THE USE OF THE MAGNETIC NEEDLE IN SURVEYING. 1 55 



assistant in charge of the computing division, United States Coast Survey 

 office, Washington, D. C. : 



The influence which produced the increase of magnetic west declination on our 

 Atlantic coast was first recognized in the north-east, extending itself in time toward 

 the south-west. The minimum west declination occurred at Portland, Me., about 1765 ; 

 at Cambridge, Mass., about 1783 ; at New York, about 1795 ; at Savannah, Ga., about 

 1817-; at New Orleans. La., about 1831 ; and at the city of Mexico about 1838, appear- 

 ing at the last three places as a maximum east declination. The same influence will 

 possibly soon reach our Pacific coast, where at present the east declination is still 

 slowly on the increase. Sub-periods or subordinate waves in the secular change have 

 been recognized in the observed declinations at Cambridge, Mass., at Hatboro', Penn. 

 (near Philadelphia) , and at other places ; and they are also noted in the observed dips 

 at Washington, and Toronto, Canada. 



Taking this view of the subject, the phenomenon of the secular change is a complex 

 one ; and the numerical formulae designed for expressing it must, for the present, 

 retain their tentative and hence provisional character; and they should not be used 

 (either way) much beyond the time for which they are supported by observations. 



The declination at Hanover in 1840 was 9 20' west, and the annual 

 increase at that time 5 ' .2. But the whole change since then is only 2, 

 being an average of 3 '.5 per annum ; and recent observations show that 

 the annual increase during the last decade has been less than 3'. The 

 present rate of change will not be accurately known until the observa- 

 tions made in September, 1873, shall be repeated. It is probably not 

 more than 2' or 2^.5 per annum, which indicates a probability that the 

 westerly declination will reach a maximum here about the close of the 

 present century. If this estimate should prove correct, and the period 

 of decrease should be as long as that of increase, the time required for 

 the declination to pass from a minimum to a maximum and to return to 

 the minimum, will be about two hundred and forty or two hundred and 

 fifty years. We have not, however, at present, sufficient data to deter- 

 mine this period with accuracy, nor are the causes which produce the 

 change well known. 



The amount of the secular variation is very different in different parts 

 of the earth. At the Cape of Good Hope, in two hundred and forty-six 

 years, ending 1850, the declination had changed from 30' east to 29 i8'.8 

 west, and was at that date slightly increasing. This shows a longer 

 period and much greater change than in the United States. In New 



