ON THE MAGNETIC INTENSITY OF THE EARTH. 9 
~ The observations of the voyage of 1818 were published in 
the Phil. Trans. for 1819; those of the voyage of 1819-20, 
partly in the appendix to the narrative of that voyage, and 
partly in my work entitled Pendulum and other Experiments, 
published in 1825. In these publications the results were 
deduced without any corrections having been made for the 
arc of vibration or the temperature of the needle. On this oc- 
casion I have introduced both these corrections. That for the 
arc has been computed by means of the table published in the 
Voyage de Dentrecasteaux, which I find to reduce the vibrations 
in the different arcs so nearly to an equality as fully to justify 
its employment. The arcs themselves are stated in the printed 
record of the observations. ‘The temperatures on the different 
days of observation are taken from the record of the external 
thermometer in the Meteorological Journal, and the corrections » 
are computed by the usual formula for that purpose, in which 
the coefficient ‘0004 has been determined by experiments with 
the same needle in high and low temperatures. 
In the voyage of 1819-1820 I furnished myself, besides the 
dipping-needle, with three horizontal needles, and an apparatus 
for their vibration. ‘These would have been of great use had it 
been our good fortune to have returned to Europe by the way of 
the Pacific; but the method of deducing the total intensity by 
meansof horizontal needles almost ceases to be available in coun- 
__tries where the dip so nearly approaches 90°, and where small 
incidental errors in the determination of the dip will so greatly 
affect the conclusion as to the force. Accordingly, I have at 
no time brought the observations with the horizontal needles 
in this voyage in comparison with the results given by the dip- 
ping-needle. ‘There is, however, an incidental purpose of some 
value which they may serve, which did not occur to me when 
the record of the observations was printed, and which is worth 
noticing, as it may be useful on similar occasions, should there 
be such. The horizontal vibrations, though inappropriate in 
such circumstances to furnish the total intensities, give as cor- 
rect measures of the relative values of the horizontal component 
the Appendix of the second Polar Expedition. From the circumstance of the 
narrative and appendix of that voyage having been published at an interval of 
_ some months apart, the copy of the narrative which reached M. Hansteen was 
unaccompanied by the appendix, which it seems he has never seen. The abs- 
tract of the results, published in another work from whence he has taken them, 
refers to the full record of the observations in the appendix, and omits their 
dates, and Mr. Hansteen has consequently been at a loss to know whether the 
vibrations were observed both before and after the voyage of 1819—1820. By 
consulting the original account, he will see that this necessary care was not 
omitted, 
