GREAT MALVERN AND LONDON. 27 



but it can never be higher than the temperature of the air : when 

 the latter falls (supposing them at the same point) the former 

 must fall also — the superfluous moisture (i. e., some portion of the 

 invisible vapour) being condensed either into dew, fog, clouds, or 

 perhaps rain. In autumn, when the temperature of the air is 

 advancing during the day, it often happens that the dew point 

 advances as much, and in the evening, when the air is again cool- 

 ing, the dew point must fall; its subsidence being accompanied 

 by a copious deposition of dew, and where the lower strata of 

 the atmosphere are chilled by radiation, by the appearance of mist 

 or fog. The form in which moisture under these circumstances is 

 deposited upon the ground, is much modified by the state of the air: 

 if the wind blows strongly, surfaces become moist and perhaps wet ; 

 it is only when the air is calm, that those minute drops, standing at 

 the very tips of the blades of grass and upon every, the minutest 

 fibre, constituting dew properly so called, can be seen. 



Air of the same temperature affects our sensations differently ; 

 the impression is greatly modified by the force of the wind and the 

 state of the dew point. When the atmosphere is calm, the tempe- 

 rature moderate, and the dew point very high, it seems close, warm, 

 and oppressive — sensations much diminished by a light breeze ; on 

 the other hand, when the temperature is moderate, the air calm, 

 and the dew point very low, the feeling is cool, bracing, and plea- 

 sant ; but if the wind blows fresh, we then feel it cold, harsh, and 

 disagreeable ; this last condition is very apt to induce catarrh, sore- 

 throat, and rheumatism in those disposed to these affections. It is 

 when the dew point is high that ladies' hair falls out of curl, a 

 ringlet is an elegant and delicate hygroscope. Every one must have 

 observed, occasionally, the visible condensation of the breath ; this 

 is a tolerable indication of the state of the vapour of the atmosphere, 

 such an appearance shewing that the temperature of the air and 

 dew point are quite, or very nearly, the same : it may sometimes be 

 remarked in the open air, before, almost always during, or just 

 after, continued heavy rain. 



During the winter season, (and the remark will apply generally 

 to the autumn and spring), a rise in the thermometer and dew point 

 at the same time, is a sure indication of clouds and rain. 



It often happens on a clear sunny day after rain, when evapora- 

 tion is going on with the utmost rapidity, that the dew point does 

 not rise, or, in other words, that the quantity of vapour in the low- 

 er regions of the atmosphere does not increase, nor do clouds form 

 in the higher ; the vapour, therefore, must be drawn off to distant 

 regions. 



