Mr Blackadder on the Formation of Dew', 5S 
the air after sunset, have their temperatures very differently 
affected. Th us the soil of a garden is less cooled than a 
gravel-walk, and the latter much less cooled than a grassy sur- 
face ; and these facts admit of the most satisfactory explanation, 
by a consideration of the respective facilities with which these 
bodies acquire supplies of heat by conduction from the ground. 
Garden-mould is a much better conductor than gravel, and the 
latter is greatly superior to grass and other vegetable bodies ; 
hence the temperature of the latter always suffers the greatest 
depression. There is, however, one circumstance connected with 
grass and other vegetable bodies which requires to be adverted 
to, and which, if it could not be satisfactorily explained, would 
render the influence of evaporation on such occasions problema- 
tical It has been observed, that the temperature of a grassy 
surface is often diminished, and sometimes remarkably reduced, 
while the surface of the blades of grass, &c. seemed to be desti- 
tute of moisture, and hence, to appearance, little fitted to sup- 
port evaporation. But this apparent difficulty vanishes, when we 
consider that vegetables have an organization which enables 
them to transpire moisture and various gaseous bodies, and that 
similar to what takes place with the insensible perspiration of 
animals, without necessarily producing any apparent wetness of 
their surfaces. 
Having thus attempted to show that evaporation is the means 
by which the temperature of grass suffers a depression on a se- 
rene evening after sunset, it remains to be explained how it after- 
wards comes to be dewed, and even at a period when the air is 
not saturated with moisture. It has been found that, on dewy 
evenings, the ground under a grassy surface is always warmer 
than the air, and consequently considerably warmer than the 
cold grass. As, therefore, the soil under the grass is moist, and 
in contact with the air, evaporation must be going forward at its 
surface; and the vapour that is there generated must, as it rises 
upwards, pass through or between the blades of grass. We 
might therefore expect that part of this vapour would be liable 
to be condensed by coming into contact with the cold grass, in 
its progress upwards. We might also be led to expect that this 
condensation would take place at an earlier or a later period, 
according to the state of the air, in respect to its greater or less 
