654 



HISTORY OF SCIENCE. 



In the northern hemisphere the barometric pressure is less on the left of 

 the wind's course than on the right. In the southern hemisphere the 

 pressure is less on the right of the wind than on the left. 



The observation of temperature is important to the meteorologist, 

 and the improvements which in modern times have been made in the 

 construction of thermometers have greatly facilitated the obtaining of 

 correct data. There are some differences of opinion and of practice 

 as to the proper conditions for finding the true temperature by a ther- 

 mometer. The results are in general liable to be complicated with 



FIG. 324. 



FIG. 325. 



effects of radiation. As such questions do not here concern us, we 

 pass them over to consider the principal improvement in all classes of 

 thermometers. This consists in engraving the scale on the glass tube 

 itself. For good instruments tubes are selected which have been filled 

 many years before. The bore of the tube is gauged by causing a short 

 column of the mercury to occupy various parts of its length succes- 

 sively. From the length of the little mercury column the capacity of 

 tube is noted, and the corresponding allowance is made in the gradu- 

 ation. After the tube has been filled with the mercury and sealed, 

 the first step is to immerse it in powdered ice, as shown in Fig. 324. 



