184 M. Melloni on the Theory of Dew. 



by communication will become cold by radiation when they 

 are covered with dew, while the varnished points placed un- 

 der the small disc cannot well be cooled but by contact. The 

 central part of the varnished circle continues always dry, and 

 the metallic zone which surrounds it is moistened to the edge, 

 if the atmosphere be excessively humid. 



But what we would not expect at first sight is this, — ^tlie 

 exact reproduction of the same appearances on the surface 

 of the large disc turned towards the ground. The dew begins 

 by shewing itself on this surface in the points opposite to the 

 small annular exterior band of the varnish, and we perceive a 

 slight whitish circle, which appears suddenly on the obscure 

 field of the polished metal, reminding us of the daguerrian 

 figures. This circle becomes stronger and dilates by degrees, 

 sometimes reaches the edges, but never extends to the cen- 

 tral part, which always preserves its dryness and metallic 

 brilliancy, like the corresponding portion of the other side of 

 the disc, and the small circular roof which covers without 

 touching it. 



• This experiment, so simple, so economical, and so easy to 

 be performed even by those who have never handled a philo- 

 sophical instrument, is, so to express it, a speaking synopsis 

 of the discussion of natural philosophers on the origin and 

 nature of the nocturnal meteor of which we are now treating. 

 Does dew fall from the sky 1 No, for the superior disc is 

 always dry, and the largest part of the inferior disc moistened. 

 Does it rise from the ground 1 Assuredly not, for if the 

 middle part of the surface of the large disc turned towards 

 the earth be covered with dew, there is always a dry and 

 brilliant space towards the centre. Do metals repel the 

 aqueous vapour which constitutes dew, or do they cause its 

 evaporation as it is precipitated on their surface 1 Neither 

 the one or the other, for you see metallic parts completely 

 moistened, and other parts perfectly dry. 



The first appearance of the dew on the band covered with 

 varnish, and its successive propagation to the adjacent and 

 opposite parts of the great disc, taken in connection with the 

 i^inking of the temperature which is seen in the varnislied 

 apparatus of the thermometers exposed to the air, finally 



