1848.] Observations made on a Botanical Excursion. 365 



I have registered the black-bulb Thermometer at 1 50°, a temperature 

 and difference so little short of what has ever been observed in higher 

 latitudes that we must look to other causes than distance from the 

 Poles for the generally diminished power of the sun's rays in and near 

 the tropics. The low results cited by Daniel* were all obtained from 

 Pelagic stations, as are Capt. Campbell's, compared with my own; nor 

 have I on the tropical and sub-tropical coasts of Africa and S. America, 

 or on the ocean at a distance from land, ever obtained results at all to 

 be compared with these. It is much to be regretted that an instrument 

 so simple and easy of observation should be so neglected. The value 

 of its indications are approximate only, but not the less necessary, as 

 may be gathered from the circumstances of the few experiments I have 

 been enabled to make tending to invalidate a theory grounded on a 

 comparison of all the observations hitherto made in low latitudes. f 



* Meteorological Essays, Ed. 2. v. 2. p. 110. 



f Since writing the above I have met with a paper by the Rev. Mr. Everest 

 " On the Meteorology of Ghazipur :" in which a record is contained of observations 

 taken with a Thermometer laid on black wool and freely exposed to the sun in the 

 months of September and October. (As. Journ. 1833, p. 605.) The range of the ex- 

 posed Thermometer in these observations coincides very nearly with my own. The 

 maximum being attained at 11 a. m. and the greatest difference observed is also at 

 that hour (50°. 6). 



Dr. McLelland,* who has made some excellent analyses of the meteorological 

 phenomena of India, attributes the haze of the atmosphere during the N. W. winds 

 of this season, wholly to the suspended earthy particles. That such may be the case 

 to a great degree is clear, for the amount of the haze is evidently proportioned to 

 the force of the wind during the prevalence of the Diurnal breeze. But the haze 

 is always present, even in the calmest weather, when it is only to be accounted for 

 by the hygrometric state of the atmosphere. Extreme dryness, (which here is so 

 marked that there is no deposition of dew,) is in all parts of the world usually 

 accompanied by an obscure horizon. 



Capt. Campbell also objects to the conclusiveness of Dr. McLelland's theory, 

 citing those parts of Southern India which are least likely to be visited by dust 

 storms, as possessing an equally hazy atmosphere, and further denies its being in- 

 fluenced by the hygrometric state of the atmosphere. (Cal. Journ. Nat. His. 

 v. 2. p. 44). I have observed the same phenomenon in oceanic islands, when the 

 surface rocks were powerfully heated by a tropical sun, and the air extremely dry, 

 and I have further remained a brilliantly clear atmosphere with a similarly low Dew 

 point in the Antarctic Ocean, where the horizon was ice-bound : hence it is pro- 

 bably not so much the amount of vapor as its tension that determines the trans- 

 parency of the atmosphere. 



When on this subject I may add that even on the ocean the air is sometime so 

 brilliantly clear that Venus is visible at mid-day during a strong sun-light. I have 

 seen that planet in the north tropical Atlantic under similar circumstances to what 

 Dr. Campbell did at Kemedy, (Cal. Journ. Nat. His. v. 2, p. 279,) but have not 

 with me the date or corresponding observations. 



* Cal. Journ. Nat. His. v. 1, p, 52. 



3 c 2 



