July 16 5 .• 



. 22 



Sept. 24 6 •• 



. 22 



Oct. 8 3 •• 



• 50 



Introduction. ' xxiii 



Tables have been formed from these vahies, and the known values of the scale 

 divisions, by which the readings in scale divisions have been reduced to angular 

 measure. 



22. Mr Russell had determined the time of vibration of the declination magnet 

 to be nearly 18 seconds, which was accordingly used in the observations. 



The results below were obtained afterwards. 



June 24-1 20^' 1842, 60 vibrations give a mean of 17 89 



1780 



17-84 



17-82 



23. The points of the scale which coincided with the vertical wire of the reading 

 telescope, were noted 18^ before the minute of observation, at the minute, and 18* 

 after the minute. The readings at these periods being a^ 6, and c ; the mean is de- 

 duced by the formula ^ 



The observations of declination in this volume are given in minutes and de- 

 cimals. The absolute declination is obtained from them by adding 23° 20' ; or, if 

 100' be subtracted, the absolute declination is obtained by prefixing 25°. 



t 

 § 4. BiFiLAE OE Horizontal Force Magnetometer. 



24. This instrument is also by Grubb of Dublin, and is similar, in its general 

 construction, to the Declinometer. 



The magnet, whose dimensions are 15 inches, f inch, and \ inch, is placed in a 

 stirrup, which carries below it the lens and scale connected by a tube, forming a close 

 collimator ; the axle of a grooved wheel fits into the suspension eyes of the stirrup, 

 the whole being borne by a silver wire passing round the grooved wheel, and having 

 its two extremities fixed to a suspension roller ; the roller is supported by the tor- 

 sion circle, which also bears beneath the roller a micrometer-headed screw, right- 

 handed where it meets one wire (or portion of the wire), and left-handed where it 

 meets the other. The screw is for the purpose of making the distance of the wires 

 at the top equal to that at the grooved wheel. 



25. In the adjustment of the instrument, the magnet is forced to a position at 

 right angles to the magnetic meridian, by turning the arms of the torsion circle. 

 As, in forcing the magnet from the meridian, the upper extremities of the wire will 

 move through a greater angle than the lower extremities, the wires will be no longer 

 vertical, and the magnet and appendages will be raised ; the forces producing equi- 

 librium, therefore, being the weight suspended endeavouring to attain the lowest 

 point, and the horizontal component of the earth's magnetic intensity acting on the 

 free magnetism, of the bar. 



