S62 Mr Barlow on the Diurnal Variation 



Here, referring to fig. 2. and repeating the calculations already indi- 

 cated, we find, 



PL = 16°46' angle PLM = 124° ZQ = 54°51' 

 PM= 4 angle LPM= 10 23' Qm == 25 49 



The two latter, converted into time, give 3^ 40"™ p. m. for the time 

 the sun is on the ma^etic meridian of Port Bowen, and 10^ 40"" a. m. 

 for the time when he is perpendicular to the same. 



It is remarkable, that these times are nearly the reverse of those in . 

 London, and cannot, therefore, but furnish an excellent test of the hypo- 

 thesis in question. According to these the needle ought to have its 

 greatest westerly variation, ^vhich is, however, due to an actually east- 

 ward motion at 10^ 40™ a. m., and its least westerly at lO'^ 40™ p. m., 

 whereas the times stated from a mean of all the observations is 1 1^ 49™ 

 A. M. and 10^ 1™ p. M., which is as close an approximation as can be 

 expected. Again, according to this calculation, the time of the sun 

 traversing the magnetic meridian, when the needle ought to be found 

 in its natural meridian, is 3^ 40 p. m. and 3^ 40™ a. m. And the 

 time stated of the needle passing what is called the mean daily zero is 

 &^ 13™ A. M. and 4^ 37™ p. m., and as this time was merely an ap- 

 proximate assumption of the mean meridian by taking it at half the ex- 

 treme range for the day, it is far from being unsatisfactory. 



The times of greatest and least intensity of the horizontal needle are 

 not so well defined as some of the other points. By referring to the 

 table we have given, it will, however, be found to be greatest from 

 about three or four o'clock afternoon to about six or seven ; and least 

 at about the same hours in the morning ; and, according to what we 

 have computed, these effects should have taken place at 3^ 40™ p. m. 

 and A. M. precisely. 



Upon the whole, therefore, I conceive that the experiments, obser- 

 vations, and computations, are very consistent with each other, and 

 afford a strong presumption in favour of the hypothesis, particularly 

 when it is observed, that the illustration has been given wholly with 

 reference to the sun being in the equator ; and that all the circum- 

 stances of time, &c., will be different when the sun has either north or 

 south declination, and thereby produce that kind of uncertain and variable 

 results, which are so strongly marked in the general table. There can, 

 moreover, be no doubt, that all the phenomena are modified by other 

 circumstances, besides those of the sun's motion ; and that, although 

 the latter is the great primary cause, it is not the only one which is in- 

 fluential in producing the changes we have been endeavouring to ex- 

 plain. It may be proper also to add a few words with regard to the 

 amount of daily variation in different latitudes. 



Lieutenant Foster says, that he conceives a mean radius of about 

 2' or 2^' for the orbit of the daily motion of the pole, will agree very 

 well with the quantity of daily variation observed in different latitudes. 

 We woiUd, however, rather say 2^' or 3' ; and assuming this, let us 

 examine how nearly the observed and computed quantities agree with 

 each other. According to this, the daily variations at the magnetic 



