expansion of the included air, but by the barometric changes of the exterior atmosphere.

Air has not only the advantage of being extremely regular in its expansion, but also of indicating very minute changes of temperature by the great alterations of volume which it undergoes from being heated and cooled; and a modification of the air-thermometer is now very often used in delicate researches. It was invented in the year 1676, but was chiefly brought into notice by the admirable experiments of the late Sir John Leslie.

The first great improvement in the thermometer, for ordinary purposes, was made by the members of the Italian Academy del Cimento, in 1660, who substituted the expansions of a liquid as the measure of heat; enclosing it in a glass ball and tube, which was afterwards cut off from all communication with the variable atmosphere, by softening the extremity of the glass at the flame of a lamp, and hermetically sealing it: the instrument thus at once became more accurate and more manageable. Spirits of wine was the liquid first employed, and quicksilver was afterwards used by Halley and Sir Isaac Newton; both liquids are now at times employed for different purposes.

Still the thermometer wanted much of perfection; for different instruments thus constructed, could not afford comparable results, although experiments made with the same instrument were comparable with one another. For the last great improvement we are indebted to Newton. It had been observed by another eminent philosopher, Hook, that the temperature of melting ice was always

fixed and permanent; and that the temperature of boiling water was equally invariable, provided the pressure of the atmosphere did not change. The sagacity of Newton pointed out the application of these observations to the completion of the instrument. If we immerse the mercurial thermometer in melting snow or ice, the liquid will gradually contract, and sink in the stem to a certain point, and then stop; and however long we may allow it to remain in the ice, it will sink no lower. The experiment repeated at any time, or in any place, will afford the same results; the liquid will always sink to the same part, and no further. By marking this, we obtain one fixed point, which must be the same in every thermometer which is subjected to the trial. If we now remove the instrument into a vessel where it may be surrounded with boiling water, while the barometer indicates an unvarying pressure of thirty inches, the liquid in the glass will expand till the mercurial column reaches a certain height, where it will again become perfectly stationary, and afford another invariable point of comparison. The distance between these two points measures the amount of expansion of the whole quantity of the included liquid, or rather is the measure of the difference of the expansion of the liquid and the glass; for both expand. Upon the supposition that the expansion of the two is equable, and that the bore of the glass tube, in which the liquid moves, is equal throughout, the distance between the two points may be divided into any number of equal parts, and the equal amounts of expansion or contraction measured by them, may be taken to indicate equal increments or decrements of temperature, which will correspond in all instruments similarly graduated: upon the supposition, moreover, that the law of expansion continues the same both above the boiling and below the freezing point of water, an extension of the same scale of equal parts will afford the means of measuring greater and less degrees of temperature, with reference to the same fixed standard.-Daniel.

THE BAROMETER.

To the husbandman the barometer is of considerable use, by aiding and correcting the prognostics of the weather which he draws from local signs familiar to him; but its great use as a weather-glass seems to be to the mariner, who roams over the whole ocean, and is often under skies and climates altogether new to him. The watchful captain of the present day, trusting to this extraordinary monitor, is frequently enabled to take in sail, and to make ready for the storm, where, in former times, the dreadful visitation would have fallen upon him unprepared.—The marine barometer has not yet been in general use for many years, and the author was one of a numerous crew who probably owed their preservation to its almost miraculous warning. It was in a southern latitude. The sun had just set with placid appearance, closing a beautiful afternoon, and the usual mirth of the evening watch was proceeding, when the captain's order came to prepare with all haste for a storm. The barometer had begun to fall with appalling rapidity. As yet, the oldest sailors had not perceived even a threatening in the sky, and were surprised at the extent and hurry of the preparations; but the required measures were not completed, when a more awful hurricane burst upon them than the most experienced had ever braved. Nothing could withstand it. The sails, already furled and closely bound to the yards, were riven away in tatters: even the bare yards and masts were in great part disabled; and at one time the whole rigging had nearly fallen by the board. Such, for a few hours, was the mingled roar of the hurricane above, of the waves around, and of the incessant peals of thunder, that no human voice could be heard, and amidst the general consternation, even the trumpet sounded in vain. In that awful night, but for the little tube of mercury which had given warning, neither the strength of the noble ship, nor the skill and energies of the commander, could have saved one man to tell the tale. On the following morning the wind was again at rest, but the ship lay upon the yet heaving waves, an unsightly wreck.