’ 530 
Same Station, August 25, 1858 ; from 9° 40™ A. I, to 105. 
Poles Direct. Poles Reversed. 
B. North End. A. North End. 
E Limb. W. W. Limb. E 
70° 6 | 109° 43' || 109° 57’ 70° 28' 
6 44 57 30 
8 43 58 30 
17 48 56 33 
18 49 57 36 
19 49, 57 36 
* 70° 20' 109° 59 109° 27’ 69° 40’ 
18 58 28 40 
20 58 25 408 
23 57 31 51 
21 57 31 51 
24 57 29 52 
Mean of Means or Dip = 70° 12°75 
Dip corrected for daily range, 70 10° 4 
In the sixteen sets of readings which make up the observation of 
the 24th August, that least embarrassed by the injurious action of the 
Ys, there is but one case in which this discordance amounts to 2’, while 
there are four cases of complete accordance. In the eleven others, a 
difference of one minute of arc occurred. On the other hand, in the ob- 
servation of the 25th August, but two cases of complete accordance will 
be found, while the difference three times rises to 3’, and in five other 
cases it is 2’. 
The injurious effect of the hitching produced by the Ys is equally 
apparent, if, instead of scrutinizing the individual observations, we 
combine parts of different observations, so as to bring out numbers 
which ought to be identical in sets. I have done this in various ways 
with the series of observations made in the survey of the southern half 
of Ireland in 1858, and found in several instances, instead of identical 
numbers, differences of one or two minutes, and occasionally, though 
rarely, of three minutes. These differences are doubtless in part due 
to magnetical causes; but they clearly point to a mechanical origin 
also, since they lean in a marked manner towards those positions of the 
needle in which the hitching of the Ys was most troublesome. 
If anything else be necessary to show how much dip observations 
are effected by a careless design or construction of the lifting apparatus, 
it will be found in the astonishing, and to me quite unexpected, accuracy 
with which the needle will roll back to its position when in any fair 
