evaporated by the boiler, and used by the cylinder, was known, and in determining, by the quantity and heat of the hot water emitted by Newcomen's engines, the quantity of steam required to work them did not lead to the improvements I afterwards made in the engine. These improvements proceeded upon the old established fact, that steam was condensed by the contact of cold bodies; and the later known one, that water boiled in vacuo at heats below 100°, and consequently that a vacuum could not be obtained unless the cylinder and its contents were cooled every stroke to below that heat."

EXPOSITION OF PHYSICAL PRINCIPLES.-THERMOMETER.-METHOD OF GRADUATING IT. FREEZING AND BOILING POINTS.-LATENT HEAT OF WATER.-QUANTITY OF HEAT NECESSARY TO CONVERT ICE INTO WATER.-QUANTITY OF HEAT GIVEN OUT BY WATER IN BEING CONVERTED INTO ICE.PROCESS OF BOILING. OF RECONVERSION OF STEAM INTO WATER.-QUANTITY OF HEAT NECESSARY TO CONVERT WATER INTO STEAM. -BOILING POINT OF WATER. -DIFFERENT IN DIFFERENT PLACES.-DEPENDS ON THE BAROMETER VARIES WITH THE PRESSURE.-EXPERIMENTAL PROOF OF THIS.-BOILS AT LOWER TEMPERATURES THAN 212° UNDER PRESSURES LESS THAN THE ATMOSPHERE. -SUM OF LATENT AND SENSIBLE HEAT OF STEAM ALWAYS THE SAME.THE FUEL NECESSARY TO EVAPORATE WATER THE SAME, WHATEVER BE THE TEMPERATURE OR PRESSURE AT WHICH IT IS EVAPORATED.-MECHANICAL FORCE OBTAINED BY EVAPORATION. THIS FORCE NEARLY THE SAME UNDER ALL CIRCUMSTANCES.

(51.) WE shall pause here to put the reader in possession of the physical and mechanical principles connected with the evaporation of water and other liquids, which are necessary to enable him to understand the full extent of the value and the merit of the discoveries of Watt, and to comprehend the

H

structure and operation of the steam engine in its improved form, as it has passed to us from his hands.

As we shall frequently have occasion to refer to the indications of a thermometer, we shall first explain the principle of that instrument as it is commonly used in this country.

The thermometer is an instrument used for the purpose of measuring and indicating the temperature or sensible heat of material substances.

Heat, like all other physical agents, can only be measured by its effects. One of these effects best suited for this purpose, is the change of dimension which all bodies undergo in consequence of their change of temperature. In general, when heat is applied to a material substance, that substance undergoes an enlargement of bulk; and if heat be abstracted from it, it suffers a diminution of bulk. This variation of magnitude is not always in the same proportion as the increase or diminution of temperature; but it is so when applied to certain substances and between certain limits. One of the substances whose expansion and contraction through an extensive range of temperature has been found to be nearly uniform, and which is attended with other convenient qualities for a thermometer, is the liquid called mercury or quicksilver. A mercurial thermometer is constructed in the following way:

A glass tube is made with a small and uniform bore: upon the end of this tube, a bulb is blown, having a magnitude very great compared with the bore of the tube. Let us suppose this bulb and a part of the tube to be filled with mercury. If the mercury contained in the bulb be heated, it will expand, and being more susceptible of expansion than the glass which contains it, the bulb will be too small for its augmented volume: the mercury in the bulb can only, therefore, obtain room for its increased bulk by pressing the mercury in the tube upwards, which it will accordingly do. The increase of volume which the mercury in the bulb therefore undergoes, will be exhibited by the increased length of the column in the tube. Since the bore of the tube is made so exceedingly minute compared with the magnitude of the bulb, a very small quantity of mercury forced

from the bulb into the tube, will cause a considerable increase of the length of the column. Small degrees of expansion will therefore be rendered very apparent, and may be accurately measured. The following is the method by which the thermometer called Fahrenheit's thermometer is graduated.

The tube and bulb being prepared and supplied with mercury, as already explained, let the instrument be plunged in a vessel of melting ice. It will be found that the mercury will stand in the tube at a certain point, from which it will not vary so long as any ice remains not completely melted in the vessel. Let a mark be made on the tube, or on a scale attached to the tube, at the point corresponding to the top of the column: the point thus marked is called the freezing point.

Now let the instrument be immersed in a vessel of boiling water, the barometer at the time having the height of thirty inches. It will be found that so long as the water is kept boiling, the column of mercury in the tube will remain stationary. Let the point corresponding with the top of the column be marked on the tube, or on the scale attached to it. This is called the boiling point. Let the space on the scale between the freezing and boiling points be now divided into 180 equal parts: each of these parts is called a degree. Let the same divisions be continued upon the scale below the freezing point, until thirty-two divisions be taken; let the lowest division be then marked 0, and let the successive divisions upwards from that be numbered 1, 2, 3, &c. In like manner, let the same divisions be continued above the boiling point, as far as the tube will admit.

It is evident that, under these circumstances, the freezing point will be marked by 32, and the boiling point by 212. It is usual to express the degrees of a thermometer in the same manner as the degrees of a circle, by placing a small above the number. Thus the freezing point is expressed by 32°, and the boiling point by 212°.

The reason the degrees were commenced at 32° below the freezing point was, because, when the thermometer was invented, that temperature was supposed to be the lowest degree of cold possible, being that of a certain mixture of

snow and salt. This, however, has since been found to be an error, very much lower temperatures being obtained by various physical expedients.

The temperature of a body is, then, that elevation to which the thermometer would rise when immersed in that body. Thus, if in plunging the thermometer in water we found the mercury to rise or fall to the division marked 100, we should then say, the temperature of the water was 100°.

Let us suppose a spirit lamp, or other regular source of heat, applied to a bath of mercury, so as to maintain the mercury at a fixed temperature of 200°, and let another vessel, containing a quantity of ice at a temperature of 20° be immersed in the mercury. Let a thermometer be placed in the mercury, and another in the ice. The following effects will then ensue. The thermometer immersed in the ice will be observed gradually to rise from 20° upwards, until it indicates the temperature of 32°. It will then become stationary, and the ice which had hitherto remained in a solid state will begin to melt and be converted into water. This process of liquefaction will continue for a considerable time, during which the thermometer immersed in the ice will constantly be maintained at 32°. At the moment, however, when the last portion of ice is liquefied, the thermometer will begin again to rise. The coincidence of this ascent of the thermometer with the completion of the liquefaction of the ice, may be very easily observed, because the ice being lighter, bulk for bulk, than water, will float on the surface, and so long as a particle of it remains unmelted it will be distinctly seen.

Now it cannot be doubted that, during the whole of this process, the mercury, supposed to be maintained at 200°, constantly imparts heat to the ice; yet, from the moment the liquefaction begins, until it is completed, no increased temperature is exhibited by the thermometer immersed in the melting ice. If during this part of the process no heat were received by the ice from the mercury, the consequence would be, that the application of the lamp would cause the temperature of the mercury to rise above 200°, which may be easily demonstrated by withdrawing the vessel of ice from the mercurial bath during the process of liquefaction. The moment