529 ; 
against the hard agate plates, and hops upon them, thus altogether de- 
ranging the adjustments which the frame was designed to secure. If, 
on the other hand, it be lowered too slowly, there are two sources of 
mischief. The two ends of the axis do not reach the agates at strictly 
the same instant; and in the intervening moment one end of the axis is 
still resting in its Y, while the other is rolling on the agate plate, and 
disturbing the adjustment. But the chief source of error seems to arise 
from the axis receiving a slight brush from the Ys in parting from 
them. This arises in a variety of ways, and can be reduced to its mini- 
mum only after some practice. There are some positions of the needle 
in which, upon some days, the effects of this friction can, by careful 
manipulation, be reduced to an insensible amount ;* but there are other 
positions of the needle in which I found it impracticable, by any care 
or patience, to evade it; and the slight alteration of the proportions of 
the parts, due to the differences of temperature from one day to another, 
is enough to make it sometimes very troublesome in almost all positions 
of the needle. These remarks will help to show that which I am anxi- 
ous to insist on, that this part of the instrument deserves all the care 
the maker can bestow on it. 
As an illustration, I will transcribe two complete observations taken 
at the same station on consecutive days,—the first of which happened to 
be more than usually free from this source of error, and in the second of 
which it was particularly troublesome. 
Parsonstown (Lord Rosse’s Demesne); August 24,1858; from 22 to 34P.M. 
Poles Direct. Poles Reversed. 
A. North End. B. North End, 
E. Limb. W. W. Limb. E. 
70° 24’ 109° 60’ 109° 44’ 70° 4 
23 59 44 5 
24 60 44 5 
31 60 51 18 
30 60 51 18 
30 60 52 lV 
69° 43’ 109° 32’ 109° 58’ 70° 16 
43 32 59 15 
43 31 59 14 
53 36 59 21 
53 36 59 20 
53 35 58 20 
Mean of Means or Dip = 70°- 10'-44. 
Dip corrected for daily range, 70 - 11° 4. 
* T.e. insensible in an instrument reading only to minutes. 
K. I. A. PROC.—VOL. VII. 4E 
