Combined Motions of the Magnetic Needle. Ixxiii 



159. These numbers give the following curious result ; — That the velocity of motion of the north end of a 

 magnet freely suspended in the direction of the magnetic force is a maximum when the sun makes its superior 

 transit of the magnetic meridian (between 10*^ and 1 1'' a.m.), and a minimum when it makes its inferior transit of 

 the same meridian (between lO'^and 11'^ p.m.). This result is the more curious that the epoch of the minimum 

 velocity of the diurnal motion is an epoch of maximum disturbance, and, in as far as the declination is con- 

 cerned, the epoch of maximum velocity of the diurnal motion is also an epoch of minimum disturbance. 



160. When we compare the results for the irregular disturbance, with reference to the separate elements 

 of magnetic declination and magnetic dip (see horizontal component), with the velocities of jnotion as deduced 

 from these figures, we find, that when the diurnal motion is inost rapid the departures from the direction of that 

 motion are least, and luhen the diurnal motion is slowest the irregular departures from, the hourly mean position are 

 greatest, 



161. It is scarcely possible to connect the previous facts of area, perimeter, or velocity of motion with the 

 laws of variation of temperature. In the mean for the whole year, the temperature changes most rapidly between 

 8*^ and 9^ a.m. ; but it changes with nearly equal rapidity between 6^ and 6'* p.m. There is no corresponding- 

 fact in the previous numbers. When we compare the variations of temperature with the variations of position 

 for the suspended magnet in the summer months, we find the difference between the two classes of facts even 

 more marked : in summer, the temperature changes most rapidly about 7^ a.m. and 7'' p.m., the change for 

 May, June, and July, from 6^-8^ a.m. being + 3°'80, and from 6'*-8'' p.m. being — 3°-54 ; for the same 

 months the mean angular motion of the needle from 6'^-8'^ a.m. =1'-00, from Q^-ll*^ a.m. =2'-12, and from 

 Q^_Qh p_jj_ _o'-74_ There is a diminution in the velocity of the motion between P and 2'' p.m. ; there is also 

 a slight diminution at the turning point, 6*^— 7^ p.m. and between 2*^ and 3** a.m. These diminutions appear 

 to be connected with the fact, that they occur at turning points in the figures. 



162. It may be remarked that the line representing the astronomical meridian, and passing through the 

 centre of gravity of the figures for the months during which the sun is north of the equator, also passes through 

 the position of greatest velocity, and nearly through that of least velocity, of the diurnal motion. 



163. General Form and Turning Points of the Diurnal Motions. — The general forms of the diurnal motion 

 vary between rude ellipses and circles. In the winter months, the principal portion, or loop of the figures, is 

 elliptical with the major axis horizontal ; near the equinoxes the figure becomes somewhat circular, and in the 

 midsummer months it again becomes rudely elliptical, with the major axis inclined about 20° or 30° west of 

 the magnetic meridian. In the usual investigations of the conventional element of declination, it has been re- 

 marked that the turning from the farthest westerly position occurs near the time of maximum temperature ; a 

 coincidence which has been supposed to indicate a real connection, though there is no similar coincidence between 

 the epoch of minimum temperature and the eastern turning point. If, however, we examine the figures indi- 

 cating the diurnal motions of a needle in its true position, such as those for the months of April, August, Oc- 

 tober, &c., we might find it difficult to say, where is a turning point and where not ; and it is difficult to see 

 why the turning points at the extremities of the horizontal diameters of these rude circles, or at the extremities 

 of a horizontal line, in the ruder ellipses, should be chosen, in preference to the turning points at the extremi- 

 ties of other lines drawn in the figures, as tests for a theory ; unless, indeed, it be explained by the accident that 

 a horizontal suspension of a magnetic needle is a convenient one for observing a certain portion of the motion 

 of a magnet, which, independently of gravity, would rest in the direction of the magnetic force. 



164. It may be noticed, chiefly with reference to the months from March to October, that a line passing 

 through the positions of noon and midnight also passes through, or nearly through, the mean position, or the centre 

 of gravity, each hour having equal weight : also a line passing through the positions about four hours before and 

 four hours after noon, passes nearly through the centre of gravity of the figures ; the former of these lines lies 

 nearly in the direction of the minor axis, the latter nearly in that of the major axis of the rude ellipses for the 

 midsummer months. The horizontal line passing through the centre of gravity, also passes nearly through the 

 positions of I'* a.m. and 1'' p.m., which, therefore, are the epochs of mean dip. (See also No. 162.) 



j 165. Angular Distances between the Hourly Positions from the Mean of all, and from the Undisturbed Days. 



' — It has been already stated, in considering the effect of disturbance on the hourly mean values of the magnetic 

 ' elements, that it is assumed that the mean of all the hourly values is unaffected, which, in the present case, is 

 \ equivalent to assuming, as has been done in Plate VIII., that the centre of gravity of the disturbed and un- 

 disturbed figures is the same; this must be very nearly true, as regards its position in declination (No. 38), 

 hut it is probable that there is some error with reference to its position in dip : it will be seen from No. 123, 

 that this error in the figures for May-June and July-August is very small ; it will also be seen from No. 

 123, that the dotted figures for the other months should be raised somewhat in the page, since the centre of 

 ! 'gravity of the dotted figure has a less dip than that of the continuous figure ; the effect of this elevation would 

 be chiefly to diminish the distance between the points about 4^ and 5r^ p.m. on the figures for March and April ; 



