INCLINOMETER. hi 
Cot 276 = cot 2 + cot 27 
Azimuth, 0° 7 = 71° 26°75 Azimuth, 90° o = 89° 5725 6 = 71° 26°75 
seseeeeee 80° 9 = 73° 25°50 eee eee 120° of = 80° 27°56 = 6 = 71° (7-78 
Sain 60° 7 = 80° 22':25 veseeeees 150° of = 73° 30°62 386 = 71° 9°65 
The dip from the observations in the magnetic meridian differs considerably 
from the values of @ obtained from the observations in other planes; the latter agree 
pretty well with those deduced from needle No. 1, April 18, 1843 (Introduction, 
1841-42, Table 22, first series.) This coincidence of the results from two needles 
seems to place the source of error in the metal of the instrument, rather than in the 
axles of the needles. 
March 31, 1846. The vertical circle of the inclinometcr was removed from 
the instrument and placed horizontally, the dip needle, No. 1, was suspended by a 
silk fibre within the circle, the needle and circle being in the same plane, the needle 
was then vibrated horizontally, the zero of the graduations being placed in different 
azimuths ; taking the zero on the right of the horizontal diameter (the circle being 
in its usual vertical position) as the commencement of the graduations, and counting 
downwards and onwards to 360, the following are the means for different positions 
of about 80 vibrations (commencing with a semiarc of 18°, and ending with a semiare 
of 6°) :-— 
N. end of needle at graduation, 50° Mean time of one vibration, corrected for are, 6-257 
decosccesssccescuansssennencisercessesiccs 30° seueteenaeencencscscecceccncccserssesesconsceassecnscoecasses 6-282 
cee eee eneceeesensesenceseseeensores senses 850° tence ceeanecencancestssccnaseceeceuenerssovesscosscsecceses 6:275 
wt ae ee eceecepecaeesensnsnesseesesscsssates 320° ven teeeeeeeeeeetee cen seusesaeessncsnsaseuscescsutasacsersses 6-278 
SOHC BORER EESERAE Cc) 00 2090 SoS SqeeBE sec ge 290° SIC CU Stn CERO Ho etig ones Sone so SBepnacos gos eenee ears 6:278 
Bee lcinsis o0\s «> a(e ree eee onsale we 260° aaleicis-sohions(ahaen|ssslanicleslslicicasleeceesneeeusisseness=sceesve 6 1° 4.8 
The time of vibration varies little with the exception of that for the first posi- 
tion ; the last two means are for positions occupied by the needle in the observations 
of inclination, given above. 
84. The true dip, and the cause of the differences in different azimuths, is still 
a question. No correction has been applied to the observations in this volume. The 
difficulties noticed in connection with the lifter in the Introduction for 1843 were 
experienced more or less in 1845 and 1846, but considerably less than in 1844.* 
* Observations were made in the following manner for the determination of the magnetic dip 
in February 1846. The dipping needle having been placed on its supports in the inclinometer, it was 
deflected by a magnet placed at known distances, in order to determine the ratio of the magnetic 
moment of the deflecting bar to the vertical component of the earth’s magnetism ; the moment of the 
bar was obtamed from observations of deflection and vibration for the absolute horizontal intensity, 
whence the vertical component could be determined, and the dip from the ratio of the two com- 
ponents. The advantage of this method over others, consists in the capability of using a powerful 
deflecting bar whose moment can be determined with the accuracy of the observations for the hori- 
zontal intensity. 
