" Meteoroho^k al Observations. 145 



hucketfull of the water and sUrring^ round the thermometer in it. When- 

 ever a change to the extent of 2° Fahr. appears to have taken place since the 

 last observation, a fresh hucketfull should be taken up and the observation 

 repeated. It should also be noticed whether rain has fallen since the last ob- 

 servation. A sudden depression of 3° or 4° indicates the near approach of 

 land. In a voyage from England, lately made by a member of this commit" 

 tee, the temperature of the surface water fell at once it" Fahr. on approaching 

 within a few miles of the entrance of Table Bay. 



The temperature of the sea at considerable depths can hardly be regarded 

 as a subject of ordinary meteorological inquiry and regular registry, though 

 undoubtedly one of much physical interest, for which reason it is not consi- 

 dered necessary to dwell further on it. 



-Of the Hygrometer^ ^c — In the absence of Daniell's Hygrometer, or of 

 ether to cool it, the degree of dryness of the air may be ascertained by ob- 

 serving the temperatures marked by two thermometers suspended freely side 

 by side (but not in contact) in the shade, and completely defended from all 

 radiation to or from the sky^ihe one having its bulb and stem naked, the other 

 with the bulb and lower part of the stem wrapped in linen or cotton, and 

 thoroughly wetted with pure spring or rain water. The temperatures indi- 

 cated by both should be noted when the wetted thermometer refuses to sink 

 lower, and the conclusions left for subsequent calculations. The naked ther- 

 mometer may be the " External Thermometer" itself, in which case a coated 

 thermometer 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 hygrometer be used, its points of 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 blot-i 

 ting 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 quicklime, till it refuses to indicate a 

 higher degree of dryness. 



The best measure oi the momentary evaporating power oi the KXXt seems to be the 

 depression of the wetted thermometer below the dry one. But the actual eva- 

 poration from a given surface, is quite another thing, and a question may very 

 reasonably be raised, how far any useful approximation to a knowledge of the 

 total evaporation from an extensive and diversified surface, unequally moist- 

 ened, and variously exposed to the sun, defended by clouds, or refreshed by 

 dews, can be obtained by any small or local experiments. 



The llain-gauge is an instrument of such extremely easy construction 

 that any person who lives near a tin-man can procure one. In 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 disuse in which insects 

 and dust may lodge. It will often happen, too, that the slight rain of one 

 day, if left unregistered, will be entirely lost by evaporation in the next — 

 nay, that slight and transient showers may never enter it, being evaporated 

 from it as they fall. The effect of copious dew, too, must be separated from 

 that of rain, so that the mere registry of the contents of the gauge is not oC 



VOL. XXI. NO. XLI. — JULY 1836. K 



