523 
‘The observation of the true magnetic dip does not depend upon any 
difficulty in eading the position of the needle when it is in equilibrium ; 
but only upon the question, if when the horizontal axis has been elevated 
from the agates, it always returns to the same points of division. With 
large needles, like those of Gambey, with good lenses opposite both ends 
of the needle, when the axis has been elevated from the agates, and the 
needle makes oscillations of 30, 40, or 50 minutes, I observe three conse- 
cutive extremes,—a minimum a, a maximum f, anda minimum y. Each 
of these can be observed correctly to a minute. The true position in 
m ; dy 8 |. In the oscillation of the needle, and 
its mechanical moment, every obstacle, such as alittle friction and small 
irregularities in the pivots of the axis, is overcome. Observing the same 
three extremes at the other end of the needle before a new elevation of 
the axis, I then take the mean of these two means. In the same position 
of the needle, I repeated this formerly four times; so that in the eight 
different positions of the needle and limb I had thirty-two readings of 
each end of the needle. As I found by long experience that the four 
means, in every position, differed very little from each other, I have in 
the latter two years diminished them to three. 
‘* With your little circle, with microscopes, it is necessary to wait 
till the needle is guwie¢, and to direct the wire to the upper point of 
the needle, and read both verniers, then to the lower point, and read 
again. But when the needle makes only an angle of 3, 4, or 5 minutes 
with the true position of equilibrium, its moment is so weak, that the least 
effect of friction, and irregularity in the pivots, can stop it in a false po- 
sition. I therefore regard the microscopes as unnecessary, and even inju- 
rious. It is true that a correct dip depends also upon the skill and expe- 
rience of the observer; and I confess that for a dilettante, who makes 
an observation for the first time, the microscope may be agreeable, as it 
gives an imaginary correctness. 
“The above-cited facts of the daily variation, observed with Gambey’s 
instrument, without microscopes, are a clear proof of this. I have not 
seen that any observer with the small Barrow has detected the daily 
variation, and its gradual increase from the winter to the summer sol- 
stice. 
_ ‘From April, 1859, there has arisen a period of great perturbations, 
with flashings of polar light (Aurora Borealis), and great spots in the sun. 
The daily variation is still always positive, but extraordinarily great: 
it has once amounted to 1° 2’; and the variations of the Bifilar have been 
great in proportion. As the latter instrument is always observed by 
an assistant twice in the same hour with my observations of the dip, 
I will give some examples :— 
equilibrio is then 4 
