44 



VETERINARY HYGIENE 



A Barograph is a self-recording barometer, the changes in the 

 barometric pressure with time being shown, in general, on a sheet 

 of paper attached to a revolving drum. The usual form of baro- 

 meter adapted as self-recording is the aneroid. 



The units by means of which barometric pressure is indicated 



FIG. 6. A Barograph. 



may be inches or centimetres of mercury or millibars, the pressure 

 unit on the C.G.S. system in use at the Meteorological Office. 

 (1 millibar = 1000 dynes per sq. cm. = -02953 mercury inch and 

 one-tenth of a millibar = -003 mercury inch and is the limit of 

 accuracy of a barometer under ordinary circumstances.) 



Barometric readings require to be corrected for temperature 

 and latitude, and, in order that readings may be compared one with 

 another, these should be corrected for altitude above sea level and 

 thus reduced to readings at the same level, viz., sea level. 



The term isobar refers to a line drawn through places which 

 have the same barometric reading at a given time. Thus, by means 

 of a series of isobars, regions of high and low pressures can be 

 identified, and valuable information can be derived by a study of 

 such a map. 



Low pressure regions are termed Cyclones or Depressions or 

 Lows, and those of high pressure are Anticyclones or Highs. In 

 anticyclones the weather, generally speaking, is usually fine, while 

 in cyclones it is the reverse. 



Eddy motion is also a factor of considerable moment in con- 

 nection with the mixing of air layers and production of cloud. 



CLOUDS. Clouds are characterised as "cloud sheets and cloud 

 heaps. The general distinction as regards appearance is whether 

 they form an extended horizontal sheet or not. As far as the mode 



