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VIII. Magnetic Survey of Belgium in 1871. By the Rev. Stephen J. Perry. 
Communicated by General Sir Edward Sabine, F.R.S. 
Keceived December 24, 1872, — Dead February 6, 1873. 
Previous to the year 1871 few observations had been made in Belgium for determining 
the elements of terrestrial magnetism, if we except the series which has been carried on 
without interruption at the Boyal Observatory since 1828. Before this latter date the 
Intensity and Dip had never been ascertained, and there existed only two reliable 
measures of the Declination, viz. that of 20° 35'*5 for Ostend, which Pigot observed in 
1772, and the other for Nieuport, which, at about the same date, was found by Mann to 
be 19° 48 ,- 5. Since 1828 the observations made at any other station besides Brussels 
have not been numerous. In 1854 the Dip was measured at Antwerp, Courtray, Ghent, 
Mons, and Ostend ; the Horizontal Force was found at Liege and Louvain in 1829, 
1850, and 1854, and also at Namur in 1829; and the three elements were observed in 
1859 at Ghent and Mechlin. The results of these various observations are collected in 
the work entitled “ La Physique du Globe,” by M. A. Quetelet, and in Dr. Lamont’s 
‘ U ntersucl lungen fiber die Bichtung und Starke des Erdmagnetismus in Belgien,’ Ac. 
The above being the only determinations of the magnetic elements, there is an obvious 
want of a complete series of observations at a sufficient number of stations, and the 
survey which forms the subject of the present paper was undertaken with the view of 
supplying the required series of connected values of the three elements. 
The instruments employed in this survey were the Barrow dip-circle, the Jones 
unifilar, and the Frodsham chronometer of Stonyhurst Observatory, and an excellent 
theodolite by Teoughton and Simms for determining the azimuth of the fixed points for 
the Declination. For this last instrument I was indebted to the kindness of James 
Shoolbred, Esq., C.E. All necessary information respecting the magnets and instru- 
ments will be found in the paper on the Magnetic Survey of the West of France printed 
in the Phil. Trans, of 1870. 
The methods of observation and reduction were identical with those adopted in the 
preceding surveys. 
At all the stations in Belgium the Dip was observed by Mr. W. Carlisle, the readings 
for each needle being never less than 32 ; the remaining elements were determined by 
myself. 
In forming the equations of condition for the determination of the lines of equal Dip, 
Declination, and Intensity, the Boyal Observatory of Brussels has been chosen as the 
origin of coordinates. 
mdccclxxiii. 2 z 
