APPENDIX. 153 



and that of the ship approximated towards equality in this 

 part of the voyage of the Griper. 



We are fortunately furnished with the requisite data for 

 this investigation, by the experiments on the local attraction 

 of the vessel at Green Hj-the, previous to the voyage, for 

 from these it appears that the attraction of the vessel was 

 such, (rejecting small fractions,) as to produce a deflection 

 of the needle, with the ship's head at east or west, of about 

 S°. That is, the two forces, namely, that of the earth and 

 ship, when at right angles to each other, were so related 

 that their common resultant formed with one of those forces 

 an angle of S°, and with the other an angle of S2° ; con- 

 sequently, these two forces vrere to eacli other as sine of 

 8° to sine of 82°, or as radius to tan. 82°, or as 1 to 7 

 nearly ; that is, the magnetic action of the Griper was to 

 that of the earth before the vessel left the Thames as 1 to 7. 

 But at Seahorse Point we have seen that the earth's mag- 

 netism was less effective on the horizontal needles than in 

 the Thames, in the ratio of 1 to 3 ; so that calling the 

 powers of the earth 7 in the latter place, it was only I'i 

 at the former. But besides this reduction in the effective 

 powers of the earth's magnetism, it is to be remembered 

 that the magnetism of the vessel will be increased in the 

 ratio of ^ {4.<-3 sin.' 86° 32') to aJ{4>-3 sin.^ 70°) *, or in 

 about the ratio of 100 to 1 16, so that the ratio of the mag- 

 netic powers of the iron of the vessel and of the earth, 

 were at this station to each other as 1 X 116 to 100 x 1.4 : 

 or as 116 to 140, or as 1 to 1.19, nearly, whereas at the 

 commencement of the voyage they were to each other as 

 1 to 7. 



Hence it appears that the power of the ship's magnetism 



* Essay on Magnetic Attraction, Art. 206, 



