168 Attempt to facilitate Observations of 
and differencing and dividing, /, being considered as constant, 
i 
P83 tan 3. 
? 
Loe 
Now, confining our consideration to the error arising from friction, dd=— = ¢; so 
o 
that the /imit of error in the determination of the force, in the usual method, is <’ tan é ; 
while, in that now proposed, it may be reduced to. ‘The limit of error in the 
common method, therefore, is to that of the method now proposed, in the ratio of 
the tangent of the dip to unity ; that is, in our latitude, as 2.75 to 1, nearly. 
In the instrument in my possession, constructed by Mr. Roginson, of Devon- 
shire-street, London, I consider the limit of error arising from friction, in the ordi- 
nary position of the needle, not to exceed 5’. With this instrument, therefore, 
¢ =5'=.0015, nearly, radius being unity; so that the error in the determination of 
the force does not exceed the .0015 part of the entire quantity. 
In order to satisfy myself more fully as to the accuracy of this method, the follow- 
ing series of observations was taken. The position of the needle (Needle I of the 
preceding experiments) was observed, first without any counterpoise, and secondly 
with a counterpoise which brought it into a position nearly perpendicular to the line 
of the dip. ‘The observations were taken in the same spot of the room, and as nearly 
as possible at the same temperature ; the magnetism of the needle was not interfered 
with during the observations. 
TABLE IIL. Containing the results of Four Observations taken with Needle I. in 
the Philosophy School of Trinity College. Therm. = 61°.5. 
! 
| oe | I. E marked side Ww | II. E marked side Ww 
ple ————oe ————— a eo 
<4 ls s N s | oN s N s 
(Ef eM% OF WUABofy TI 1B FI Wi pomislo MMO) 7A oMoweT 
é- 
lw! 70 35 70 30 | 70 25 70 22) 70 30. 70 22 | 70 80 70 28 
#510 -sdvsavelso opis pasts inlay oleliy! (7 paws Ise 
6 
= —19 40 —19 37 | —18 15 —18 5|—19 40¥—J0 .45"|)'=1s8" 16°=18 
[Mean | $= 70° 51.8, | 9=—17° 37'.4 8 = 70° 50; 0=—17° 32’ 
Liee.3 ill. E marked side WwW DV; E marked side Ww 
Sue — a SSS aa 
}<H | oN S N Ss N iS N s 
| — . 
8g To O° “MOM eis” Tie Sor me a! 7 IS) zi 1 ee 
| tw) 70 30 70 25 | 70 30. 70.25) 70 30 70 22 | 70 30 70 30 
| 52-16 65°=17...00 | =15 15 +15 -17 22-17 35 | —15 20 —15 20 
0 
Uwi-l9 30 —19 30 | —17 45 —17 45|-19 45 —19 45 |-17 55 —17 55 
Mean | § = 70° 50'.6, @—-17° 20.9 $= 70° 50.2, @=-17° 371 
