On the douHe Images caufid by Jtme/pkeVical Refrailieh. JOJ 



The greateft deviation produced in this cafe meafured about half a degree. 



Exper. 6. By a red hot bar of iron, 30 inches long, the refradlions were much 

 greater, the extreme deviations amounting to full 1^ degree. 



The refractions obferved in thefe experiments may at firft view be thought greater than 

 could be caufed by difference of temperature alone, being in one inftance more than double 

 the greateft horizontal refraction of the heavenly bodies ; in which cafe, as the rays enter 

 from a vacuum, the greateft poffible efFe£l of the atmofphere might be expedted. But it 

 muft be remembered, that when a ftar appears in the horizon, its rays interfedl the 

 fuperior ftrata of the atmofphere at an inclination of feveral degrees, and that they pafs 

 but once through the variations from rarity to denfity ; but, on the contrary, that in the 

 experiments with red hot iron, the rays may pafs a£tually in the direftion of the ftrata, 

 and that they are refradted not only by their entrance from the denfer into the rarer 

 medium, but the effeCl is doubled, fince the refraction caufed by their emergence is equal 

 to that produced by their incidence. 



■ Although a ftratum of air, heated by, thefe means to fo great a degree, affords an ere£l, 

 as well as an inverted, image of objedts feen through it, the more moderate warmth com- 

 municated to it from bodies heated by the aCtion of the fun upon them, feems infufiicient 

 to produce both images; but the inverted image may generally be feen when the fun fhines 

 upon a brick wallj or other darker coloured furface. 



While the eye of the obferver is placed nearly in a line with the wall, if another perfon, 

 at 30 or 40 yards diftance, extends any object towards the wall, an image fimilar to it- 

 will appear to come out to m'eet it. 



It would be difficult to afcertain with accuracy the degree of rarefaCtion capable of 

 fhewing this appearance, but it may be of fome ufe to future obfervers to mention the 

 different degrees of heat which I obferved. 



in one inftance, a thermometer, in contaCt with the wall, ftood at 96° ; but, at ^ of 

 an inch diftance, 82°. 



One morning, when the fun fhone bright, I examined the temperatures and refraClion 

 produced at the furface of a deal bar painted green, about eight feet long. 



A fmall thermometer, in contaCt with the bar, rofe to g.6° ; at l of an inch diftance, 

 it ftood at 73°. 



The refraction at the fame time exceeded 20 minutes. 



To explain why red hot iron occafions two images, while folar heat produces but one, 

 I imagine that the intenfe heat in the former cafe rarefies the air for fome fmall diftance 

 uniformly, and thereby affords the fame ferles of variations as between other fluids of 

 uniform denfity; but that, in the latter, the heat is conveyed off as as faft as it is gene- 

 rated; fo that, as there is no extent of medium uniformly rare, the denfities cor- 

 refponding to the concave portion r m. Fig. 5, of the curve before defcribed do 

 not take place, but the phenomena occafioned by the convex part m d are alone pro- 

 duced. ' 



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