98 



PHYSIOGRAPHY. 



[chap. 



cannot therefore be compared together until we know 

 at what elevations the instruments are situated ; one observer 

 may live on high ground and another on low, one may have 

 his barometer up stairs and another down stairs. Hence it 

 has come to be understood that all barometric readings 

 shall be reduced to what they would be if the instrument 

 were at the sea-level, which gives, of course, a fixed datum 

 line. Then, again, the barometer needs correction for 

 temperature. Mercury, in common with other liquids, 

 expands by heat, and expands much* more than the glass 

 tube which holds it ; hence the barometer will rise on a 

 hot day and fall on a cold day, although the atmospheric 

 pressure may not have changed. It is essential therefore 

 that all barometric readings should be reduced to the same 

 temperature, and the standard taken for this purpose is the 

 freezing-point of water, or 32° of Fahrenheit's thermometer. 

 All the figures given in the newspaper charts are conse- 

 quently reduced to sea-level and to 32°. 



The Standard, instead of giving diagrams or charts, 

 publishes reports which, in addition to barometric readings, 

 convey to the reader a good deal of useful information 

 relating to the weather. This will be seen in the following 

 example extracted from to-day's issue (March 31st, 1877) : — 



Date. 



Mar. 



25 

 26 

 27 

 28 

 29 

 30 



*»* At Two A.M. the Barometer had risen to 30-17. 



