THE MERCURIAL THERMOMETER. This consists of a glass bulb terminating in a long capillary tube. The bulb and lower portion of the tube are filled with clean mercury, while the upper portion is supposed to be quite empty and to be hermet ically sealed. When the bulb is warmed the mercury is seen to rise in the tube, because it expands more than the glass of the bulb. This difference of expansion is not regular, nor is it the same for all kinds of glass or for impure mercury. If we wish to construct an independ ent standard mercurial thermometer, we im merse the bulb in melting ice and make a slight scratch on the glass tube to mark the height of the mercurial column. A similar mark is made for boiling water and the space between the two is divided into a hundred equal parts for centigrade degrees, or a hundred and eighty equal parts for Fahrenheit degrees. This intermediate space is called the fundamental distance. Owing to the irregular expansion of mercury in glass, the so-called standard mercurial thermometer will differ from the standard air thermometer by as much as half a degree about midway be tween freezing and boiling water. Therefore for accurate work the mercurial should be carefully compared with the air thermometer and the sulting system of corrections throughout the en tire length of the scale should be well determined and carefully applied. Owing to the gradual contraction of the glass bulbs, which goes on for several years after they are freshly made, the mercurial thermometer usually has an error that increaAes with the lapse of time. This error
is due to the change in the volume of the bulb, and should be determined after any important work has been done by immediately making a new determination of the freezing point. In stead of this procedure the error can be cal culated theoretically, but the actual new deter mination is far preferable.
A thermometer is not considered first class whose errors of division exceed one or two tenths of a degree centigrade. or two or three tenths Fahrenheit. In the temperature of a liquid by the use of the mercurial thermom eter one must. be very careful to keep the liquid thoroughly stirred, because when permitted to rest the colder portions of the liquid settle. In meteorological work all thermometers are ex posed in some sort of protecting cage through which the wind may blow freely, but into which the sun's rays, or any obnoxious radiation, can not penetrate. It is believed that after making due allowance for the sluggishness of the ther mometers and the inefficiency of the screens and the ventilating apparatus, it is still true that the various meteorological services of the world obtain the temperature of the air within a half degree Fahrenheit or two-tenths Centigrade. A general comparison between the Centigrade and Fahrenheit scales is given in the accompanying table: