512 



EXPEDITION TO POINT BARROW, ALASKA. 



ary maximum about 6 a. in. as exhibited at Toronto, and more strongly at Philadelphia at 5J a. 

 m. where it constitutes the principal maximum, the secondary occurring at 4 p. m. The maximum 

 at Toronto takes place between 4 and 5 p. m. and the minimum about 10 a. m. 



The diurnal inequality in the whole deflecting force acting in the horizontal plane may bo 

 exhibited graphically both in direction and magnitude as in the annexed diagram. 



The origin of the co-ordinates represents the normal declination and horizontal force, and any 

 line drawn from it to any part of the curve will represent in direction and magnitude (according 

 to scale of diagram) the deflecting force acting at the time as marked against that point. If for any 

 time the angle y equals the westerly deflection of the horizontal needle the deflecting force produc- 

 ing the same is IT sin y, and when expressed "in parts of the horizontal force simply sin </. A deflec- 

 tion of v minutes corresponds to ,- or 0.000291 y, parts nearly. The table of the solar-diur- 



64:61. 7 



nal variation of the declination contains the values of y for every hour of the day, and the corre- 

 sponding change in the force at right angles thereto is contained in the preceding table of the 

 variations of the horizontal force ; these two components, the westerly and northerly, appear com- 

 bined in the diagram. It will be seen that the disturbing forces act more energetically in a plane 

 approaching closer to the true than to the magnetic meridian, and that the usual character of the 

 representation is changed by their action, that half of the curve containing the hours 21 (9 p. in.) 

 to 2Ja. m. being thrown far to the westward, forming a loop, and beyond the branch containing noon ; 

 on the other hand, the great extension of the deflecting force between 7 and 8 a. m. is wholly due 

 to the great activity of the easterly disturbances about these hours. This will become clear when 

 the disturbances have been separated from the normal deflecting forces, and a diagram for the 

 latter alone is presented. 



DIURNAL VARIATION IN THE WHOLE DEFLECTING FORCE ACTING IN THE HORIZONTAL PLANE. 



[The intensity of the total horizontal deflecting force is expressed in parts of H and all its disturbances are included.] 



o 



o 



V 







So c 

 (q i* 



o fl ei 



Wcaurly deflcctiry farce. 



S 



I 



