32 Mr. Ivory /'// rephj to the Historical Sketch 



attack. I should however have left it, as I now do, to the 

 decision of every reader. But the sequel of Mr. Lindley's 

 preface does so much injustice to all who have pursued botany 

 in this country, and so absurdly contrasts the Natural Orders 

 of Jussieu (chiefly indeed those of Linnaeus) with the artifi- 

 cial system of Linnajus, things which no experienced or learned 

 botanist could ever put in comjietition, that I could not but 

 be struck with the injustice of the representation altogether. 

 On this subject I also " appeal to the world," and having done 

 so, I shall return to my own occupations. Having been the 

 first to make known to my countrymen, in two different works, 

 the transcendent merits of my illustrious and constant friend 

 M. De Jussieu, I cannot be reproached witli insensibility to 

 liis wordi, and I am sure he would be one of the last to charge 

 me with it. 



Hoping, gendemen, that I shall have no further occasion 

 to trouble you on any controversial subjects, 



I remain, your constant reader, 

 Norwich, Jan. 18, 1825. J. E. Smith. 



VI. A Letter in reply to the Historical Sketch of the Problem 

 of Atmospherical Refraction in the last Number of the Journal 

 of Science. By J. Ivory, Esq. M.A. F.R.S. 



To the Editors of the Philosophical Magazifie and Journal. 



Gentlemen, 



T ADDRESS you to request you will insert in your next 

 -*• publication the following observations, occasioned by an 

 liistorical sketch of the problem of Atmospherical Refraction 

 that appeared in the last Number of the Journal of Science 

 edited at the Royal Institution. 



1. It is said, p. 375, that the exponent | whicli I have 

 chosen in my solution of the problem, is derived from obser- 

 vations on mountains at small heights. This is incorrect. In 

 the average I have adopted, all the greatest known heights are 

 taken into account, not excepting Gay-Lussac's ascent. The 

 result obtained is then compared to the same ascent, which is 

 the greatest height hitherto attained by man ; and the dif- 

 ference is shown to be very inconsiderable. 



Great stress is laid upon the extreme accuracy of the French 

 Horizontal Refraction. Laplace has adopted this quantity as a 

 mean of the results of astronomers, although he allows that 

 the quantity of this element has been very imperfectly ascer- 

 tained. If we turn to the Tables Astronomiqucs, 1806, we shall 

 find the horizontal refractions adopted by six eminent astro- 

 nomers : 



