156 PHYSICAL GEOGRAPHY. 



England the whole change is probably between 6 and 8. The following 

 declinations, copied from the United States Coast Survey Report for 

 1855, were observed at Cambridge, Mass., and show the change at that 

 place since 1708: 



1708 9 oo' west. 1782 6 45' west. 1840 9 18' west. 



1742 8 oo' west. 1783 6 52' west. 1842 9 34'. 9 west. 



1757 7 20' west. 1788 6 38' west. 1844 9 39' west. 



1761 7 14' west. 1810 7 30' west. 1852 10 08' west. 



1763 7 oo' west. 1835 8 51' west. 1854 10 39' west. 



1780 7 02' west. 1837 9 09' west. 1855 10 54' west. 



From these facts will appear the importance of recording, with the 

 minutes of every survey, the declination of the needle at the time and 

 place. To do this the surveyor must know the declination, which he 

 cannot do without some trouble and labor. He must frequently try his 

 compass by some well established meridian, which, if he cannot find 

 already determined, he must locate for himself. Neither should he lose 

 any opportunity to take the bearing of any old line whose former bearing 

 he may find in the record of some previous, perhaps the original, survey. 

 By continuing such observations, he will learn not only the amount of 

 the declination at the time of the former survey, but, also, its rate of 

 change, and the whole change that has occurred since the running of the 

 old lines with which he has compared his needle ; and he will thus gain 

 information which will render his services invaluable in disputes relating 

 to division lines. 



3. Diurnal change in the Declination. If hourly observations be made 

 upon the delicately suspended needle of a magnetometer, or, still better, 

 if we use a self-registering instrument by which a continuous record is 

 made of the changes in the direction of the needle, we shall notice a 

 diurnal variation of the declination, in northern latitudes, substantially 

 as follows : During the night the needle will be comparatively quiet ; but 

 at dawn of day the north end will move toward the east, and will 

 continue this decrease of declination till about 8 o'clock A. M., when it 

 will commence a westerly motion, and will come to its maximum west 

 declination at about 2 o'clock p. M. It will then return toward the east 

 until some hours after sunset, when it will again remain quiet till the next 

 dawn. But while this is in general true, it must be taken with much 

 allowance. In the first place, it must not be understood that the needle 

 is stationary even at night, for it seldom, if ever, fails to show more or 



