341 



net 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 1^ a.m. and 7'' p.m., the change for May, 

 June, and July, from 6''-8" a.m., being + 3°-80. and from 6^-S^ 

 P.M., being —3-54 ; for the same months, the mean angular motion 

 of the needle from 6'>-8»'a.m. =1'00, from 9'"-ll»' a.m. =2'-12, 

 and from 6''-8'' p.m. =0''74. There is a diminution in the velo- 

 city of the motion between l*" 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. 



It may be remarked that the line representing the astrono- 

 mical meridian, and passing through the centre of gravity of the 

 perimeters of the figures, for the months during which the sun is 

 north of the equator, also passes through the position of greatest velo- 

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



General Form and Turning Pouits of the Diurnal Motions. 

 — The general forms of the diurnal motion vary between rude el- 

 lipses 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 remarked, that the turning from the farthest westerly posi- 

 tion 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 tempera- 

 ture, and the eastern turning point. If, however, we examine the 

 figures indicating the diurnal motions of a needle in its frwe position, 

 such as those for the months of April, August, October, &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 extremi- 

 ties of the horizontal diameters of these rude circles, or at the extremi- 

 ties of a horizontal line, in the ruder ellipses, should be chosen, in 

 preference to the turning points at the extremities of other lines 

 drawn in the figures, as tests for a theory ; unless, indeed, it be ex- 

 plained by the accident that a horizontal suspension of a magnetic 

 needle, is a convenient one for observing a certain portion of the 



