Astronomical Metliods of Observation, 129 



tract published by Messrs. Troughton and Simms,) to de- 

 termine the obliquity of the ecliptic at the summer solstice, 

 the first two terms of Delambre's formula would be suf- 

 ficient, though Dr. Young's table, recommended by Dr. 

 Pearson, and more lately approved by Mr. Simms, would, 

 at 24 minutes from the meridian, give results erroneous to 

 about 7", a quantity quite inadmissible, though this problem 

 is just such a one as is, under the given circumstances, 

 suited to the smaller class of altitude and azimuth circles, 

 generally in the hands of astronomical students, and re- 

 peating circles previously alluded to. 



If, however, the horary distance from the meridian be, 

 under such circumstances, restricted to 12 minutes of time, 

 which will admit of a sufficiently extensive number of re- 

 petitions useful to exterminate casual errors of observation, 

 reading and dividing; two terms of Delambre's formula 

 will be fully adequate for the purpose, while the error 

 arising from the use of Dr. Young's table will not exceed 

 half a second. 



With regard to the most eligible size of an instrument, 

 it is difficult to come at an accurate conclusion. That must, 

 in a great degree, be regulated by the purposes for which 

 it is intended. I am strongly inclined to think, however, 

 that circles of moderate size, and of the most simple yet 

 substantial construction are the most likely to give satis- 

 faction. Very large mural circles that do not revolve in 

 azimuth, especially when employed to make observations 

 on the sun, are liable to suffer unequal expansions from 

 heat on that side next the sun, being acted on powerfully if 

 not shaded, which it is difficult to do completely, while the 

 opposite side is slighty affected by its position in the shade 

 of the other, and it is doubtful, in my opinion, whether a 

 considerable number of microscopes except under peculiar 

 circumstances will correct the errors arising from this 

 cause. On the other hand, a much smaller instrument re- 

 volving in azimuth, and by that means having its different 

 sides, though as much shaded as possible, exposed partially 

 in succession to the sun will expand much more equally, 

 and when the mean of three or four verniers or microscopes 

 read at each observation, which may be repeated two or 

 three times in pairs of double observations, within the 



VOL. IV. K 



