g REPORT—1848. 
case of the magnetic declination, very nearly, by the hours 6 a.m., 2 P.M., 10 P.M. ; 
which give, moreover, the maximum and minimum of temperature, and of the ten- 
sion of vapour, nearly, and the maximum pressure of the gaseous atmosphere; and 
if we add the intermediate hours 10 a.M., 6 P.M., we shall have, nearly, the prin- 
cipal maxima and minima of the other two magnetical elements. The author ac- 
cordingly proposes, as the best hours of observation in a limited system, 
6 a.M., 10, 2 p.m., 6, 10. 
The case is different where the course of the diurnal curve has been already obtained 
from a more extended system of observations. In this case the mean of the day 
may be inferred from observations taken at any hours whatever; and the hours of 
observation should therefore be chosen, chiefly, if not exclusively, with reference to 
the diurnal range of the observed elements. 
The author proceeded, in the next place, to consider the course to be pursued in 
the reduction of a more extended system of observations (such as that prescribed by 
the Royal Society in 1839, and adopted by all the magnetical observatories), when 
some of the observations are deficient. He showed that, in this case, in deducing 
the daily means from the remaining observations, we must attend, not only to the 
elimination of the regular diurnal variation, but also to that of the irregular changes 
of longer periods, which are sometimes (as in the case of the atmospheric pressure) 
more influential in the result. With this view he determined the values of the 
mean daily fluctuation for each of the elements already referred to; and compared 
the mean values of the horary changes thence arising with that resulting from the 
regular diurnal variation. 
The author showed, finally, in what manner the monthly means of the results ob- 
tained at any hour are to be corrected in the case of deficient observations, so as to 
render them comparable with those in which none are wanting ; and he deduced the 
probable values of these corrections for each element, with the view of ascertaining 
in what cases the correction may be disregarded, and in what it is indispensable. 
Account of Experiments belonging to a new Magnetic Action. 
By Herr PLickeEr. 
A crystal with one optical axis being brought between the two poles of a magnet, 
there will be a repulsive force going out from each of the poles and acting upon the 
optical axis. According to this action, the crystal, if suspended, will take such a 
position that its optical axis is placed within the equatorial plane. When the crystal 
has two optical axes there will be the same action on both, according to which the 
line bisecting the acute angle, formed by the axis, will turn into the equatorial 
plane. When the crystal is suspended in such a way that it may freely move round 
any line whatever of the plane containing both axes, this plane will take the equa- 
torial position. Thus in a crystal which is neither transparent nor shows any trace 
of its crystalline structure, we may, by means of a magnet, find the optical axes : 
at the same time we get a new proof of the connexion between light and magnetism. 
When light is passing through a crystal, there are in general two directions where 
it is affected in a quite distinct way: these same directions are acted upon by a 
magnet. 
On an Explanation of the “ Beads” and “ Threads” in Annular Eclipses. 
By the Rev. Prof. Powe rt, F.R.S. 
The principles on which this explanation is suggested are the following :— 
1. The fact estabished by Mr. Airy, that the intensity of the sun’s light increases 
rapidly from the extreme edge of the disc to some short distance inwards. [See 
Ast. Soc. Notices, vol. v. p. 216.] é 
2. The admitted law that irradiation increases with the intensity of the light. 
On these grounds, when the irregularities of the moon’s edge have their summits 
beyond the limb of the sun, but their depressions within it, so that they leave patches 
of light, these will be enlarged by irradiation, and will be much more enlarged as the 
depression is deeper within the sun’s limb, where the light is so much more intense ; 
they will thus appear elongated in a direction perpendicular to the limb, if actually 
