INTRODUCTION. 
345 
Collecting in one view the different mean results, we have 
Ross, in 1840-41, 15 in. magnets 4*573 
Kay, in 1841, 15 in. magnets 4*553 
Kay, in 1842, 15 in. magnets 4*513 
Kay, in 1843, 12 in. magnets 4*520 
Kay, in 1843, 9*18 and 7'50 in. magnets 4*501 
Kay, in 1844-45, magnets of various lengths, 9*18 to 2*45 in. 4*499 
These results exhibit (with one exception) a progressive decrease, but between 
those of 1840-41, and subsequent years, there is a very great difference. The incli- 
nation has decreased from —70° 40'*7, observed in 1840-41*, to —70° 37 , ‘6, which is 
the mean of the results obtained twice in each week at the Hobarton Observatory in 
the first nine months of 1845. Assuming the total force at Hobarton as constant, 
the horizontal component should have been increased rather than diminished by the 
small secular change which appears to have taken place in the Inclination. The 
discrepancy between the earlier and later results of the absolute determinations 
cannot therefore be a consequence of secular change in the Inclination ; nor is it 
probable that the total force should have undergone a decrease of such magnitude. 
Presuming the results of 1840-41, with the 15-inch magnets, to have been affected 
with error from some cause as yet unexplained, (possibly from an erroneous value 
having been taken for the moment of inertia of the magnet,) the subsequent results 
exhibit only such differences as cannot be regarded as excessive. They have all to 
undergo recalculation, as Lieut. Kay does not consider the elements of reduction as 
yet finally determined ; and they will all, in common with all the other determina- 
tions of the absolute horizontal force given in these Contributions, have to receive a 
small correction for the difference of the magnetic moment of the deflecting bar, 
caused by the earth’s inducing action in the different positions in which the bar is 
placed in the experiments of deflection and vibration. If, therefore, we assume pro- 
visionally the mean of the four last results, or 4*508, as the best approximation to 
which we have yet arrived for the horizontal component at Hobarton, and —70° 39' 
as the corresponding Inclination, we have the total force in the arbitrary scale 1*797 ; 
and we may hence conclude, that influenced by the earlier determinations (those of 
1840-41), the provisional value of the total force at Hobarton, employed in the Vth 
and Vlth Numbers of the Contributions (1*82), was taken too high, and that all the 
values of the force dependent on Hobarton will require a correction to be applied, 
in amount about —0*02, before they are combined in the general map of the southern 
hemisphere. For Lieut. Smith’s and Lieut. Dayman’s observations, Lieut. Clerk 
has taken a base value of 1*80 at Hobarton. 
A subsequent number of these Contributions will contain the Magnetic Observa- 
tions of the Erebus and Terror in the summer of 1843-1844, between the meridians 
* Philosophical Transactions, 1843, p. 165. 
