NATURAL PHILOSOPHY. 175 



Therefore, the effect of these bodies is equivalent to the addition of 

 117 grains to the 2,000 grains of water, and 1-17 must be added as a 

 correction to all temperatures obtained by the instrument ; in other 

 words, the value of a degree will be increased from 31 to 33 Fahr. 

 A piece of fire clay may be substituted for the platinum, and a direct 

 experiment, either with this, or the platinum, will give the value of a 

 degree. The author found that a piece of Stourbridge clay, 200 

 grains weight, plunged in melted silver, and then into the tinned ves- 

 sel containing 2,000 grains of water, raised the temperature of the 

 water 41. 



Now if 1,890 Fahr., the melting point of silver, be divided by 41, 

 we obtain 56 as the equivalent to 1 of this pyrometer. 



In measuring temperature by means of platinum, it must not be 

 plunged into melted metals, as it readily alloys even with melted lead, 

 it must be put in the fire, or the clay employed instead. 



In practice it will be found best to determine at once the value of a 

 degree in the manner we have described, viz : by submitting the piece 

 of metal, or clay, to some heat already known, and then observing 

 how much the temperature of the water is raised ; always employing 

 the same vessel, and using the same thermometer. Brewster's Journal. 



MELLONI OX DEW. 



TVs copy the following observations on dew, from the recently 

 published memoir of M. Melloni. 



Dew is not an immediate effect of the cooling produced by the 

 nocturnal radiation of vegetables on the vapor of the atmosphere, as 

 most treatises on physics and meteorology assume, but the result of a 

 series of actions and reactions betAveen the cold due to the radiation 

 of plants, and the cold transmitted to the surrounding air. The 

 grass is cooled but little below the temperature of the air, but it very 

 quickly communicates to it a portion of the acquired cold ; and since 

 the difference of temperature between the radiating body and the 

 surrounding medium is independent of the absolute value of the pre- 

 vailing temperature, the grass surrounded by a colder air still further 

 lowers its temperature, and communicates a new degree of cold to the 

 air, which reacts in its turn, on the grass, and compels it to acquire a 

 temperature still lower, and so on in succession. Meanwhile the medium 

 loses its state of equilibrium, and acquires a sort of vertical circula- 

 tion, in consequence of the descending motion of the portions con- 

 densed by the cold of the upper foliage, and the ascending motion of 

 the portions which have touched the surface of the earth. Now, the 

 gradual cooling and the contact of the soil evidently tend to augment 

 the humidity of the stratum, and thus bring it by degrees towards the 

 point of saturation. Then, the feeble degree of cold produced directly 

 by the radiation of bodies, suffices to condense the vapor contained in 

 the air which surrounds them ; and since the causes which give rise to 

 the circulating movement, and to the humidity of the air, continue 

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