48 VETERINARY HYGIENE 



governed by the distribution of pressure " (Meteorological 

 Glossary). 



Monsoons. These are chiefly manifest in India and China. 

 Their direction is reversed with the seasons; in summer they are 

 south-west and in winter north-east. They are of great importance 

 from an economic and climatic point of view in the regions over 

 which they blow. 



ATMOSPHERIC ELECTRICITY AND LIGHTNING. Franklin by fly- 

 ing a kite with an iron point was able to get large sparks of electri- 

 city when the string was sufficiently wet to become conducting. He 

 thus showed that there exists electricity in the atmosphere. During 

 a thunderstorm the effects may be very great and we may have a 

 discharge between two clouds or between a cloud and the earth or 

 an earth-connected body. 



There exists normally in the atmosphere a difference of potential 

 between two points at different vertical distances apart, and the 

 upper point is usually at the higher potential. 



In certain regions, however, and under abnormal conditions in 

 any particular region the higher point may be at the lower potential. 



In Edinburgh the average value of the potential gradient during 

 1912 was 167 volts per metre and the direction is usually reversed 

 during bad weather, e.g., a rainstorm, as is the case at most stations. 

 This gradient has diurnal and annual variations. 



A lightning conductor, which acts as a protection for a building, 

 consists of a metal point situated above the building and connected 

 by a metallic conductor to a " good earth." It is better to have all 

 the exposed points protected by rods which are connected to several 

 " good earths." The ideal system is to have the building covered 

 with a wire mesh-work electrically connected to earth. 



Exposed country is a more dangerous place to be in than a town 

 during a thunderstorm. A tree by itself is more likely to be struck 

 than any given one forming part of a wood, and trees differ, 

 according to kind, in the frequency with which they are struck. 

 Oak trees are very dangerous. The order for a number of different 

 trees as regards danger (beginning with the most dangerous) is 

 oak, Scotch pine, spruce, beech. 



WEATHER FORECAST. If the values of the various meteoro- 

 logical phenomena from different regions at a given time are plotted 

 on a map of these regions then it is possible to forecast the weather. 

 This map is known as a " weather map " or " synoptic chart," 

 and the Meteorological Office in general forecasts for the future 

 24 hours, whilst general conditions of the weather may be given 

 for a longer period. The data are collected by a central office and 



