1869.] On Spectroscopic Observations of the Sun. 



415 



starboard tack, should have brought us to a point with Wicklow Head, 

 lying W. by N., twenty-one miles distant." 



It is clear from this that the captain made an allowance for the point 

 of error he had discovered. Had he applied it in the opposite direc- 

 tion, he would undoubtedly have mentioned that he did so and why he 

 did so. 



The particular conclusions, then, which I draw from the facts of the case 

 are these : — 



1. There must have been a large heeling error affecting the steering- 

 compass of the ' Glenorchy,' which, on the courses steered, would be a 

 westerly deviation on the starboard tack, an easterly deviation on the 

 port tack. 



2. The westerly deviation detected on the starboard tack was this 

 heeling error. 



3. The true construction to be put on the captain's statement is, that 

 when on the port tack he allowed for the point of deviation which he had 

 detected on the starboard tack as a point of westerly deviation, not as a 

 point of easterly deviation, as he would have done had he known the 

 cause and the law of the deviation which he had detected. 



4. That, in consequence, his supposed course was in error one point 

 plus the heeling deviation, which, on a S.W. by W. course, was probably 

 about one point more. 



The general conclusions to be drawn from the history of the ship- 

 wreck seem to me to be : — 



1. The great importance of selecting a position for the navigating- 

 compass where the force of the ship's magnetism is moderate and uniform. 



2. The importance of extending the usual process of "adjustment" of a 

 compass to the ascertaining and (if necessary) the correcting of the heeling 

 error. This is a matter of no difficulty if the compass-adjuster is duly 

 instructed and supplied with the requisite instruments. 



III. " Spectroscopic Observations of the Sun. — No. IV." By J. 

 Norman Lockyer, F.R.A.S. Communicated by Dr. Sharpey, 

 Sec.R.S. Received April 14, 1869. 



I beg to lay before the Royal Society very briefly the results of observa- 

 tions made on the 1 1th instant in the neighbourhood of a fine spot, situated 

 not very far from the sun's limb. 



L Under certain conditions the C and F lines may be observed bright 

 on the sun, and in the spot-spectrum also, as in prominences or in the 

 chromosphere. 



II. Under certain conditions, although they are not observed as bright 

 lines, the corresponding Fraunhofer lines are blotted out. 



III. The accompanying changes of refrangibility of the lines in question 



