PROCEEDINGS OF THE POLYTECHNIC ASSOCIATION. 643 



prints these changes at stated intervals, at the same time descri- 

 bing the curve of variation. At the end of every twenty-four 

 hours a leaf of figures and diagrams is completed, which forms a 

 part of the annual volume of automatic records. 



TiiE Silvered Object-Glass. 

 M. Leverrier informs the French Academy that M. Foucault's 

 process for weakening the solar radiation by covering the object- 

 glass with a film of silver, has been successfully applied to a glass 

 twenty-five centimeters in diameter. The interposition of the film 

 of silver does not appear to materially alter its optical qualities. 

 The spectroscope shows that all the rays of the spectrum ar(^ 

 present except the extreme red-^the orange yellow and green 

 undergoing partial extinction. Under favorable conditions of th© 

 atmosphere, a power of 300 diameters could be applied, and the 

 entire surface of the sun was seen to be covered with those numer- 

 ous details known to experienced observers. The transmitted light 

 is blueish, to which the eye soon becomes accustomed. 



Meteorological Observations in Great Britain. 

 The Board of Trade, upon the recommendation of the Royal 

 Society, has followed the example set by the Imperial Observa- 

 tory of Paris, and ordered the Observatory at Kew to be enlarged 

 and made the Central Observatory station, w^hich is to be in 

 connection with subordinate stations at Falmouth, Stouyhurst, 

 Armagh, Glasgow and Aberdeen. Each of the stations is to be 

 provided with self-recording instruments. Observations are also 

 to be taken at sixty other stations, lighthouses &c. Outline maps 

 will be published every day, showing the direction of the wind at 

 each station. Forecasts are to be discontinued as untrustworthy, 

 while the storm-warnings are to be kept up; but the Board requires 

 that the ofiicer who hoists the signals shall note down at the time 

 all the reasons which guided him in making the prediction. 



Modes of Crystalizing. 

 M. E. Fremy, in a communication to the French Academy, 

 describes various methods for slowly precipitating certain chemi- 

 cal compounds which are amorphous owing to the suddenness of 

 their formation, thus enabling him to obtain them in a chrystaliue 

 form. In some cases the bodies to be decomposed were intro- 

 duced into liquids of dififerent density, containing gum, sugar, 



