THERMOMETER. 217 THERMOMETER. cooled. To this end the bulb and larger part of the surrounding objects, and especially reflected the stem of the thernionieter are immersed in a thin glass cup nearly full of sulphuric ether. A current of air is driven rapidly through the ether, causing it to evaporate and therefore to cool. The thermometer follows the falling tem- perature of the ether until the observer per sunshine by means of screens. Under these con- ditions the solar radiation thermometer, or the so-called Arago-Davy aclinometer may be used in two ways: (1) The static method. The ther- mometers are allowed to attain the highest tem- peratures they can in the full sunshine; the dif- ceives the deposition of dew on the outside of ference between their readings expresses approxi- the glass cup. The current of air is then cut mately the intensity of the solar radiation, the off. The ether and glass cup begin to warm; exact value of which may be more exactly com- the dew evaporates from the surface, and as puted by the method and formula! given by Pro- it disappears the temperature is again read off. fessor William Ferrel in his memoir "Tem- The mean of the two readings of the thermometer perature of the Atmosphere and Earth's Sur- face," pages 34-50, Professional Papers of the Signnl Service .Vo. 13 (Washington, 1884). (2) The dynamic method. The bright and black bulb thermometers are successively exposed to the full sunshine and shaded from it a minute at a time, so that one may determine the initial rate of in- corresponds closely to the dew point. See Hy- drometer. The Wet Bulb Thermometer. This is an ordinary thermometer whose bulb is covered very neatly with the thinnest muslin which is kept wet. The wet bulb thermometer should either be rapidly whirled in the air or else exposed crease of temperature in the sunshine and the to a current of air at the rate of five or ten rate of cooling in the shade. From these rates feet a second. The evaporation of the water the solar radiation is calculated by the formulae cools the thermometer to the temperature of first applied by Pouillet to his actinometer. evaporation, which is about half way between the temperature of the air and the temperature of the dew point. The combination of a dry bulb and a wet bulb thprmometer constitutes a psy- chrometer, and the whirled or ventilated psy- chrometer is the ordinary apparatus used by meteorologists for determining the humidity of the atmosphere. The formulae and tables used by the Weather Bureau in this work are those prepared by Professor C. F. ^larvin and pub- lished as Weather Buieau Document Xo. 2.35 (Washington, 1900). See Hygrometer for il- lustration and further description. The Alcohol Thermometer. This differs from the mercurial thermometer only in that the in- terior liquid is alcohol. But this difference Is important from many points of view, since one class of errors disappears and another is intro- duced. Alcohol or ether must be used for the measurement of very low temperatures, since mercury freezes at about — 40° Fahrenheit. The principal error in using the alcohol ther- mometer for loAV temperatures arises from the fact that the liquid becomes viscid, adheres to the sides of the glass tube, and requires a long time to drain down, which indeed it never does per- fectly. The alcohol thermometer cannot be used for temperatures above its boiling point or 80° Centigrade, whereas the mercurial can be used up to a temperature of about 350° Centigrade. SoLAK Radiation Therjiometer, or Black Bulb Thermometer. This is essentially an or- dinary mercurial thermometer, whose bulb is bottom of the seale so that the mercury that has covered with a thin layer of non-reflecting and ptjshed up past this point cannot easily slide almost perfectly absorbing lampblack. Under back. After the maximum temperature has been U. S. WEATHER BUREAr MAXIMUM AND MINIMUM THERMOMETER. The Maximum Thermometer. This is usually a mercurial thermometer having a device by which the top of tlie column of mercury remains at the highest point that it attains during any given interval of time until the observer has recorded the reading and adjusts the instrument for another observation. In the Rutherford maximum the stem of the thermometer lies near- ly horizontal. The rising mercury pushes a little steel index upward within the bore of the tube. When the temperature falls the index is left in place until the observer draws it back to the top of the mercurial column by means of a magnet. In the Phillips or Walferdin ther- mometer the thermometer stem is horizontal. A small bubble of air makes a permanent break in the upper part of the mercurial column. When the temperature begins to fall the lower column contracts into the bulb, but the upper portion re- mains and registers the maximum temperature. The Negretti and Zambra maximum thermometer has the bore of the tube constricted below the the assumption that the black bulb thermometer absorbs and radiates more easily than the plain glass bulb, and especially more so than a polished silver bulb, the difference between the readings of two such thermometers exposed to the sunshine will depend primarily upon the intensity of the radiation received by them. In order to meas- ure the solar radiation, it is necessary, first, to diminish as much as possible the conduction and convection of heat to or from thermometer bulbs. They are, therefore, inclosed in large thin glass bulbs within which there is as perfect a vacuum as it is possible to make. It is also necessary recorded the observer forces the column of mer- cury back by whirling the thermometer in a circle. The clinical thermometers of physicians are made after this pattern. The Minimum Thermometer. The best form of this is the Rutherford alcohol minimum, in which the stem is kept nearly horizontal, and the contracting column of alcohol draws down- ward a light steel or glass index, which remains in place when the temperature rises. The ob- server sets the index anew by drawing it back bv means of a magnet. Combined Maximum and Minimum ob Six'.s to cut off as much as possible of radiation from Thermometer. This consists of a U-shaped tube
