238 Instructions for the Scientific Expedition 



where t denotes the actual temperature (in degrees of Fahrenheit) at 

 the time of observation, and q the relative change of the magnetic 

 moment of the needle corresponding to one degree. As in the case 

 of the other instruments, however, it is not in general necessary to 

 apply these reductions to the individual results. 



Times of Observation. 



Tlie objects of inquiry in terrestrial magnetism may be naturally 

 classed under two heads, according as they relate, 1 . to the absolute 

 values of the magnetic elements at a given epoch, or their mean va- 

 lues for a given period ; or 2. to the variations which these elements 

 undergo from one epoch to another. It will be convenient to con- 

 sider separately the observations relating to these two branches of 

 the subject. 



Absolute Determinations. 



By the method of observation which has been suggested for the 

 absolute declination, every determination of the position of the decli- 

 nation bar is rendered absolute. We have only to consider the va- 

 rying angle between the magnetic axis of the bar and the line of col- 

 limation of the fixed telescope, as a correction to be applied to the 

 constant angle (already determined) between the latter line and the 

 meridian. It is manifest that if the fixity of the line of collimation 

 of the telescope could be depended on, a single determination of the 

 latter angle would be sufficient. But this is not to be trusted for 

 any considerable period ; and it will be therefore necessary, from 

 time to time, to refer the line of collimation of the telescope to the 

 meridian, by means of the transit instrument. This observation 

 may be repeated once a month, or more frequently if any change in 

 the position of the telescope be suspected. 



In the case of the intensity, there is another source of eiTor (be- 

 sides that due to a change in the position of the instruments) which 

 can only be guarded against by a repetition of absolute measure- 

 ments. The magnetic moment of the magnet itself may alter ; and 

 the observations of intensity changes afford no means of separating 

 this portion of the effect from that due to a change in the earth's 

 magnetism. This separation can only be effected by means analo- 

 gous to those employed in the determination of the absolute value of 

 the horizontal intensity ; and accordingly one or other (or both) of 

 the methods proposed for this determination should be occasionally 

 resorted to. It is desirable that this observation should be repeated 

 once in every month ; and more frequently, whenever the changes ob- 

 served with the horizontal force magnetometer indicate, by their 

 progressive character, a change in the magnetic moment of the sus- 

 pended bar. 



It would be easy, in theory, to devise a method by which the ver- 

 tical force magnetometer might be made to serve in determining the 

 absolute value of the vertical intensity. The means which at pre- 

 sent offer themselves appear, however, to be surrounded with prac- 



