Photography at Greenwich Observatory. 17 



observatory, and, as to the magnetic department, are three in 

 number. We say ' c were placed/' for recently a change has 

 been made in their positions, to be referred to presently ; but 

 it will be more convenient to describe the arrangements as 

 they existed for many years previously. It has already been 

 stated that the inclination or direction of a magnetized bar can 

 be resolved into two forces acting in different directions, and 

 the instruments used are designed to ascertain the variations 

 from time to time, first in the direction of a needle, free to 

 move in azimuth (that is, E. or W. with regard to the meri- 

 dian) ; secondly, the variations in the dip or vertical plane ; 

 and thirdly, in the intensity of its horizontal force. The 

 declination magnetometer is used for the first purpose, viz., to 

 measure the variation, or, as it is now termed, the declination 

 of the magnet, and the incessant alterations which are going 

 on in this element. The instrument is placed in the southern 

 arm of the cross- shaped building, and consists of a massive 

 bar magnet two feet long, carried by a stirrup suspended by 

 silken threads with as little torsion as possible, and surrounded 

 by double gilt boxes, having holes covered with glass for the 

 observations, to avoid currents of air, and an oval copper bar 

 or damper to lessen the vibration. At one end of the magneb 

 is a frame carrying a cross of cobwebs, and at the other end a 

 lens, which renders the rays from the cross parallel. In the 

 centre of the room is a transit theodolite, by means of which, 

 directed to the stars through a shutter in the roof, the true 

 meridian can be ascertained, and the readings of the direction 

 in which the cross on the magnet is seen through the lens, as 

 compared with the readings of the circle given by stars on the 

 meridian, will give the amount of the declination at the times 

 of observation. The cross is lighted either by a reflector in 

 daylight or a lamp at night, as may be necessary for the 

 purpose. 



The dip or balance magnetometer occupied the western arm 

 of the cross building, and, as its name implies, measures the 

 angle formed by a magnet freely suspended on knife edges, 

 and at liberty to move in a vertical direction. The dip, like 

 the other magnetic elements, is in a state of constant change, 

 some of its more important alterations having already been 

 noticed. 



In all ordinary suspended magnets, the dip is counteracted 

 by an increased weight at the other end, and in the balance 

 magnetometer, which is placed nearly at right angles to the 

 magnetic meridian, this inclination is almost counterpoised, 

 the object being not so much to determine the absolute dip, 

 which is done by separate instruments at stated times, as the 

 variation in the vertical force. In order to measure this 

 vol. viii. — no. i. c 



