[Illustration: FIG. 123.--Comparison of centigrade and Fahrenheit scales.] =143. Comparison of Thermometer Scales.=--It is often necessary to express in centigrade degrees a temperature for which the Fahrenheit reading is given or _vice versa_. Since there are 180 Fahrenheit degrees between the "fixed points" and 100 centigrade degrees, the Fahrenheit degrees are smaller than the centigrade, or 1°F. = 5/9°C. and 1°C. = 9/5°F. One must also take into account the fact that the melting point of ice on the Fahrenheit scale is marked 32°. Hence the following rule: To change a Fahrenheit reading to centigrade subtract 32 and take 5/9 of the remainder, while to change centigrade to Fahrenheit multiply the centigrade by 9/5 and add 32 to the product. These two rules are expressed by the following formulas. (F.° - 32)5/9 = C.°, 9C.°/5 + 32° = F.° Another method of changing from one thermometric scale to another is as follows: A temperature of -40°F. is also _represented_ by -40°C., therefore to change a Fahrenheit reading into centigrade, we add 40 to the given reading, then divide by 1.8 after which subtract 40. To change from a centigrade to Fahrenheit reading the only difference in this method is to multiply by 1.8 or C. = (F. + 40)/1.8 - 40 and F. = 1.8(C. + 40) - 40. [Illustration: FIG. 124--Comparison of absolute, centigrade and Fahrenheit scales.] =144. The Absolute Scale of Temperature.=--One often hears the statement "as cold as ice." This expresses the incorrect idea that ice cannot become colder than its freezing temperature. The fact is that ice _may be cooled_ below freezing down to the temperature of its surroundings. If a piece of ice is placed where the temperature is below the melting point, the ice, like any other solid, cools to the temperature of the surrounding space. For example, a piece of ice out of doors is at 10°F. when the air is at this temperature. It follows then, that when ice has been cooled below the freezing temperature that heat is required to warm the ice up to its melting point; or in other words that ice at its melting temperature possesses some heat. The temperature at which absolutely no heat exists is called _absolute zero_. There has been devised an _absolute scale of_ temperature. This scale is based upon the centigrade scale, _i.e._, with 100° between the two fixed points; the scale, however, extends down, below the centigrade zero, 273°, to what is called _absolute zero_. It follows therefore that upon the absolute scale, the melting point of ice, and the boiling point of water are 273° and 373° respectively. (See Fig. 124.)