with Geographic Botany. — Humidity. 319 



of one solitary observed fact, for a variety of situations where 

 the progression is by no means regular ; it can only be re- 

 garded as an approximation, in the absence of regular obser- 

 vations. Several remarks might be elicited by it, but it will 

 be sufficient to place by its side a few observed results on the 

 same subject. 



At Cumana Lat. X. 10° 28' the annual evaporation is 100 inches. 



Guadaloupe ,, 15 39 ,, 97 „ 



Toulon , 43 7 „ 40 „ 



Paris „ 48 50 „ 32 „ 



London ... „ 51 31 „ 24 ,, 



I am especially desirous of drawing a line between the 

 theoretical deductions of the closet and the real practical 

 results of observations. Without disparaging the labours of 

 those who have devoted their time and abilities to the construc- 

 tion of theoretical tables, we must add, that a close adherence 

 to them is more likely to lead to false than correct conclusions. 

 There is a "fatal facility" about the formation of some of them 

 in leading us to important conclusions, that it is not surpri- 

 sing we have tables for mean temperatures, and the annual 

 amounts of evaporation, rain, &c., for every latitude from the 

 equator to the poles, in many of which we are greatly deficient 

 in any practical observations whatever. The use of them con- 

 sists in offering approximations to what is the real condition 

 of the subject when observations are wanting, and this is their 

 chief advantage ; no modifying circumstances are taken into 

 consideration, yet in the case of rain we can select a parallel 

 of latitude where in one part of it rain never falls, and in 

 another a dry day is an unusual luxury. It cannot even be 

 allowed that some of these tables furnish a correct mean for 

 any given latitude, after setting aside all those circumstances 

 which are continually combating a regular progression. If 

 they did so far unfold circumstances they would prove of the 

 greatest possible use, as henceforth every modifying agent 

 could have its proper value applied. 



In England some pains have been taken to estimate the 

 amount of evaporation during the different months of the year, 

 and to discover their several evaporating powers. Mr. Hoyle 

 and Mr. Dalton used a method which is perhaps as little objec- 

 tionable as any for this purpose is hkely to be ; it is thus de- 

 scribed : — " A cylindrical vessel of tinned iron, ten inches in 

 diameter and three feet deep, having tubes soldered to it for 

 conveying off" into bottles the water which is received, was 

 buried in the ground in an open situation, and then filled 

 with gravel, sand and soil ; the whole being covered w ith grass 

 and other vegetables, it was allowed to receive the rain, and 



