0] T II K M 1GXETIC FORC E 



The differences in the successive annual means indicate that the progressive 

 change may be assumed to have been uniform from year to year, and applying the 

 usual method we find an annual progressive change of 220 scale divisions. 



Introduction of the Horizontal Intensity in absolute measure and separation of the 



effect of the loss of Magnetism of the Bifilar hue from the effect due to the secular 

 change of the Horizontal Intensity. — Although some experiments were made to 

 determine the gradual loss of magnetism of the bar, as, for instance, in January, 

 1841, when the amount was found to be 0.9601 of the force in May, 1840, and 

 again in June, 1841, when the amount was 0.9686 of its amount in January, 1841, 

 yet the experiments do not extend over the whole period of observation, and con- 

 sequently we are obliged to deduce the effect of the secular change of the horizontal 

 intensity from other independent means, and, after converting it into scale divisions, 

 Ave can assign the proper proportion of what is due to secular change and to loss 

 of magnetism, in the whole progressive change of 220 scale divisions in a year. 



In connection with the operations of the U. S. Coast Survey, Assistant Schott has 

 investigated 1 the secular change of the horizontal intensity at a number of stations 

 on the Atlantic and Pacific coasts. At several stations the results were subsequently 

 improved by a discussion of my observations for intensity, made in part in connection 

 with a magnetic survey of Pennsylvania, and also extending into adjoining States, 

 and, in one of the journeys, into Canada. From the complete material the values 

 in the following table of observed horizontal and total intensities have been collected. 

 The horizontal intensity A and the total intensity <\, are expressed in absolute mea- 

 sure (grains and feet). 



1 Report to Superintendent, dated January 19, 1861. 



