356 Observations made on a Botanical Excursion. [Oct. 



For the wet-bulb observations, distilled water was invariably employ- 

 ed ; and the minimum temperature taken, which is not indicated if the 

 bulb be loaded with water, as is too often the case. 



The observations for nocturnal radiation are not so accurate as if a 

 parabolic reflector were used ; they are however sufficiently demon- 

 strative of the state of the atmosphere. 



Those taken by exposing a naked thermometer on a non-radiating 

 substance, removed from the surface of the earth, as the top of a 

 broad brimmed Sholahat (the bulb quite free) may I think, be depend- 

 ed upon. 



Those again indicative of the radiation from grass, whether dewed or 

 dry, are not strictly comparable ; not only does the power of radiation 

 vary with the species, but much more with the luxuriance and length 

 of the blades, with the situation, whether on a plane surface or raised, 

 and with the soil upon which it grows. Of the great effect of the 

 surrounding and subjacent soil I had frequent instances ; similar tufts of 

 the same species of grass, radiating more powerfully on the dry sandy 

 bed of the Soane, than on the alluvium on its banks ; the exposure 

 being equal in both instances. 



Experiments for the surface Temperature of the soil itself, are least 

 satisfactory of any : — adjoining localities being no less affected by the 

 nature, than by the state of disintegration of the surface, and amount 

 of vegetation in proximity to the Instrument. 



Such observations however are not useless : the mean of a number 

 taken synchronously with those for the Temperature of grass and 

 for free radiation, affording valuable results, especially if compared 

 with the power of absorption by the same soil of the sun's heat during 

 the day. 



The power of the sun's rays is so considerable, and protracted 

 through so long a period of the day, that I have not found the temper- 

 ature of running water, even in large deep streams, so constant as was 

 to be expected. 



On a few occasions the temperature of the soil at considerable depths 

 was obtained by sinking holes. My daily progression and the exceed- 

 ing hardness of the baked alluvial soil, prevented this being fully 

 accomplished, except on a few occasions, and as connected with the 

 Register the observations will be detailed. 



