248 



The Data on which we have to depend 



such lands often drain large tracts of country^ but I cannot 

 understand how Mr. Thorn should imagine that fifty-four 

 inclies of rain fall in Paisley. This town is only seven miles 

 distant from Grlasgow, and, according to the meteorological 

 tables, the rainfall for this city is twenty-one inches. 



Mr. Thorn's observations and experiments, therefore, could 

 not have been conducted with much regard to scientific 

 accuracy, when he computes the available rainfall at thirty- 

 nine inches out of an annual rainfall of twenty-one inches, 

 and they contrast rather singularly with Dr. Thomson's 

 observations and experiments, which give four inches of 

 available rain out of twenty-one inches for the Clyde district. 



Since Dr. W ells published his well known Essay on Dew, 

 his theory of its formation has been almost universally re- 

 ceived as correct. 



The production of dew occurs in the following manner. 

 The quantity of aqueous vapour that can exist in the atmosphere 

 depends entirely on temperature. 



During a clear calm night, all bodies that are fully exposed 

 in the air become more or less rapidly cooled by radiation of 

 heat from their surface. The air in contact with such bodies 

 suffers a corresponding loss of heat, and, as soon as its 

 temperature reaches the dew point, the moisture, which can 

 no longer retain the form of vapour, is condensed in the form 

 of dew. Thus, those bodies which radiate most heat and 

 conduct least, condense most dew, and it is found that all 

 bodies which are good conductors and good reflectors of heat 

 from their surface, are bad radiators. 



The metals, therefore, condense dew very sparingly. Water, 

 though a bad conductor of heat when applied to its surface, is 

 from the extreme mobility of its particles, the most rapid 

 conductor of heat and cold, ivhcn these are apphed with due 

 regai'd to its peculiar laws. 



Water in this sense may be regarded as strictly analogous 

 to the metals, and, being a good conductor and reflector of 

 heat, it is necessarily a bad radiator, and the dew is not formed 

 on any surface whose temperature is not cooled by radiation 

 below the dew point, which ranges from 5*^ to 20^ below the 

 temperature of the air. Unless, therefore, the surface of 

 water be cooled by radiation below the dew point, it is quite 

 clear that no dew can be condensed but its density, which 

 varies with every change of temperature, and its fluidity 

 operate to prevent any reduction of its temperature until the 

 whole mass is similarly affected. 



The temperature of water is thus very slowly reduced by 



