MANUAL OF AGRICULTURE. 259 



temperature is measured and indicated by the thermometer, 

 which contains mercury or fluid in a closed glass tube, from which 

 the atmospheric air has been extracted. Its contents expand and 

 contract under the influence of heat and cold ; and the amount of 

 these respectively is indicated by the standard scale of degrees 

 according to which the instrument is graduated. They are also 

 constructed with an arrangement for registering the maximum 

 amount of cold w^hich has been reached during any fixed period. 

 The hygrometer indicates the amount of moisture contained in 

 the atmosphere immediately surrounding it ; whilst the hygro- 

 scope again merely indicates its presence. Anemometers are 

 instruments used for measuring the amount of the wind's force 

 and velocity. As exhibiting the influence of temperature upon 

 the distribution of plant life in the earth, we find the face of the 

 globe in physical atlases, from the equator to the pole, roughly 

 divided into eight isothermal zones, with characteristic plants^, as 

 follows : — 



1st, The region of palms and bananas— Equatorial zone— equator to lat, 15° from max. temp, to 78* 



The same applies to the vertical isothermal lines, i.e., those 

 indicative of similarity of temperature in distance above sea-leveL 

 We can see the effects of temperature on the choice of cultivated 

 plants exemplified within the limited area of our own country, 

 in the presence and absence of certain field crops, as we travel 

 from the southern to the northern extremity of our island. The 

 important bearing which the science of meteorology brings more 

 immediately upon agricultural practice has until recently been 

 almost entirely overlooked ; but now its principles, and their 

 connection with the flourishing of farm crops and the welfare of 

 live-stock, are more entirely appreciated. And there is all the 

 more reason for this, from the peculiar situation of our kingdom 

 subjecting it to numerous, extreme, and sudden changes of 

 weather. The farmer can now, at the same time, more success- 

 fully combat with these, through the agency of the widely-dis- 

 seminated weather-charts and forecasts, the fruits of the system 

 so admirably organised by the Government department of the 

 Board of Trade. 



Cn AFTER VTI. 



Of the Lcadinfj Scientific lYiiiciplcs of the Art of Agriculture. 



The perusal of the foregoing chapters will have prepared the 

 student for tracing the relation of the various physical sciences 



