3S2 Instructions for Making and Registering 



pliv^icnl interest, for which reason it is not considered necessary 

 to dvveil furtliar on it. 



Of the Hygrometer, Sfc. 



Ii the ab'^ence of Diitiiolls' Hvfrronicter, or of ether to cool it, 

 thi! dcgiec f)l dryness of the air may be asccrlinned by obscrvitiff 

 the (eniporaturcs marked by two thcrmometeis, btispoiided 

 freelv side i)y side, (but not in contact) in the shade, and com- 

 plct(.'ly defended from all radiation to or from tiie sky^ tiie one 

 hiving its bulb and stem naked, the other with the bulb and 

 Itiwer part of the stem wrapped in linen or cotton, and 

 thoroua;hly wetted with ))ure sprino- or rain water. Tlic tem- 

 peratures indicated by b»)th, should be noted when the wetted 

 thermometer lefuses to sink lower, and the conclusions left for 

 subsequent calculations. The naked thermometer may be the 

 " External Thermometer " itself, in which case a coated tlier- 

 moraeter may be kept always suspended near it, completely 

 screened as above mentioned, and wetted some minutes previous 

 to the regular daily readings. 



If a hair Hyij^rometer be used, its points oi absolute moisture 

 and dryness should be frequently ascertained, as they are apt 

 to change The former may be found by keeping it some time 

 in a close covered jar lined with wet blotting paper, and having 

 water in it, and noting the point of moisture beyond which it 

 refuses to go. The latter, by keeping it in the same manner in 

 a jar perfectly air-tight, over fresh burnt quick-lime, till it 

 refuses In indicate a higher degree of dryness. 



The best measure of the momentary evuporativi/ poiver of 

 th'> air, seems to be the depression of the wetted thermometer 

 below the dry one. But the mtunl evaporat on from a given 

 surface, is quite another thing, and a question may very reason- 

 ably be raised, how far any useful approximation to a knowledge 

 of the total evaporation from an extensive and diversified 

 surl'ace, unequally moistened, and variously exposed to the sun, 

 d' fended by clouds, or refreshed by dews, tan be obtained by 

 any small or local experiments. 



Tiie Rain Guagc is an instrument of such extremely easy 

 construction, that any person who li\cs near a tin-man can 

 procure one. Iii a climate so arid as that of Africa, however, 

 it must be remembered that it will often need examination and 

 cleansing, owing to long intervals of disiise in whi;;h insects and 

 dust may lodge. It will often happen, too, that the ilight rain 

 of ono day, if left unregistered, will be entirely lost by evapora- 

 tion in the next --nay, that slight and transient showers may 

 never enter ii, being evaporated from it as they fall. The 

 effect of copious dew, too, must be separated from that of rain, 

 60 that the mere registry of the contents of the guage is not of 



