18 Photography at Greenwich Observatory. 



change, which requires much more delicate means than the 

 variations of the declination magnetometer, a small plane- 

 mirror is attached a little out of the centre of the bar, which 

 reflects the divisions of a vertical scale attached to the wall of 

 the room, and these reflected divisions are observed with a 

 telescope fixed near the theodolite, the motion of the magnet 

 having been thus much magnified. 



The absolute force of the magnet at Greenwich, like the 

 absolute dip, is determined periodically by separate experi- 

 ments ; but the remaining element, or the horizontal force of 

 the magnet, is measured by the third instrument contained in 

 the eastern arm of the room, and called the bifilar magneto- 

 meter. It consists of a magnetized bar, like the other instru- 

 ments, which is suspended by two parallel sets of silk threads. 

 If the strings and the magnet were in the same vertical plane 

 the whole system would remain at rest ; but the force of tor- 

 sion is ingeniously brought into play to measure the horizontal 

 force of the magnet. For this purpose the suspending skeins 

 of silk pass under two pulleys, attached to a plate turning on 

 a graduated circle with much friction, bymovingwhichthe magnet 

 suspended to the circle is twisted out of the meridian to a posi- 

 tion nearly at right angles with it. The force of torsion is, there- 

 fore, now acting antagonistically to that of the magnet, which 

 endeavours to regain its normal bearing ; and as the torsion 

 force is constant, and that of the magnet subject to ceaseless 

 variations, the magnet is always taking up fresh positions, 

 and these angular changes can be connected mathematically 

 with the forces producing them, and the share of the 

 magnetic force in them deduced by calculation. The bar 

 carries a small mirror, as in the last- described instrument, 

 by which the reflection of a fixed horizontal scale is ob- 

 served in another telescope precisely as with the balance 

 magnetometer. 



In the original arrangements of the observatory, all these 

 instruments were observed every two hours, and on certain 

 days in the year at every hour, and even more frequently still, 

 which, in addition to other work of the department, fully 

 employed Mr. Glaisher and several assistants, and was 

 monotonous and wearisome in the extreme, as, unlike the 

 astronomical assistants, bad weather brought them no relief of 

 their labours. Even assuming that the observations were 

 punctually made, it is obvious that many most important 

 phenomena might escape notice during the two hours' interval, 

 especially in the case of magnetic storms, and some system of 

 continuous and automatic registration became a great deside- 

 ratum. It was obvious that no mechanical arrangement, such 

 as the movement of pencils by the magnets, was admissible ; 



