20 EEPORT 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 also in 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. 



