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 [Pg098] 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:—