562 Bejport on the Magnetic Survey. [No. 6. 



doostan. It was during the first experiments covered with a 

 stratum of the sandy soil about one centimetre thick, but in the ex- 

 periments at Umballa a little hole was filled with mercury and 

 this covered with the stratum of soil. By this arrangement the 

 thermometer was kept through the medium of the mercury, in a 

 much more intimate contact with the particles of the soil, the tem- 

 perature of which it was to indicate. 



8 and 9. Metals exposed to a tropical sun feel remarkably hot 

 when touched, but this is due in a great measure to their conducting 

 power. Their real temperature seemed an interesting object for 

 direct determination. I therefore exposed two metal vessels, the 

 one an iron bottle, well screwed and filled with mercury to two- 

 thirds of its volume ; the other a flat square copper basin (used 

 generally as an artificial horison,) blackened, with about two cubic 

 centimetres of mercury distributed over it in isolated drops. Both 

 were put upon a soft and thick layer of cotton. In making the obser- 

 vation with the copper basin, the cotton was slightly pressed down 

 at one side, so as to incline the basin without the necessity of 

 touching it, causing the mercury to collect in one corner. The ther- 

 mometer was then dipped into the mercury, which it might be 

 expected had assumed the temperature of the metallic surface with 

 which it was in contact. 



The following Table contains the reading of the different ther- 

 mometers at Umballa ; the correction for index errors are applied to 

 the reading. The instruments with black bulbs and on black wool 

 were read with a telescope from 4 to 5 feet distance, since any near 

 approach to read them with a magnifier altered very suddenly the 

 indications of the thermometers. 



