134 ESSAY 



turbidness of the air in the morning shewing, 

 that it must have contained, during the preced- 

 ing night, a considerable quantity of moisture. 



Thirdly ; I have observed dew to be unusually 

 plentiful on a clear morning, which had suc- 

 ceeded a cloudy night. For the air, having in 

 the course of the night lost little or no moisture, 

 was in the morning more charged with watery 

 vapour, than it would have been, if the night 

 had also been clear. 



Fourthly ; heat of the atmosphere, if other 

 circumstances are favourable, which, according 

 to my experience, they seldom are in this 

 country, occasions a great formation of dew. 

 For, as the power of the air, to retain watery 

 vapour in a pellucid state, increases considerably 

 faster, while its temperature is rising, than in 

 proportion to the heat acquired, a decrease of 

 its heat, in any small given quantity, during the 

 night, must bring it, if the temperature be high, 

 much nearer to the point of repletion, before it 

 be acted upon by the immediate cause of dew, 

 than if the temperature were low. We read, 

 accordingly, in the writings of those, who have 

 travelled into hot climates, of a copiousness of 

 dew frequently observed by them there, which 

 very much exceeds what occurs, at any time, in 

 this country. But even here, dew, though for 

 the most part scanty in our hottest season, is 



