20 REPORT OF THE SECRETARY. 
ever, is not entirely satisfactory on account of the large disturbances 
due to variations of temperature, changes of the magnetism and ad- 
justment of the instrument. 
The third part of the fourth section, or the twelfth part of the 
entire series, contains the result of the observations made with a 
portable dip circle constructed by Robinson, of London, the same 
which had been previously used by Professor Bache in his magnetic 
observations in Pennsylvania and adjacent States, and alsoin Europe. 
The observations were made weekly during a period of nearly two 
and a half years; the monthly and annual mean observations of the 
dip were tabulated and were found to indicate an annual decrease of 
one minute and two-tenths in this element. The same paper con- 
tains a collection of observations on the dip at different points in 
Philadelphia by different observers, from which a similar change in 
the dip has been deduced. The least dip occurred in January, 1840, 
and increased for several years after that date. It is probable, how- 
ever, from some subsequent investigations by Mr. Schott of observa- 
tions at other places, that the minimum obtained at Philadelphia, 
above mentioned, was of a secondary character, and that a still 
smaller dip will hereafter be observed. But this point will be cleared 
up in a few years by observations now in process of collection. The 
discussion of this part, and indeed the whole of the series, ends 
with a table of magnetic constants for Girard College, namely: of 
the declination, or variation, as it is sometimes called; of the dip; 
of the horizontal, vertical, and total force, all expressed in absolute 
measures, for five different epochs and for one mean epoch, that for 
January, 1843, for which the declination is 3° 32’ W., the dip 71° 59’ 
N., the horizontal force 4.173, the vertical force 12.83, and the total 
force 13.49, in units of one foot, one grain, and one second of mean 
time. 
From all the investigations on this subject up to the present time 
we may infer, first, that the earth is a great magnet, having a natural, 
and in one sense a permanent, polarity; second, that this polarity 
is disturbed in intensity and direction by the varying effect of the 
heat of the sun; third, that the magnetism of the earth is affected 
by that of the sun and moon; and fourth, it is probable that magnetic 
polarity is common to all the bodies of the solar system. 
The second paper in the thirteenth volume of the Contributions-— 
that on the silvered glass telescope—is fully described in an article 
at the end of this report, copied from the ‘‘ Intellectual Observer,’’ 
of London. 
