SIR W. THOMSON ON THE MAGNETIC DIP. 291 



before they can escape to propagate their species ; and it 

 certainly ought to be part of a qualification for a forester 

 or park-keeper that he should know the life-history of these 

 wood-boring beetles, and the plans that have been adopted 

 for their destruction. 



XXIV. On a New Form of the Dynamic Method for 

 Measuring the Magnetic Dip. By Sir William Thom- 

 son, M.A., D.C.L., F.E.S., &c.. Honorary Member of 

 the Society. 



Eead April i6th, 1867. 



Seven years ago an apparatus was constructed for the 

 natural philosophy class of the University of Glasgow for 

 illustrating the induction of electric currents by the motion 

 of a conductor across the lines of terrestrial magnetic force. 

 This instrument consisted of a large circular coil of many 

 turns of fine copper wire, made to rotate by wheelwork 

 about an axis, which can be set to positions inclined at all 

 angles to the vertical. A fixed circle, parallel to the plane 

 containing these positions, measured the angles between 

 them. The ends of the coil were connected with fixed 

 electrodes, so adjusted as to reverse the connexions every 

 time the plane of the coil passes through the position per- 

 pendicular to that plane. When in use, the instrument 

 should be set as nearly as may be in the magnetic meridian. 

 The fixed electrodes being joined to the two ends of a coil 

 of a delicate galvanometer, a large deflection is observed 

 when the axis of rotation forms any considerable angle with 

 the line of magnetic dip. On first trying the instrument 

 I perceived that its sensibility was such as to promise an 

 extremely sensitive means for measuring the dip. Ac- 



