We have combined the two (_see_ Fig. 22.) The diagram will, we have no doubt, prove exceedingly useful to scientific readers and experimentalists. There is also another system of notation, adopted by the French, called the _centigrade_, but it is not much referred to in Great Britain. In the centigrade thermometer 0 zero is the freezing point, and 100 the boiling point. Fahrenheit's scale is generally preferred. Reaumur's is mostly used in Germany. Of Fahrenheit's scale 32 is the freezing point, 55 is moderate heat, 76 summer heat in Great Britain, 98 is blood heat, and 212 is the boiling point. Mr. Wedgwood has invented a thermometer for testing _high temperatures_, each degree of which answers to l30 degrees of Fahrenheit. According to his scale cast iron melts at 2,786 deg.; fine gold at 2,016 deg.; fine silver 1,873 deg.; brass melts at 1,869 deg.; red heat is visible by day at 980 deg.; lead melts 612 deg.; bismuth melts 476 deg.; tin melts 412 deg.; and there is a curious fact with regard to the three metals, lead, bismuth, and tin, that if they are mixed in the proportions of 5, 8, and 3 parts respectively, the mixture (after previous fusion) will melt at a heat below that of boiling water. 712. _What is the difference between the thermometer and the barometer?_ In the thermometer the column of mercury is much smaller than in the barometer, and is sealed from the air; while in the barometer the column of mercury is open at one end to atmospheric influence. 713. _Why does the mercury in the thermometer, being sealed up, indicate the external temperature?_ Because the heat passes through the glass, in which the mercury is enclosed, and _expanding or contracting the metal within the bulb_, causes the small column above it to _rise_ or _fall_. [Verse: "Blessed is the people that know the joyful sound: they shall walk, O Lord, in the light of thy countenance."--PSALM LXXXIX.] 714. _When does the thermometer vary most in its indication of natural temperature?_ It varies more in the _winter_ than in the _summer_ season. 715. _Why does it vary more in the winter than in the summer?_ Because the temperature of our climate _differs more from the temperature of the torrid zones in the winter_ than it does in the _summer_, and the _inequalities of temperature_ cause frequent changes in the degree of prevailing heat. The same remarks (714, 715,) apply to the barometer. CHAPTER XXXIV. 716. _What is sound?_ Sound is an _impression produced upon the ear_ by _vibrations_ of _the air_. 717. _What causes the air to vibrate and produce sounds?_ The atoms of _elastic bodies_ being caused to _vibrate_ by the application of some kind of force, _the vibrations of those atoms are imparted to the air_, and sound is produced.