THERMOMETER. 216 THERMOMETER. mercurial column is measured by the difference in the level of the mercury in the right-hand and left-hand tubes. The scale for this meas- urement is graduated on a strip of mirror set into the vertical framework that supports the bulb and the tubes. i v///,y/:////////M W//M Fig. 2. chappuis air thermometer. In Fig. 2 we see an arrangement of the air thermometer for more exact measurements, etc. The thermometric substance (nitrogen) is in- closed in the platinum bulb A, which is sup- ported inside of a bath, B, which is so ar- ranged that the hot gas or liquid whose tem- perature is to be measured may completely sur- round the bulb by flowing in at E and out at F. D D are metal screens. There are a number of mercurial thermometers, T, which are at the same tempcratvire as the bulb. The bulb commu- nicates with the barometer at G by the tube C, the connection being shown enlarged at V. The pressure of the mercurial column needed to keep the gas in the bulb at a constant volume is that due to the height of the column from X to Y, and may amount to several atmospheres, or to a height of many feet. The lower end of the mer- curial column, or the tube H, I, J, K, is fixed, but the upper tube, P Q, can be raised indefi- nitel}'. As Q rises inside of the large tube, J, more mercury may be needed, and this is sup- plied from the reservoir, E. through the flexible tube, L. The difference in height between the mercurial surfaces at X and Z is measured by a reading telescope or cathetometer. JI is the upright standard carrying the apparatus. N a movable cradle for the I)arometer tube worked by the adjusting screw O. S is a similar ar- rangement for the reservoir, R, with its adjust- ing screw, U. The Mercubiai 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 himdred 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 nnich as lialf 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 re- sulting system of corrections throughout the en- tire Icngtli of the scale should be well determined and carefully applied. Owing to the gradual contraction of the glass bulbs, which goes on for several 3-ears after they are freshly made, the mercurial thermometer usually has an error that increases with the lapse of time. This error is due to tile change in tlie 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 determining the temperature of a liquid by the use of the mercurial thermom- eter one nuist be very careful to keep the liquid thoroughly stirred, becaiise 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 sluggi-shness 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 : Cent. Fahr. 212 167 -|-125'> 100 75 60 -f 25
— 17.8 — 25 — 40 —273 122 -4- 77 -j- 32
— 13 - 40 —491 The Dew Point Thermometer. This is an ordinary thermometer specially arranged for the determination of the temperature at which dew is formed in the atmosphere when the air is
