490 Mr. Charles Tomlinson's Remarks 



there be kept near its maximum of humidity ; and the slightest 

 degree of cold will suffice to precipitate the aqueous vapour 

 on the bodies which are immersed therein. 



In the same way may be explained the large amount of 

 cooling which takes place when a thermometer loosely en- 

 veloped in wool, or cotton-wool, is exposed to the clear noc- 

 turnal sky. The air cooled by coming in contact with these 

 envelopes penetrates into them, and tends to fall by virtue of 

 its greater density ; but the mechanical resistance and the 

 attraction offered by the innumerable tangled fibres retain it 

 for some time in the midst of those parts which are radiating 

 towards the sky. A series of actions and reactions similar to 

 those just described then takes place, and the mixture of air 

 and of wool cools much more than a simple layer of varnish 

 or of lampblack applied to the thermometer. In the same 

 way plants with velvety leaves acquire a somewhat lower 

 temperature than plants with smooth ones, and consequently 

 become covered with a greater quantity of dew. Now in 

 studying the phenomena of cooling in the meadow, if the 

 greatest cold be produced at the top of the grass, the cooler 

 air would, by its superior density, soon sink to a lower level. 

 We may imagine three layers of air — one in contact with the 

 points of the grass ; the second immediately below it, where 

 the blades are more numerous, and more or less exposed to 

 the zenith ; and the third entangled in the matted portion, 

 which is entirely sheltered from the sky. The points of the 

 grass are in a condition most favourable for free radiation, 

 but the blades are there fewer in number, and the air is ex- 

 posed to slight disturbances which diminish the radiating 

 effect ; but the middle portion, where the blades are most 

 numerous and the disturbance less, radiates most powerfully 

 and produces the greatest cooling. The lowest layer of air 

 being sheltered, will at first have a higher temperature than 

 the other two ; but these, being cooler and denser, will descend 

 and react on the radiating portions of the grass, and the more 

 so in proportion as the movement is slow. In time the middle 

 portion will contract the greatest cold, and in descending will 

 displace the somewhat warmer air, and in the end the lower 

 stratum will be colder than the first, so that the blades and 

 stems which are least exposed to the aspect of the sky will be 

 colder than the points of the blades, and the thermometer 

 buried in the grass will mark a lower temperature than one in 

 contact with the surface. And here, too, the formation of dew 

 will be most abundant, not on the surface, but just below it. 

 But as the cooling goes on, the lower layer of air will be 

 again displaced, and radiation continuing from the soil, a 



