32 SEVENTH REPORT—1837. 
pressed in geographical miles, and 0 the difference of latitude 
also in geographical miles. 
If we combine the five equations so formed for the five dip 
stations by the method of least squares, giving each equation a 
weight proportioned to the number of observations which it re- 
presents, we obtain by the usual process of summing and eli- 
mination 
§ = 68° 42’; r = — 0013608, 
the latter being equivalent to 75°5 geographical miles to one 
degree of dip. With these we may compute the dip for each of 
the horizontal stations ; and having the values of the horizontal 
component we may deduce the total intensity. ‘The dips and 
intensities for the North American stations in Table 3 are thus 
computed. 
Mr. Douglas mentions that the dip he observed i in the crater 
of Kiraueah was 2! greater than at Byron’s Bay; I have there- 
fore entered it in Table 3 as 38°00’. The dip at Oahu is from 
Capt. de Freycinet’s observations at the adjacent island of Mowi, 
and must be regarded as uncertain for Oahu to some minutes ; 
but in so low a magnetic latitude an error of that amount would 
have very little influence on the calculation of the intensity. 
The horizontal intensity at Oahu was very well determined, the 
four needles being employed, a few months only after their vi- 
bration in London, 
Fitz Roy, 18351-1836.— We come next to a series which must 
rank amongst the most important contributions to magnetical 
science, and which we owe to Capt. Fitz Rey, R.N., and the of- 
ficers of H.M. ship Beagle, employed in the years above-men- 
tioned in the survey of the coasts of South America, and in a 
voyage of circumnavigation performed chiefly in the southern 
hemisphere, having for its primary object the determination 
of differences of longitude by a number of chronometers. 
Capt. Fitz Roy had the precaution to furnish himself with a 
dipping needle of Gambey, whose instruments of this kind, 
though not always without fault, are universally acknowledged ~ 
to be the best that are made, and superior to those of our own © 
artists inmodern times. For the intensity he received fromCapt. — 
King the horizontal needle with which that officer had been © 
supplied by M. Hansteen. This needle, which in Capt. King’s 
voyage had lost from time to time considerable portions of its — 
magnetism, appears to have very nearly attained a permanent — 
magnetic state when Capt. Fitz Roy received it. By observa- _ 
tions at Plymouth in 1831 and 1836, and at Port Praya in 1832 
and 1836, its time of vibration is shown to have varied to a very 
