360 Mr Barlow 07i the Diurnal Variation 



of tlie parts of the earth successively exposed to its influence. And, 

 moreover, it seems by no means improbable that the annual variation 

 of the position of the magnetic pole, may ultimately be traced to the 

 same universal cause. 



" I have not attempted to enter into any minute calculations on this 

 subject, but I believe it will be found, that, if the radius of the circle, 

 described by the pole of the general magnetic axis of the earth during 

 the day, be supposed to subtend at the centre an angle of 2 or S* mi- 

 nutes, it will reconcile, to a considerable degree of precision, nearly ali 

 the observations on the daily variation of the direction, and daily change 

 of intensity of the horizontal needle, made both in Europe and with- 

 in the Arctic Circle." 



In order to illustrate the very ingenious hypotheses which Lieute- 

 nant Foster has thus deduced from the experiments last reported, it 

 will be best to refer to Fig. 1. In this, P represents the terrestrial pole, 

 M the magnetic pole of any place L, of which the dip and variation 

 are given ; Join M L, which is the magnetic meridian of the place L, 

 and produce M L to /, m Q L representing the equator ; as also, P I-. 

 be produced to meet the equator in Q, then MLQ will be the me- 

 ridian of the place L ; and from M draw Mm, perpendicular to ML /. 

 Then, confining our illustrations to the time when the sun is in the 

 equator, the arc Q /, converted into time, will give the time when the 

 sun is on the magnetic meridian of the place L ; Q ?w, converted into 

 time, will give the time when the sun is perpendicular to that meridian, 

 and, of course, from these may be readily determined the time when 

 the sun is again on the magnetic meridian, and when again he is per- 

 pendicular to the same ; let us, therefore, go through the necessary cal- 

 culations, and see how nearly the several phenomena which have been 

 recorded, agree with the hypotheses in question. 



First, let L represent London, Lat. STST, dip 70°34', Long 0°0'. 



TT • V o ,1 tan 70°34' ^^o^o. 

 Here, since tan dip = 2 tan mag. lat. we have — ^ ^^ 54<°4o' 



mag. lat. conseq. ML = 35^12' PL = 38°29', and PLM = 24^30' 

 variation. 



From these we readily find MP = 14t'58' colat. of magnetic pole, 

 and angle LPM = 67"41' =-. long, west of magnetic pole. In the tri- 

 angle LQ/, right angled at Q, we have LQ = 51°31', and the angles 

 QL / = 24°30', whence we find IQ= 19°38', which, converted into 

 time, gives about 10*^ 40"^ a. m. for the time when the sun is on the 

 magnetic meridian of London ; and, in a similar manner, we - find 

 Qm = 55^24' equal to about 3'^ 40™ p. m., the time when the sun is 

 perpendicular to the magnetic meridian of London. 



These being determined, let us proceed to a comparison of recorded 

 phenomena with the hypotheses in question. Lieutenant Foster con-" 

 ceives, that the sun, by some influence or other, causes the magnetic 

 pole M to describe, in the course of 24 hours, a little circle about the 

 mean point M, whose diameter is about 5' or 6', the pole bein^ con- 

 stantly deflected towards the sun. Now, first, according to this sup- 

 position, at 10*^ 40"^ A. M. M will be deflected towards L, the magne- 

 tic colat. will be diminished, and, consequently, the dip increased ; and 

 this increase of dip will induce a less magnetic intensity on the hori- 



