1856.] Report on the Magnetic Survey. 17 



No. 3. — White wool (very fine white colour). 



No* 4. — Black vegetable earth (as formed naturally in little con- 

 cavities of the rocks, not quite black, a little reddish). 



No. 5. — Quartz sand from a river, formed of decomposed sand- 

 stone rocks. 



No. 6. — Short grass, imitating the natural surface in the flat of 

 Cherra by being cut off and arranged in the box points upwards. 



No. 7. — Dark grey slate from Myrung with a very uniform smooth 

 surface. 



These substances were exposed in the following way. A double 

 stratum of light bamboo mats was spread over short grass ; length 

 of the mats 25 meters, breadth 1.4 meters. The paper boxes were 

 disposed so as to have the greatest possible distance from each other 

 and from the borders of the mats. 



The empty paper-box, weighed at the very beginning and at the 

 very end, was during the night protected from radiation and dew by 

 being placed on the grass, supported by a large cake of wax, and 

 covered by a bamboo umbrella with a stick of 0.4 meter height. 



In this way we obtained the changes of weight of the paper cases, 

 produced by absorption during the night and evaporation during the 

 stay in the room. 



Weights. 



Increase 



of 

 Weights 



in 

 Grammes. 



1. — Empty box weighed first and last for giving a correc- 

 tion for the absorption of moisture by the paper, . . . , 0.174 



2.— Black wool, 4.019 



3.— White wool, , 3.791 



4.— Black vegetable earth, 2.211 



5. — Quartz Sand, 1.965 



6. — Grass, „ , 2.631 



7. — Dark grey Slate, 0.904 



The change of weight in the test paper box having been 0.174 

 grammes, this amount is to be deducted from all the substances 

 from No. 2 to No. 6 inclusive. Besides this, the weights of the two 

 kinds of wool and the grass must be corrected for a small, but appre- 

 ciable, quantity of moisture lost during the stay in the room before 



D 



