18 Report on the Magnetic Survey. [No. 1. 



their turn came to be weighed ; this quantity was ascertained by- 

 weighing them a second time, and putting the loss thus ascertained, 

 during a given difference of time, proportional to the time which 

 elapsed between the moment when they were brought into the room 

 and the moment when their turn for being weighed came. 

 The corrections thus obtained are : — 



grammes. 



For No. 2.— Black wool weighed first, 0.000 



3.— White wool, 0.079 



4.— Black earth, 0.012 



5.— Quartz sand, , 0.020 



6.— Grass, 0.310 



Dark grey slate could not be managed in the same way, the water 

 being taken off by blotting. The loss may be considered as in- 

 appreciable, the water forming well defined drops not extended by 

 capillarity over so large a surface as in the other substances. 

 The corrected increase of weight is therefore ; — 



Difference be- Correction Correction Real 



tween 1st & 2nd for absorp- for evapo- increase 



Weighing, tion gram- ration grammes, 



grammes. mes. grammes. 



For No. 2.— Black wool,. . . . 4.019 — 0.174 0.000 3.845 



3.— "White wool, .. 3.791 —0.174 +0.079 3.696 



4.— Black earth, .. 2.211 —0.174 +0.012 2.049 



5.— Quartz sand, .. 1.965 —0.174 +0.020 1.711 



6.— Grass, 2.631 —0.174 +0.310 2.767 



7.— Dark grey slate, 0.904 0.904 



To facilitate the conversion of the weights in vertical heights 

 of the stratum of water deposited in the form of dew, the boxes 

 containing the different substances, as well as the stones, were made 

 as nearly equal to a square decimeter as we could, but the moisture 

 allowed the boxes to extend their edges, and it was not possible to 

 mark the stones with sufficient accuracy without a useless waste of 

 time, the real surfaces had therefore to be ascertained after the 

 experiments. This was done by putting them on a paper covered 

 with square centimeters and square millimeters, tracing the upper 

 contour line, and counting the number of little squares thus en- 

 closed. 



