76 Intelligence and Miscellaneous Articles. 



we may conclude from it, with sufficient certainty, that the extreme 

 limit could not have been more than 20 metres lower. Moreover, 

 the experiments made in the Lake of Geneva have shown that 

 the dispersion of sunlight by a light layer of clouds does not cause 

 any considerable diminution in the depth to which it can penetrate 

 in water. 



We believe we are justified in concluding from our experiments 

 that, in the month of March, in the middle of the day and in bright 

 sunlight, the last glimmers of daylight are extinguished at 400 

 metres from the surface, in the Mediterranean. 



After these results, those of the experiments which we have still 

 continued in the Lake of Geneva, since the publication of our previous 

 notice, have scarcely more than a local interest. To the absorption 

 peculiar to the water is in this case added that resulting from 

 particles in suspension, more or less abundant according to the 

 level. We hope, however, to determine an interesting point relative 

 to the influence exercised by the seasons on the degree of trans- 

 parency of these waters. 



We know that the experiments of M. Forel have shown that 

 albumenized silver paper is blackened, in winter, at a depth of 100 

 metres, whilst in summer it undergoes no alteration at a depth of 

 45 metres. It would be interesting to ascertain whether this 

 variation of transparency with the season is peculiar to the super- 

 ficial layers, or if the same law also holds good at lower levels. 



On the 18th of March of this year we repaired to the middle of 

 the Lake in the 'Sachem,' a steam-yacht belonging to M.E.Eeverdin, 

 which her owner had kindly placed at our disposal. As in the 

 former experiments on the lake, M. F. A. Eorel kindly volunteered 

 to accompany and assist us. The weather was fairly bright ; a 

 light layer of clouds dispersed the light without completely stopping 

 the direct rays of the sun. The exposure of the following plates 

 was made in the manner described in our former paper : — 



Plate 10 : from 9h 20m to 9.30, at a depth of 158 metres. 



Plate 11 : from 10.0 to 10.10, at a depth of 192 metres. 



Plate 12 : from 10.30 to 10.40, at a depth of 235 metres. 



Plate 13 : from 11.10 to 11.20, at from 240 to 245 metres. 



Plate 14: from 11.48 to 12.23, at from 280 to 300 metres. 



The time of exposure was therefore uniformly ten minutes in 

 each case excepting the last, which remained uncovered, at 280 

 m., during thirty-five minutes. Nevertheless not the slightest trace 

 of an impression is visible, either upon this plate or upon plates 

 13 and 12. Plate 11 shows a very faint impression, somewhat like 

 that of plate e at 380 m. in the sea. Lastly, plate 10 at 158 m. is 

 acted on to about the same extent as plate e. We place the ex- 

 treme limit of penetration of daylight in the Lake of Geneva in 

 winter at about 200 metres. 



It follows from a comparison between this series of experiments 

 and the preceding, that light does not penetrate in March more 

 than 20 or 30 m. lower than in September ; in the month of August 

 the difference is perhaps a little greater. Accordingly the strata of 



