meter is readily connected with that of the thermometer by immersing the register in boiling mercury, whose temperature is as constant as that of boiling water, and has been accurately determined by the thermometer. The amount of expansion for a known number of degrees is thus determined, and the value of all other expansions may be considered as proportional.

§ 147. The melting point of cast iron has been thus

or cylindrical piece of porcelain; d the platinum band, with its wedge, e.

2. Is the scale by which the expansion is measured; ff' is the greater rule upon which the smaller, g, is fixed square. The project

ing arm, h, is also fitted square to the ledge, under the platinum band, d. D is the arm which carries the graduated arc of the circle fixed to the rule, ff', and moveable upon the centre, i. c is the lighter bar, fixed to the first, and moving upon the centre, k. H is the nonius at one of its extremities, and m the steel point at the other. The rule g admits of adjustment upon ff", so that the arm h may be adjusted to the centre, i, in order that at the commencement of an experiment the nonius may rest at the beginning of the scale.

ascertained to be 2786°, and the highest temperature of a good wind furnace about 3300°; points which were estimated by Mr. Wedgwood at 20577° and 32277° respectively.

Mr. Wedgwood, indeed, himself makes an observation which is calculated to throw suspicion upon the accuracy of his results, for he says, "We see at once how small a portion (of the range of heat) is concerned in animal and vegetable life, and in the ordinary operations of nature. From freezing to vital heat is barely both part of the scale: a quantity so inconsiderable relative to the whole, that in the higher stages of ignition ten times as much might be added or taken away, without the least difference being discoverable in any of the appearances from which the intensity of fire has hitherto been judged of."

Now this is utterly unlike the gradual progression by which the operations of nature are generally carried on: and the fact is, that a regular transition may be traced from one remarkable point of temperature to another.

Thus, from the freezing of water, 32°, to vital heat in man, is 60°.

§ 148. Being now in possession of accurate measures for the intensity of heat, we are prepared to proceed with our investigation of its effects upon solids, liquids, and aëriform bodies, we may drop the vague language of sensation,—of hot, and cold, and warm, and such like expressions, and substitute the precise determinations of thermometric degrees.

It has been lately proved by the careful experiments of MM. Dulong and Petit, that beyond 212°, or the boiling-point of water, the expansion of solids, for equal degrees of heat, compared with that of air, increases as the heat rises; any differences in this respect between 32° and 212° being so small as to escape observation. The principal results of their investigation are shown in the following tables:

TABLE XI. Increasing Dilatation of Metals by Heat.

572° 10878 702°.5 31660 623°.8 05340 592°.9

It appears that at temperatures beyond 212°, glass expands in a greater ratio than mercury; and as the mercurial thermometer measures the difference of the expansion of the glass and metal, upon the supposition that they are both uniform, its degrees require a correction in the upper part of the scale, when compared with the equal dilatations of air. Thus the temperature of 586° on the mercurial thermometer, corresponds with 572° on the air thermometer: while from the third column of Table X. it appears that the expansion of glass alone, supposed to be equable, would indicate 667° for the same point. The second, fourth, and sixth columns of Table XI. show the mean expansion, for each degree, of iron, copper, and platinum, when heated from 32° to 212°, compared with that from 32° to 572°; the third, fifth, and seventh columns indicate the degrees on thermometric bars of these metals, corresponding to the temperature of 572° on an air thermometer.

§ 149. Before the invention of the register pyrometer, the expansion of solids had never been ascertained beyond the temperature of 572°; the following table exhibits the pro

gressive amount of several metals to their points of fusion as determined by that instrument:

TABLE XII. Progressive Dilatation of Solids.

§ 150. The amount of the force which produces these expansions and contractions measured by any opposing force, that of cohesion for instance, is enormous. Some idea may be formed of it, when it is understood that it is equal to the mechanical force which would be necessary to produce similar effects in stretching or compressing the solids in which they take place; thus a bar of iron heated so as to increase its length a quarter of an inch, by this slow and quiet process exerts a power against any obstacle by which it may be attempted to confine it, equal to that which would be required to reduce its length by compression to an equal amount. On withdrawing the heat, it would exert an equal power in returning to its former dimensions.

Such a force as this is capable of being applied to a variety of useful purposes when properly directed, and of producing very injurious effects in constructions of art, when not properly provided against.

The wheelwright applies this power by making the iron band, or tire as it is called, of a carriage-wheel a little smaller than the wooden circle, or fellies, and then expanding it by heat, he fits it on to the latter: he afterwards suddenly cools it by throwing water upon it, and the contraction binds the

whole together with enormous force. Upon the same principle the cooper heats the iron hoops of his casks, and firmly draws the staves together by cooling them in their places. The plates of the boilers of steam-engines and of iron ships are rivetted together with hot rivets, which, when cool, draw them together and render their junction perfectly compact.

M. Molard, by an application of the same force, ingeniously restored the equilibrium of a building in Paris, the walls of which had been pressed outwards by the incumbent weight. He introduced several strong iron bars through holes in the opposite walls, and screwed strong plates, or washers as they are called, on the projecting ends, so as to bear tightly upon the walls. The bars were then heated by lamps, and consequently lengthened; in this state the washers were again screwed up, and when the bars were allowed to cool, they contracted and drew the walls together to a small amount: a repetition of the same process gradually and safely effected the purpose.

The same process has been since applied in the restoration of the Cathedral at Armagh.

§151. Experience has taught engineers that it is dangerous to attempt to confine such a force as this, and that it is necessary to make provision for these expansions and contractions, particularly in the metallic constructions which are now so common. In iron pipes for the conveyance of gas and water, when the lengths are very considerable, some of the junctions are rendered moveable; so that by the end of one pipe sliding into that of another, the accidental changes due to temperature are provided for.

The arches of the Southwark iron bridge rise and fall about an inch in the usual range of atmospheric temperature, and if provision had not been made for this, destructive consequences must have ensued.

The steeple of Bow Church has within these few years been nearly thrown down by the alternate expansion and contraction of some rods of iron which were built into it to give it stability. The rods in hot weather lengthened, and lifted the incumbent mass of masonry; they returned in cold weather to their former dimensions, leaving the stones upraised; dust and small particles of matter made their way into the cracks thus produced; the rods again lengthened, and lifted the mass a little higher; till,