600 



THE SOUTHERN PLANTED, 



[AtGlJSf 



thus formed, if the expression iiiaj be al- 

 lowed, a thermal telescope, with which the : 

 heat of a cloud of the apparent size of the 

 Bioon was readily perceptible. When this 

 instrument was directed first to the clear 

 sky in the vicinity o^' a cloud, and then im- 

 mediately after to the cloud itself, the nee- 

 dle of the galvanometer attached to the 

 thermo-electric pile in the tube alway© de- 

 viated several degrees. At first sight it 

 might appear, from this experimeiit, that 

 the heat of the cloud was greater than that 

 of the transparent air in which it was Moat- 

 ing, but this was not necessarily the case ; 

 the rays of heat from the apparatus, when 

 it was directed into the clear sky, passed off 

 into celestial space, while, when the instru- 

 ment was directed to the cloud, they w-ere 

 absorbed-and radiated back. It is probable, 

 however, that the lower surface of the cloud 

 is really a little warmer than the air m which 

 it is floating from the radiation of beat by 

 the earth, while the upper surface is proba- 

 bly colder on account of the uncompensated 

 radiation into space. But, be this as it may, 

 the counter radiation of the clouds prevents 

 the cooling down of the bodies at the sur- 

 face of the earth sufficient for the deposition 

 of dew, or at least to allow of the formatioii 

 of 'a copious quantity. A haziness of the 

 atmosphere, and it is probable a large amount 

 of invisible vapor will retard the radiation, 

 and hence a still, cloudless night, without a 

 deposition of dew is considered a sign of 

 rain. The amount of deposition of dew will 

 also depend upon the stillness of the atmos- 

 phere ; for, if a brisk wind be blowing at 

 the time, the different strata of air will be 

 mingled together, and that which rests upot? 

 the surface of the ground will be so (luickly 

 displaced as not to have time to cool down 

 sufficiently, to produce the deposition. 



Again, the deposition will be more co- 

 pious on bodies the surfaces of which are 

 most cooled by the radiati-on. It is well 

 known that different substances have differ- 

 ent radiating powers. The following table 

 from Becquerel exhibits the proportional 

 tendency of different substances to promote 

 the deposition of dew. The figures do not 

 represent the relative emissive power, but 

 the combined effects of emission and con- 

 duction ; 



1. Lamp blacTs, - - 100 



2. Grasses, - - - 103 



3. SiHcious sand; - 103 



4. Leaves of the elm and the poplar, 101 



5. Poplar sawdtet, - - 99 



6. Varnish, - - ^ 97 



7. Olass, . - - 93 



8. Vegetable earth, - - 92 



Polished metals sre, of all stibstances, the 

 worst radiators they reflect the rays of heat 

 as they do fhose of light, and it would ap- 

 pear that by internal reflection the escape 

 of heat is prevented from the capacity of 

 the metal. In order that the surface of a 

 body should cool dow^ to the lowest degree, 

 it is necessary that it should be a good ra- 

 diator and a bad eorjductor, particularly if 

 it be in a large mass and uninsulated. Thus 

 a surface of a mass of metal coated with 

 laD>p black, tl^ough it radiates best freely, 

 will not be as much erooled under a clear 

 aky as a surface of glass, since the heut lost 

 at the surface is almost immediately supplied 

 by conduction from within. If, however, 

 a very small quantity of metal, such as gold 

 leaf, be suspended by fiiie threads, the de^V 

 will be deposited, because the heat which is 

 radiated is not supplied by conduction froiu 

 any other source, and hefice the tempera- 

 ture will sink to low degree. 



M. Melloni has, within a few yeirs past^ 

 repeated the experiment of Wells, establish- 

 ed the correctness of his conclusions, and 

 added some particulars of interest. He 

 found that the apparent temperature of the 

 grass, which in some cases was 8° to 10° 

 lower than that of the air at the height of 3 

 to 4 feet, was not entirely due to the actual 

 cooling of the air to that degree, but to the 

 radiation and cooling of the thermometer 

 itself, the glass bulb of which is a powerful 

 radiator. To obviate this source of error 

 in estimating the temperature he placed the 

 bulbs of his thermometer ia a small conical 

 envelope of polished metal of about the 

 size of an ordinary sewing thimble, Thi& 

 prevented a radiation, and, by contact with 

 the air, indicated its true temperature. Lie 

 found, with thermometers thus guarded, that 

 the solid body was in no case cooled down 

 more than 2° below the temperature of the 

 surrounding air, and that the amount of ra- 

 diation was nearly the same at all tempera- 

 tiires. The explanation, therefore, of the 

 great cold of the air between the blades of 

 grass is as follows : By the radiation of the 

 heat, the grass is at first cooled two degrees- 

 lower than the air at the surface of the 

 earth, and next the thin stratum of ak' 



