AND INSTRUMENTS AT MUNICH. 51S 
magnets, and that its influence will be so much the greater, as 
large bars have not so considerable a magnetic moment in pro- 
portion to their size as the smaller bars. I have, however, 
made but few experiments on this part of the subject, because I 
deemed that when the fitness of small magnets shall once be 
recognized, the employment of large bars will be less frequent, 
on account of their great inconvenience, even if it be possible to 
protect them perfectly from the influence of currents of air. 
The experiments hitherto narrated all relate to the declination 
instrument. After I had satisfied myself of the applicability of 
small magnets for exact observations, and of the advantages 
which their employment presents, I constructed a system of 
magnetic instruments, furnishing the means of making all mea- 
surements belonging to the horizontal part of the earth’s mag- 
netism. These instruments are, 
1. A differential apparatus for the declination. 
2. A differential apparatus for horizontal intensity. 
3. An apparatus for the absolute declination, 
4. An apparatus for absolute horizontal intensity. 
The differential instruments are designed to give the daily and 
yearly variations: they may be set up in an ordinary dwelling- 
house as well as in a magnetic observatory, avoiding only move- 
able masses of iron; and when once established they do not 
require further examination or check, as the construction is such 
that small alterations have no influence on the direction of the 
magnet. The only checks required are for the scale and the 
telescope. 
The absolute instruments, when used from time to time for 
absolute determinations, may be put up either in the open air or 
in an observatory, or in a magnetic pavilion made of wood. 
Without giving in this place a description of the instruments 
themselves, I will notice briefly their essential conditions, and 
will show, by extracts from our observation journals, the degree 
of accuracy attainable with them under ordinary circum- 
stances, 
1. The differential apparatus for declination is the small 
needle figured in page 508, furnished with a mirror, and inclosed, 
not in a bell-glass, but in a case partly of glass partly of metal, 
with an arrangement by which the needle is protected from 
the influence of currents of air. The telescope has only eight 
lines aperture; the scale is of glass: the agreement between 
different instruments of this kind is shown in pages 511, 512. 
