[ 418 .] 



LXIX. Letter on the Astronomical Refractions. By J. Ivory, 

 Esq. M.A. F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



T DO not find that there is much to remark upon in the 

 ■■■ Notice addressed to me in your last Number. I am still 

 under no little astonishment at the critique on my Table of 

 Refractions that appeared in the last Journal of Science. At 

 present, I can conjecture no rational purpose it could be sup- 

 posed to serve; but time, which is a great revealer of se- 

 crets, may possibly disclose what was really intended by it. 

 I was far from wishing to put the Secretary of the Board of 

 Longitude to the trouble of examining any of my produc- 

 tions ; and I confess I do not see the propriety of such offi- 

 cial interference unless specially called for, more particularly 

 under such suspicious circumstances as attended the present 

 instance. 



In my letter in your Number for April, I used the Table 

 in N. A. according to the directions given with it. I hope 

 there will not be found any material errors in my calculations. 



The standard temperature of a Table of refractions is fixed 

 by that of the elementary quantities employed in its construc- 

 tion. In this respect there can possibly be no uncertainty. 

 The difficulty is to find the real temperature of every parti- 

 cular observation. Whether the estimation is to be made by 

 the thermometer within, or by that without, or at some in- 

 termediate degree between the two ; is the question to be de- 

 termined, and which every astronomer answers for himself 

 the best way he can. The Table in N. A. has the same 

 standard temperature with mine. Now when the S. B. L., 

 for the express purpose of lessening the errors of his own 

 Table, lowered its temperature from 50° to 48° or 47°, which 

 is equivalent to bringing the exterior thermometer two or 

 three degrees nearer the interior one ; did it not occur to him 

 that the errors of the other Table would likewise be lessened 

 by the same device? The reason of the practice applies 

 equally to both Tables ; namely, the exterior temperature is 

 not really that of the observations, and gives corrections that 

 are too great. But perhaps this is one of those little ad- 

 vantages which the Secretary reserves to himself as a pecu- 

 liar prerogative of his office. 



After heaping calculation upon calculation, and trying every 

 means of involving a very simple matter, the conclusion in 

 the last paragraph of the Notice is just the same with what I 



have 



