PRELIMINARY REPORT ON TERRESTRIAL MAGNETISM. 17 
the defects of more recent patterns have had less time to become generally 
recognised. The lesser cost of the small magnet Eschenhagen magne- 
tograph, and to some extent, doubtless, the greater pushfulness of German 
makers and magneticians, have led to the adoption of German magneto- 
graphs in the majority of the more recent observatories. Again, the 
tendency has been to replace dip-circles in observatory work by dip- 
inductors, for which a higher degree of accuracy seems fairly to be claimed, 
and the great majority of these instruments have been made in Germany. 
The number of new magnetic instruments wanted in a single year by all the 
observatories of the world may seem a trifle from the point of view of a 
large manufacturer. The construction of these instruments is not an 
industry which promises large financial returns. On the other hand, the 
country that dominates the market, such as it is, is the one most likely to 
exert an influence on magnetic development, 7.e., to be successful in what is 
known as ‘ peaceful penetration.’ Thus the position created by the war 
calls for consideration. 
_ §4. Amongst the questions on which, at one time or another, the 
International Magnetic Committee has expressed opinions, mention may be 
made of the time scale and the sensitiveness of magnetographs. A 
resolution was once passed recommending 15 mm. per hour as the best time 
scale. Originally the time scale in the Kew magnetograph was three 
quarters of an inch per hour, but this was reduced to 0-6 inch (15-2 mm.), 
which is in close agreement with the resolution. All recent Kew-pattern 
magnetographs have adopted the more contracted time scale, and the same 
is true of the Watson type magnetographs. A more open scale, 20 mm. 
per hour, has, however, been generally adopted for the Eschenhagen 
instruments. 
Again, the sensitiveness of 1mm. = 5y once received international 
approval. This may have been suggested by the fact that if 1 mm. 
represent 1’ in the declination magnetograph—a convenient round figure— 
this represents not far from 5y at an average European station. Another 
consideration was probably the risk, when high sensitiveness is adopted, of 
loss of trace during disturbances. Of late years, however, many observa- 
tories using Eschenhagen instruments have adopted a considerably higher 
 Sensitiveness. These instruments use a wider photographic sheet than the 
ordinary Kew-pattern magnetograph, and the H magnet carries two 
mirrors inclined at a small angle. In this way, greater sensitiveness can be 
secured without increasing the risk of loss of trace. ; 
A common time scale is a great convenience for the comparison of 
disturbed curves from different stations, and there are also advantages in 
an approach to uniformity in the sensitiveness. Thus both subjects are of 
importance for international co-operation. 
. A point, however, that should not be lost sight of, is that the advan- 
tages of great sensitiveness are thrown away and may become positive 
disadvantages at stations in high latitudes which are naturally disturbed, 
- or at stations in temperate latitudes which are exposed to artificial sources 
of disturbance. In the former case, in the Eschenhagen pattern, confusion 
ensues from crossing of the traces ; in the latter case the attention Is 
distracted by the artificial movements. 
§5. There are two questions of considerable importance relating to 
diurnal inequalities, which invite discussion. It may seem that the five 
* quiet ’ days provide an absolutely common set of data for all observatories. 
1917. c 
