NATURAL HISTORY NINETEENTH CENTURY. 655 



While the ice is melting, the mercury is stationary at a point which is 

 marked by a scratch on the stem. The tube is then exposed to the 

 vapour of water boiling in a metallic vessel when the pressure is at a 

 certain standard (otherwise a correction is made). The apparatus 

 used is represented in Fig. 325. In this way the upper fixed point is 

 found and marked. The total internal capacity of the tube between 

 the two fixed points is then divided into a number of points of equal 

 capacity, and a dividing engine is used in marking the divisions in the 

 tube. 



There are several kinds of self-registering thermometers much used 



FIG. 326. 



FIG. 327. 



in meteorological observation. In Six's arrangement the tube is (J- 

 shaped, the bend of the U being occupied with mercury, separating 

 two portions of spirit. The readings are taken from the positions of 

 two small fragments of iron wire, which are moved by the motions of 

 the mercury, and are replaced, after the reading has been taken, by 

 means of a magnet. Rutherford's spirit thermometer, for registering 

 minimum temperature, is a well-known instrument. Casella's is 

 another ingenious instrument for the same purpose. Phillips, and 

 also Negretti, devised maximum registering thermometers, which are 

 much used in this country. 



Next to pressure and temperature, the most important point to be 

 ascertained as to the condition of the atmosphere is the amount of 



