358 Remarks on an Error in a Table 



We have seen that all experiments confirm the fact, first no- 

 ticed by Captain Flinders, that although the effect of local at- 

 traction is the same in quantity in north and south latitudes, 

 where the dips are equal, yet it is directly opposite in direction, 

 the north end of the needle being in one case attracted by the 

 vessel, and in the other the south. For example : Suppose a 

 ship's head to be east, at the Norc, and that the local attraction 

 of the vessel at the maNimum is half a point, then the north end 

 of the needle will be drawn half a point from its true bearing to- 

 wards the east. Now, at Port Jackson the dip is as nearly as 

 possible the same as in England; but it being the south end that 

 dips, the effect will still i)e half a point the contrary way ; that is, 

 the north point will now be deflected half a point to the west, 

 the ship's head being east as before. 



This contrary effect has place in both cases till we arrive at 

 the equator, where according to Captain Flinders it vanisiies en- 

 tirely; but according to Dr. Young's table (although the effect 

 must change as we cross the magnetic equator) it does not va- 

 nish, being at the equator itself equal to about one half v. hat it is 

 in England. This was the difficulty of my young friend, and it 

 is certainly one which I am unable to explain ; and I will add 

 that it is a result absokuely impossible in itself, and contrary to 

 that principle maintained by all philosophers, that the operations 

 of nature are never made per saltns. To ])ut the case in a 

 stronger light, let us suppose three vessels, whose local attractions 

 are equal, the maximum of each being half a point at the Nore: 

 then according to Dr. Young's table, the attraction of each at 

 and very near the magnetic equator will be about 3" Let us 

 suppose these three vessels to be a mile distance from each other, 

 one being on the eqtiator and the other two a mile on each side 

 of it, the one in north and the other in south magnetic latitude; 

 and let us further suppose, that they are all sailing due east : 

 and that each takes an azimuth observation at the same instant. 

 No one can doubt for a moment that these azimuths will be the 

 same in each vessel, and let us suppose that they are such as to 

 give the computed variation 10°. , If now each vessel correct 

 her variation according to Dr. Young's table, the one in south 

 latitude will make her corrected variation 10" — 3" = 7° west, 

 and the one in no-th latitude will make hers 10" + 3° = 18" 

 west, a difference of G'^ : while the vessel on the equator will 

 have no guide to know in what direction her correction is to be 

 applied. The case which we have here supposed is by no means 

 one advanced for the purpose o exaggerating the amount of 

 the error ; it is a case that would actually occur in sailing from 

 Pernambuco to the Congo river, and it is not improbable might 

 be the means of running one of the vessels down immediately on 



' the 



