30 WRITINGS OF JOSEPH HENRY. [1855- 



The next element given in the tables is the mean monthly 

 temperature. By comparing this with the average deduced 

 from a number of years' observations we are enabled to 

 ascertain the variations of each month from the normal 

 temperature of the same month as deduced from a series 

 of 3'ears, and to compare the temperature of the "growing" 

 portions of different years with each other. When experi- 

 ments shall have been made upon the amount and distri- 

 bution of heat necessary to give the best development to 

 particular plants, by a table of this kind we are enabled to 

 select the months best suited to their cultivation. More- 

 over, each plant requires a certain amount of heat for its 

 proper growth, though this amount may vary considerably 

 in intensity ; for example, a comparatively low degree of 

 heat may be compensated by its longer continuance. This 

 rule however is confined within certain limits ; for if the 

 temperature rises above a given degree, or falls below a par- 

 ticular point, the vitality of the plant may be destroyed. 

 B}'^ a well-conducted series of experiments and observations 

 the agriculturist may be enabled to determine, without a 

 ruinous series of actual trials, what plant may be safely culti- 

 vated in a given place. 



Besides the mean temperature, the extremes are also given, 

 and these are of essential importance in determining the 

 variations of temperature to which the plant is to be sub- 

 jected. The length of the growing summer in a given year, 

 and in a particular place, may for instance be measured by 

 the interval which occurs between two killing frosts. 



The next element in order, presented in the accompany- 

 ing tables, is that of the moisture; and this is of much im- 

 portance in judging of the productiveness of different years 

 and different places. Unfortunately however, comparatively 

 few observations are regularly made on the variations of 

 moisture in the atmosphere, in the United States. It is to 

 be hoped that our returns for another year will indicate an 

 increased number of the stations where valuable observa- 

 tions of this kind are taken. The figures in the tables do 

 not indicate the actual amount of water, for example, in a 



