384 



Observations made on a Botanical Excursion. 



[Oct. 



Australian or African desert to a miniature. Its surface in the day is 

 heated above that of the neighbouring country, at night cooled below it. 

 The stars appeared to twinkle more clearly on its banks, and I thought 

 I could during the early morning detect a current of air flowing from 

 its cooled atmosphere to that surrounding the warmer alluvial plains. 

 Rhamnece, Carissa, Olax, Acacia, Menispermun and a tall stiff and dry 

 Malva, formed the pervailing vegetation, with Cuscuta, Cassytha, a few 

 Asclepiadece and withered grass. Though this is the coldest season, 

 the sand was heated to 110° and upwards where sheltered from the 

 wind, and to 104° on the broad bed of the river. 



To compare the rapidity and depth to which the heat is communicat- 

 ed by pure sand, and by the tough alluvium, I took the temperature at 

 some inches depth in both. The mean of a good many observations at 

 different holes, gave the following differences between the temperature 

 of a column of sand in situ 16 inches thick, at 2 p.m. and 5 a.m. 

 the following morning. 



Feb. 14th 2 p. m. 



15th, 5 a 



Air in shade, 81° 



62 



Surface, 108 



43 



l^inch, .. 100 



50 



3± „ 85 



57 



6 „ 73 



67 



16*.... 72* 



68 



* Sand wet at this depth. 





M 



Diff. 



18° 

 64.5 

 50 

 28 



6 



4 



Maximum of black-bulb 

 therm, during the day 126°. 

 Min. of radiation at 5 a. m. 

 from a naked bulb therm. 

 48.2. (exposed over the sand). 



That the alluvium both conducts the heat better, and retains it longer, 

 would appear from the following, the only observations I could make 

 owing to the tenacity of the soil.* 



Hard alluvial bank of river. 



2 p. m. Surface 104°. 

 2^inch, 93°. 



5 „ 88°. Sand at this depth, 78°. 

 5 a. m. Surface 51°. 

 28 inches, 68°.5. 



* The plan I adopted was suddenly to remove a large clod of alluvium and insert 

 a very small thermometer bulb into a perpendicular side of the hole thus made. I 

 should be glad that any one could suggest to me a better method, feasible for a tra- 

 veller. The increment or decrement of heat is so rapid for a few inches below the 

 surface as to render its determination with any accuracy very difficult. 



