Appendix I 347 



the freezing-point is called 32, the boiling-point 212, the space be- 

 tween being divided into 180 equal degrees, which are continued 

 downward and upward (F, Fig. 65). On the Reaumur scale, used 

 popularly in Germany and Russia, the space between freezing and 

 boiling point is divided into 80 degrees. The centigrade scale is 

 used in scientific work all over the world, except for meteorological 

 observations in English-speaking countries, for which the Fahrenheit 

 scale presents too many advantages to be discarded. It is convenient 

 to remember a quick way of translating centigrade into Fahrenheit 

 degrees. Miiltiply by 2, subtract one-tenth of the result, and add 32. 

 For example, to translate 15 C., 15x2 = 30, subtracting one-tenth 

 30 - 3 = 27, adding, 27 + 32 = 59 F. Since mercury freezes at - 40 (a 

 temperature which happens to be expressed by the same figure 

 on both centigrade and Fahrenheit scales), alcohol thermometers are 

 used for measuring lower temperatures, such as those of the winter at 

 Verkhoyansk. No two common thermometers read exactly alike, 

 and those employed for accurate observations are always compared 

 with standard instruments (those of Kew Observatory for the United 

 Kingdom), and have their error ascertained and allowed for. 

 Thermographs are constructed on the principle of the barograph, 

 to furnish a continuous record of changes of temperature. Deep-sea 

 thermometers require to be protected against the pressure at great 

 depths by surrounding the bulb by a glass sheath partly filled with 

 mercury or other liquid. They are constructed either to leave an 

 index sticking in the tube at the points of highest and lowest 

 temperature encountered while submerged, or to be inverted by 

 appropriate mechanism, and so caused to register the temperature 

 at any given point. (See article "Thermometer" in Encyclopedia 

 Britannica, 9th edition.) 



441. Hygrometers measure the amount of water -vapour in the 

 atmosphere by finding either at what rate the air is taking up 

 vapour by evaporation at its actual temperature, or how far the air 

 must be cooled in order that its vapour may be saturated. The 

 commonest form consists of two thermometers placed side by side, 

 the bulb of one being left dry, while that of the other is covered by 

 a piece of fine muslin, and kept wet by a thread dipping into a 

 vessel of water. The farther the vapour of the air is from satura- 

 tion the more rapid is the evaporation from the wet bulb, and since 

 evaporation withdraws heat (71, 157), the temperature shown by the 

 wet-bulb thermometer is lower than that shown by the dry. The 

 greater the difference between the readings of the two, the smaller is 



