Lovering and Bond on Magnetic Observations at Cambridge. 71 
observations of the magnetic elements which are now in progress 
will soon relieve the author of the theory from the embarrassments 
to which the want of accurate data subjected him; and enable him 
to predict with certainty concerning the magnetic state of any part 
of the globe, though it has never been approached by man. 
We shall close this communication with a brief description of the 
new Magnetic Observatory at Cambridge. The buildings which 
have been erected for this purpose are represented on the right 
hand of Plate I. They connect with the observer’s house at the 
northwest corner through a covered passage. Their shape as seen 
on the ground plan was determined by the positions required for 
instruments which they were to contain. The impossibility of ascer- 
taining the dip with accuracy has caused it to be dropped as one of 
the primitive Elements of Terrestrial Magnetism and the horizontal 
and vertical forces have been substituted in its place. The three in- 
struments in use at the Magnetic Observatories established by the 
Royal Society, and recommended to all the allies, are the Declina- 
tion Magnetometer, and the Horizontal and Vertical Force Magnet- 
ometers. The first two are inventions of Professor Lloyd, and 
correspond in their objects and part of their construction to the 
Gauss Declination Magnetometer and his Bifilar Magetometer. In- 
stead of the mirror, and the scale to be reflected from it, these bars 
are furnished with two sliding pieces, one at each end. That far- 
thest from the observer contains a finely divided scale of glass; the 
other, an achromatic lens. The distances of the two are so arranged 
that the scale shall be in the focus of the lens. It is evident that 
the apparatus thus adjusted forms a moving collimator. The scale is 
carried by the bar in all its changes of position and, being read off 
by a fixed telescope at a suitable distance, records the variations of 
magnetic declination. It is extremely easy to pass from the read- 
