378 Asironomical and Nautical Collections. 



tions of the French Tables are sufficiently established ; though, as 

 Mr. Ivory has discovered, they were actually computed for the 

 freezing temperature, and not very perfectly reduced to the mean 

 temperature to which they are assigned ; but they are much better 

 than the labour of many years could procure from the observations 

 of a single astronomer only. It may, however, still be advisable 

 to retain the theoretical correction for temperature in the Nautical 

 Almanac, because a work of that kind, which is likely to be con- 

 sulted in a variety of climates, is required to represent the probable 

 results of the mean constitution of an atmosphere in equilibrium at 

 different temperatures depending on climate, and not the temporary 

 effects of the seasons only, or of the weather at the moment, or of 

 the alternations of day and night. 



The grounds of Dr. Young's latest method of computing the 

 refraction will be obvious, from comparing this paper with the 

 demonstrations contained in the XlVth number of the Astronomical 



Collections, in wliich the equations 1/ = 2^ and y = z^ ^jg both 

 shown to afford finite expressions for the refraction ; and it appears 



33 1 



that their combination, m the form ?/ = — 2- — — 2^ belongs to 



an atmosphere which might be expected, from Mr. Ivory's investi- 

 gation, to represent the refraction with extreme accuracy, though 

 it is probably more dense than the true atmosphere at great heights, 

 and yet terminates too abruptly. But the unexpected advantage 

 of combining a perfect representation of the true decrement of heat 

 at the earth's surface, with a very accurate expression of the refrac- 

 tion, in an equation of a finite form, and not laborious in its appli- 

 cation, must, at least give this hypothesis some claim to the atten- 

 tion of those who feel any remaining objection to the approxima- 

 tien that has been employed in the Nautical Almanac. 



Professor Schumacher is desirous of having it explicitly under- 

 stood, that the omission of the passage relating to the preference 

 of the exterior thermometer, in his edition of the English explanation 

 of Dr. Young's Table, was completely accidental; it is retained in 

 the German translation, and Professor Schumacher fully coincides 

 in the opinion that it expresses. 



