TERRESTRIAL MAGNETIC INTENSITY. 29 



49. The lines of equal dip and equal horizontal intensity being known, the 

 direction of the lines of equal total intensity may be deduced geometrically. I 

 am, however, too well aware of the great uncertainty which a small error in the 

 elements produces, to attempt to assign a result which might prove very erro- 

 neous indeed. 



Postscript. 



Since this paper was written, and the results made public, a .suggestion has 

 been made in a quarter entitled to attention, as to how far the apparent diminu- 

 tion with height may be due to the hour of the day at which observations at great 

 heights have usually been made. I have already stated, that I have attempted 

 no correction for the hour of the day, owing to the want of accurate data, but I 

 thought it worth while to inquire how far there was any general ground for such 

 an explanation of the observed difference. I accordingly divided my observations 

 into 18 series above 4000 feet, and 22 below that height. I found that the mean 

 hour at which the former were made was 11'' 12", the latter at 12'' 42". Ac- 

 cording to the best observations, the intensity would be somewhat less at the 

 former period than the latter, and would so far give a false indication of dimi- 

 nished intensity with height. But the variation for l*" 40" would undoubtedly be 

 trifling, compared even with the small variation which the preceding paper as- 

 signs for 5110 feet, which corresponds to the mean difference of height for the 

 two series, the mean height for the first being 7160 feet, and for the second 2050. 



