40 AGRICULTURAL TEXT-BOOK. 



lines ; the second depends on local circumstances, such as moun 

 tains, table lands, large forests, lakes, &c., which affect the dis 

 tribution of heat and ram; and which may render the local 

 climate better or worse than the general one. In the United 

 States, the local climate is found to change as the country is 

 settled and cleared; and it often varies sufficiently, within a 

 comparatively small area, to influence the sort of crops to be 

 grown, and the stock to be kept 



Iso-thcrmal lines are lines of equal temperature ; a term applied to 

 imaginary lines drawn on a map connecting all those places on the sur 

 face of the globe which have the same mean temperature. Lines drawn 

 through places having the same summer temperature, are termed Iso- 

 theral; those drawn through places having the same winter temperature, 

 are called Iso cheimal; while lines drawn through places having other 

 common temperatures, receive other appropriate names. Fully to ex 

 plain this to the student, it will bo necessary for the teacher to exhibit a 

 physical atlas of the globe, where the various lines are laid down. 



126. The climate may &quot;be practically changed, to a certain 

 extent, by the farmer, by means of cutting down forests, drain 

 ing, deep plowing, high walls, and plantations of trees ; and by 

 the gardener, to any extent, by means of hot-beds, hot-walls, 

 glass-houses, manures, &c. 



127. The following changes take place, owing to the culti 

 vation of a new country : (a.) Fogs disappear, and with them 

 agues and other diseases ; sometimes naturally ; almost always, 

 by a proper and sufficiently extensive system of drainage. * 

 (Brown & Dickeson, Prof. Johnston.) (b.) Dews are less 

 heavy ; the quantity of running water is diminished ; lakes and 

 marshes dry up ; hail-storms become more frequent ; early and 

 late frosts are more injurious to vegetation. (Bomsingault.) 



128. On the contrary, by neglect* a previously healthy conn- 



* A very striking; instance of the improvement of health, and the ceasing of mias 

 matic diseases throughout a whole township, in consequence of draining, in Beverly, 

 Mass., is given in the Abstract from the Returns of Jgricul. Soc. in Mau., for 1846, 



