18 EEPORT OF THE SECRETARY. 



inclined to the horizon, at all places north and south of the magnetic 

 equator, which direction is called the magnetic dip or inclination. 

 The force with which the needle is drawn into this position may bo 

 resolved into two forces — one in the plane of the horizon, and the 

 other perpendicular to this plane. By knowing the horizontal com- 

 ponent, and the angle of the dip, the total force may be readily cal- 

 culated, and as this element can be much more accurately determined 

 than that in the line of the dip, it is the one which is generally made 

 the object of observation. 



In the case of the observations made by Professor Bache, the in- 

 strument employed was one of Gauss's large bifilar magnetometers, 

 which consisted of a magnetic bar Aveighing twenty-five pounds, 

 upwards of thirty-six inches long, and suspended horizontally by two 

 long fine wires slightly diverging from parallelism. This magnetic 

 bar instead of being allowed to take a north and south position, or to 

 settle in the direction of the magnetic meridian, was forced to turn 

 nearly at right angles to this position by twisting the pair of suspen- 

 sion wires so as to overcome the directive force of the magnetism of 

 the earth. In this position the bar was in a state of equilibrium 

 between two forces, viz : the torsion of the wires tending to 

 turn the north end of the needle round towards the west, and the 

 horizontal magnetic force of the earth which tended to draw it back 

 into the meridian. In this condition, if the magnetism of the earth 

 diminishes, the force of torsion will prevail, and the bar will move 

 from the meridian. If the magnetism of the earth increases, the 

 torsion will be relatively weaker, and the bar will move in an opposite 

 direction. Attached to this bar was a mirror which, reflecting the 

 image of the divisions of a scale into the object glass of a telescope, 

 enabled the observer to perceive a variation in the intensity of the 

 force, equivalent to a ten-thousandth part of the whole force. 



The indications of this instrument were corrected for variations in 

 magnetism produced by changes of temperature, and for a constant 

 small diminution in the intensit}' of the bar, from an actual loss of its 

 magnetic force. But besides these changes in the magnetism of the 

 bar itself, there is a progressive change in the horizontal component 

 of the earth's magnetism, which may be due either to a change in the 

 direction of the force, or to a change of its intensity. After allowing 

 for these, from observations made in various parts of the earth on 

 changes of direction and intensity, the next step was to separate the 

 large disturbances, which have been called magnetic storms, from the 



