-1859] WRITINGS OF JOSEPH HENRY. 183 



the northern hemisphere. From all the observations Pouil- 

 let gives the following deductions : 



1. The diurnal variations are not perceptible at depths 

 greater than about 40 inches. 



2. The mean annual temperature of the different strata 

 dififers little from the mean annual temperature of the air. 



3. The differences between the maxima and minima of 

 the different strata decrease in a geometrical progression, 

 while the depths increase in an arithmetical progression. 



4. From all the observations it appears that at a depth 

 of from 26 to 29 feet, the annual variation is only 1°'8 F ; at 

 from 49 to 52 feet, it is but 0°"18 F. ; and at a depth of from 

 65 to 81 feet, it becomes only 0°-02 F. 



5. At the depth of about 26 feet, or where the variation is 

 2° F., the seasons are precisely reversed ; that is, the maxi- 

 mum temperature occurs about the 1st of January, and the 

 minimum about the end of June. 



Effect vf heat on plants. — We have stated that all the trans- 

 formations of matter going on around us, the power exhib- 

 ited in the growth of the plants, in the functions and motions 

 of animals as well as in the winds, — are referable to im- 

 pulses received from the sun; but the mere continuance of 

 the heat of a body at a certain temperature does not produce 

 a continuous change in it; for example, a piece of metal, 

 when kept at the same temperature, may remain unchanged 

 for years, provided the intensity of the heat is not sufficient 

 to melt it. In order therefore that heat may do work, or 

 effect a permanent change in matter, it is necessary that it 

 be applied by means of some mechanical arrangement anal- 

 ogous to a machine. In most cases, an intermediate agent 

 (such as steam or heated air) is employed in connection with 

 the machinery, and we have a striking natural arrangement 

 of this kind in the organization of the plant. If the stem of 

 a plant were solid, and did not consist of minute cells filled 

 with evaporable liquid, the heat of the atmosphere, so long 

 as it were constant, could produce no change. To under- 

 stand this, let us suppose a tube of glass with a minute bore 

 (for instance the tube of a broken thermometer) to have 



