340 Mr. Barlow’s observations and experiments on the 
In order to examine the first of these cases, I measured 
very carefully the distance, direction, &c. of the compass 
and magnets while in the garden, and placed them in pre- 
cisely the same relative situation in the parlour ; still the 
motion in the two cases was reversed. 
To examine the second, it occurred to me that if the di- 
rection of the motion depended upon that of the light, the 
needle ought to be wholly stationary in the dark, or when 
excluded from the solar rays. I therefore kept my room 
shut for two days, and only examined the needle by the light 
of a wax taper ; but although there was certainly less mo- 
tion on those days than usual, yet I could come to no satis- 
factory conclusion ; but I still think that farther observations 
will show that the solar 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 mo- 
* I am sorry I have not the necessary apparatus for repeating Morjchini’s 
experiment 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 described, by which the magnetic property of the. ray, 
if it possessed any, could not fail of showing itself. 
