160 DISCOURSE ON THE STUDY 



(164.) But, in the case of the night dew, is this a 

 real cause is it a fact that the object dewed is colder 

 than the air ? Certainly not, one would at first be 

 inclined to say ; for what is to make it so ? But the 

 analogies are cogent and unanimous ; and, therefore, 

 (pursuant to Rule 3. 148.) we are not to discard their 

 indications ; and, besides, the experiment is easy : we 

 have only to lay a thermometer in contact with the 

 dewed substance, and hang one at a little distance 

 above it out of reach of its influence. The experi- 

 ment has been therefore made; the question has 

 been asked, and the answer has been invariably in the 

 affirmative. Whenever an object contracts dew, it is 

 colder than the air. Here, then, we have an inva- 

 riable concomitant circumstance : but is this chill an 

 effect of dew, or its cause ? That dews are accom- 

 panied with a chill is a common remark ; but vulgar 

 prejudice would make the cold the effect rather 

 than the cause. We must, therefore, collect more 

 facts, or, which comes to the same thing, vary the 

 circumstances ; since every instance in which the 

 circumstances differ is a fresh fact ; and, especially, 

 we must note the contrary or negative cases (Rule 

 4. 150.), i. e. where no dew is produced. 



(165.) Now, 1st, no dew is produced on the sur- 

 face of polished metals, but it is very copiously on 

 glass, both exposed with their faces upwards, and 

 in some cases the under side of a horizontal plate of 

 glass is also dewed ; which last circumstance (by 

 Rule 1. 146.) excludes the fall of moisture from 

 the sky in an invisible form, which would natur- 

 ally suggest itself as a cause. In the cases of 

 polished metal and polished glass, the contrast 



