THE INTENSITY OF TERRESTRIAL MAGNETISM. 257 



are affixed, is an horizontal circular disk, 4 inches in diameter, 

 divided on silver into quarter degrees. The second part con- 

 sists of an alidade, concentric with the circle, and rotating on 

 its Umb, and having two verniers indicating minutes ; a strong 

 rod, perpendicular to the plane of the circle, is firmly connected 

 with the alidade, and to this is fixed a very perfect circular 

 mirror 1^ inch in diameter, in which may be seen, through 

 a telescope placed at the distance of 16 feet, the image of a 

 portion of an horizontal scale, divided in millimetres, fixed 

 below the telescope. In this manner every change in the posi- 

 tion of the circle may be seen and measured ; small changes 

 directly, and with great accuracy by the divisions of the scale 

 seen in the telescope ; and greater ones by combining a move- 

 ment of the alidade and reading off the verniers. The third 

 portion of the apparatus is a stirrup situated beneath the circle, 

 being a double frame, in Avhich the fourth constituent part, a 

 25-pound magnet bar, is inserted. This stirrup has likewise 

 a rotatory motion round the centre of the circle, and is provided 

 with two indexes applied to the limb of the circle, by which the 

 amount of the rotation can be measured to a minute. 



If now we place the stirrup so that the apparatus preserves 

 the same position of equilibrium, whether the magnet bar be in 

 the stirrup, or a non-magnetic body of equal weight, we have 

 the first or the second of the positions distinguished above, ac- 

 cording as the magnet bar is in the dii'ect or in the reverse po- 

 sition. The first affords no particularly important practical ap- 

 })lication ; and the advantage of the second is connected with 

 the condition, that the magnetic directive force be somewhat 

 less than the directive force due to the mode of suspension. 

 With our apparatus the proportion of these forces is at present 

 nearly as ten to eleven : the resulting directive force is conse- 

 quently only the tenth part of the magnetic directive force. We 

 have consequently in this case an ai'rangement analogous to an 

 astatic magnetic needle ; and every extraneous force that dis- 

 turljs the direction of the ordinary needle is indicated here by 

 a tenfold greater effect than would take place in the case of 

 sus])ension by a single thread ; and, as will be easily perceived, 

 in the opposite direction. This then affords, among other things, 

 the solution of a problem which has been often attempted with- 

 out success, viz. that of representing the daily and hourly changes 

 of the magnetic declination under a magnified form. Numerous 

 simultaneous ol)servations of this kind, made with this appa- 



