IV. CLIMATE. 163 



above the surface of the ground, must be under the joint 

 influence of atmospheric and terrestrial temperature ; and 

 the rate of one degree of Fahrenheit's scale for one hun- 

 dred yards of ascent, may be held at present to represent 

 nearly such a mean. Suppose it to differ so much as 

 one-tenth from the true rate ; that the rate of decrease up- 

 wards should be one degree for 90 yards or for 110 yards 

 instead. Such a variation of one-tenth from the assumed 

 standard, either way, would be less than the variations 

 which arise at equal levels from local conditions ; for 

 instance, from the humidity or dryness of the ground, the 

 configuration of its surface, its inclination to or from the 

 sun, &c. 



Hitherto, the thermometer of Fahrenheit has been 

 alone looked to, in thus attempting to connect tempera- 

 ture with altitude ; and the subdivision of the degrees 

 marked on its scale lias been made only into the halves 

 and quarters, as being more familiar to English readers. 

 In what manner will it be most easy to substitute the 

 centigrade scale for that of Fahrenheit ? Fortunately, 

 the former will accord better with the supposed rate of 

 decrease in atmospheric temperature, than the scale of 

 Fahrenheit. By allowing half a degree of the centigrade 

 thermometer for each hundred yards of ascent, a rate of 

 decreasing temperature is formed, which differs about 

 one-tenth from that adaj)ted to the thermometer of 

 Fahrenheit. It will so represent a slower rate of de- 

 crease, and be more closely in accordance with several of 

 the figures before given ; those of the balloon ascents 

 excepted. By resorting to the decimal subdivisions of 

 the centigrade scale, we obtain a very facile mode of 

 applying the proposed rate to the altitudes of plants ; 

 because one-tenth of a degree will then correspond with 

 twentj' yards of ascent ; one-hundredth of a degree with 



