The principle of this thermometer is not altogether new; but the duplicate arrangement of the bars, which effectually prevents the movement of the indices by any shaking, and the application are certainly novel. Professor Trail, in the _Library of Useful Knowledge_, writes:--"In 1803, Mr. James Crighton, of Glasgow, published a new 'metallic thermometer,' in which the unequal expansion of zinc and iron is the moving power. A bar is formed by uniting a plate of zinc (fig. 71), _c d_, 8 inches long, 1 inch broad, and 1/4 inch thick, to a plate of iron, _a b_, of the same length. The lower extremity of the compound bar is firmly attached to a mahogany board at _e e_; a pin, _f_, fixed to its upper end, plays in the forked opening in the short arm of the index, _g_. When the temperature is raised, the superior expansion of the zinc, _c d_, will bend the whole bar, as in the figure; and the index, _g_, will move along the graduated arc, from right to left, in proportion to the temperature. In order to convert it into a _register thermometer_, Crighton applied two slender hands, _h h_, on the axis of the index; these lie below the index, and are pushed in opposite directions by the stud, _i_,--a contrivance seemingly borrowed from the instrument of Fitzgerald," a complicated metallic thermometer, described by the Professor previously. CHAPTER X. BOILING-POINT THERMOMETERS. =91. Ebullition.=--The temperature at which a fluid _boils_ is called the _boiling-point_ of that particular fluid. It is different for different liquids; and, moreover, in the same liquid it varies with certain changes of circumstance. Thus the same liquid in various states of purity would have its boiling temperature altered in a slight degree. There is also an intimate connection with the pressure under which a fluid is boiled, and its temperature of ebullition. Liquids boiled in the open air are subjected to the atmospheric pressure, which is well known to vary at different times and places; and the boiling-point of the liquid exhibits corresponding changes. When the pressure is increased on the surface of any fluid, the temperature of ebullition rises; and with a decrease of pressure, the boiling goes on at a lower degree of heat.