EXAMPLES OF THE FOUR METHODS. 290 



ton, etc., as eminently favorable to the contraction of dew." The Method 

 of Concomitant Variations is here, for the third time, had recourse to; 

 and, as before, from necessity, since the texture of no substance is absolute- 

 ly firm or absolutely loose. Looseness of texture, therefore, or something 

 which is the cause of that quality, is another circumstance which promotes 

 the deposition of dew; but this third course resolves itself into the first, 

 viz., the quality of resisting the passage of heat : for substances of loose 

 texture " are precisely those Avhich are best adapted for clothing, or for im- 

 peding the free passage of heat from the skin into the air, so as to allow 

 their outer surfaces to be very cold, while they remain warm within ;" and 

 this last is, therefore, an induction (from fresh instances) simply corrobora- 

 tive of a former induction. 



It thus appears that the instances in which much dew is deposited, which 

 are very various, agree in this, and, so far as we are able to observe, in this 

 only, that they either radiate heat rapidly or conduct it slowly : qualities 

 between which there is no other circumstance of agreement than that by 

 virtue of either, the body tends to lose heat from the surface more rapidly 

 than it can be restored from within. The instances, on the contrary, in 

 which no dew, or but a small quantity of it, is formed, and which are also 

 extremely various, agree (as far as we can observe) in nothing except in 

 Qiot having this same property. We seem, therefore, to have detected the 

 characteristic difference between the substances on which dew is pro- 

 duced and those on which it is not produced. And thus have been real- 

 ized the requisitions of what we have termed the Indirect Method of Dif- 

 ference, or the Joint Method of Agreement and Difference. The example 

 afforded of this indirect method, and of the manner in which the data are 

 prepared for it by the Methods of Agreement and of Concomitant Varia- 

 tions, is the most important of all the illustrations of induction afforded by 

 this interesting speculation. 



We might now consider the question, on what the deposition of dcAV de- 

 pends, to be completely solved, if we could be quite sure that the sub- 

 stances on which dew is produced differ from those on which it is not, in 

 nothing but in the property of losing heat from the surface faster than the 

 loss can be repaired fi"om within. And though we never can have that 

 complete certainty, this is not of so much importance as might at first be 

 supposed ; for we have, at all events, ascertained that even if there be any 

 other quality hitherto unobserved which is present in all the substances 

 which contract dew, and absent in those which do not, this other property 

 must be one which, in all that great number of substances, is present or ab- 

 sent exactly where the property of being a better radiator than conductor 

 is present or absent; an extent of coincidence which affords a strong pre- 

 sumption of a community of cause, and a consequent invariable co-existence 

 between the two properties ; so that the property of being a better radiator 

 than conductor, if not itself the cause, almost certainly always accompanies 

 the cause, and for purposes of prediction, no error is likely to be commit- 

 ted by treating it as if it were really such. 



Reverting now to an earlier stage of the inquiry, let us remember that 

 we had ascertained that, in every instance where dew is formed, there is 

 actual coldness of the surface below the temperature of the surrounding 

 air ; but we were not sure whether this coldness was the cause of dew, or 

 its effect. This doubt we are now able to resolve. We have found that, in 

 every such instance, the substance is one which, by its own properties or 

 laws, would, if exposed in the night, become colder than the surrounding 



