1824.] Horizontal and Dipping Needles. 169 



light,* and not the solar heat, is the principal operative agent in 

 producing the daily variation. It remained, however, to examine 

 the third query, which I attempted to do as follows. Having 

 placed the compass in its former situation in the garden, I fixed 

 on one side of it a ten inch howitzer shell, in the same direction 

 with respect to the compass as the stove had in the parlour, and 

 at such a distance that it might produce a sensible deviation in 

 the needle, and which I afterwards adjusted to zero by a slight 

 change in the position of the magnets, thus placing the needle, 

 as I imagined, under similar circumstances in both cases, with 

 respect to local attraction ; but, notwithstanding I did in this 

 way actually produce an alteration in the daily motion, changing 

 its maximum from eleven o'clock in the morning to about four 

 o'clock in the afternoon, yet the direction of the motion was the 

 reverse of what it was constantly found to be in-doors ; the 

 cause therefore of this perplexing anomaly still remains to be 

 discovered. 



It is proper to observe that Mr. Christie, having made some 

 of his observations in-doors, and some in his garden, on two 

 compasses at the same time, found the same reversion of motion 

 in the two cases. His house is a mile distant from mine ; he 

 has no stove in the room in which the in-door experiments were 

 made ; and the only resemblance of situation is, that his window, 

 like mine, is towards the north. It should be further added, 

 that this confirmation of the singular anomaly in question did 

 not arise from his simply repeating my experiment, but grew 

 naturally out of the particular mode he had adopted to prose- 

 cute the inquiry ; our experiments, with the exception of the 

 first suggestion, are independent, and, therefore, where they 

 both lead to the same result, they may be considered as con- 

 firming the accuracy of each; and where there is any difference, 

 they will at least point out those circumstances which require 

 further investigation. 



P. S. The experiments to which I have alluded in p. 16G — 167, 

 made since this article was written, seem to indicate that this 

 anomaly, as well as the circumstance there mentioned, may be 

 occasioned by the daily varying intensity of the opposing magnets. 



* I am sorry I have not the necessary apparatus for repeating Morichini's experi- 

 ment on the violet ray ; but I would suggest to those who have, that the finest test to 

 which this experiment could be submitted, would be to make use of a needle neutralized 

 as above desciibed, by which the magnetic property of the ray, if it possessed any, 

 could not fail of showing itself. 



