CHAP, i.] THE COMPASS. 527 



of the needle should remain constant, which is sufficiently the case 

 during the ordinary time occupied in topographical operations. Such 

 is the principle of the employment of the compass in land surveying. 

 But the measurement of the angles by this means is not sufficiently 

 exact, if it must be within half or a quarter of a degree ; the oscilla- 

 tion of the needle, which makes it difficult to read the angle, and the 

 diurnal and irregular variations of the declination, which are some- 

 times considerable, are the principal causes of this defect in precision. 

 Compasses have been constructed for the purposes of military 

 reconnoitring, which do not give even so good an approximation as 

 this, for the very simple reason that instead of being fixed they are 

 only held in the hand in making an observation. We only mention 

 them to call to mind this application of the declination compass. 



II. DIP CIRCLES. TERRESTRIAL MAGNETISM. 



The dip circles, or inclination compasses, have for their object 

 the measurement of the angle which the magnetic needle makes 

 with the vertical of the place. Since this element is only susceptible 

 of application in physical researches on the earth, we shall confine 

 ourselves to describing succinctly the dip circle adopted in magnetic 

 observatories. 



A metallic divided circle, in the centre of which a magnetic needle 

 is suspended so as to turn freely in the plane of the edge ; another 

 circle similarly divided, and supported on a stand with three levelling 

 screws such are the two principal parts of the apparatus represented 

 in Fig. 341. By means of a spirit level the second circle may be 

 placed in a perfectly horizontal plane. In this case the first circle, 

 which is perpendicular to the other, is vertical. It can also turn 

 with its support about the axis of the instrument, and allow the 

 needle to be placed in the magnetic meridian a sight- vane movable 

 with the support serves to fix the position. 1 In this position the 



1 This position may be found if to the apparatus we adapt a declination compass. 

 But this is not required, since all we need do is to find the position of the vertical 

 circle in which the needle at rest is vertical. The magnetic meridian makes an 

 angle of 90 with this position. By turning the vertical circle through 00, we 

 know then that we have placed it in the magnetic meridian. 



