Page:Popular Science Monthly Volume 34.djvu/323

Rh comes the little turn—in the genial tropics, where it can point out steadily and safely the path to any port.

The necessity of frequent observations for determining corrections to be applied to the compass is, therefore, evident. + The series of total deviations is generally divided into two principal parts, the quadrantal and the semicircular: the first taking its name from the fact that it arises, reaches a maximum, and again reduces to zero, all within an angular space of 90°; and the second, for a similar reason, because of its origin, growth, and decline being confined to 180° of the circle.

Frequently, means are provided for opposing the magnetism of the ship by other powerful magnets, thus permitting the needle



to point in its natural direction, however the ship may head. Such a contrivance is known as a compensating binnacle, shown in Fig. 17. Before compensation, let the needle point in the direction N' S'. A portion of this deflection is the quadrantal deviation, due to the soft iron in the ship; it is overcome by placing two large cast-iron spheres, Q and Q', at suitable distances from