172 Transactions of the American Institute. 



of the earth's magnetic force, and the needle, as we know, ever keeps 

 its length in these curves. But the object of this lecture is not to 

 prove, to support, or to discuss magnetic hypotheses, and I do not 

 give this experiment as defining my opinion as to the cause of the 

 needle's variation ; but I do give it as showing, in the most complete 

 manner, this fact, the constant fluctuation, the ebb and flow- of the 

 earth's directive influence. For, picture to yourselves the magnetic 

 curves of this axis as it sways around its circle ; they move with it, 

 and stretching beyond and enveloping yonder lantern needle, it obeys 

 their bends and flexures, and thus, as a matter of fact, sway and bend 

 the magnetic lines on the earth's surface, though the present cause 

 and future manner of such action we may be ignorant of. 



If an elastic rod is clamped at one end and bent from its natural 

 position, its free end will vibrate with a regular motion like a swing- 

 ing pendulum ; but it is also found that this free end can at the same 

 time have shorter and quicker swings, and carry these along in its 

 main vibration. Thus, likewise, vibrates the magnetic needle, for 

 this, in its 320 years' swing, does not steadily move year after year, 

 or even day after day,but, as Graham found in 1722, makes in its ground 

 vibration many minor trembles and deflections. Some of these follow 

 regular laws, others are without all law, and are called perturbations. 

 Among the former is the regular march of the daily vibration. This 

 depends upon the apparent position of the sun, and therefore follows the 

 local times of the meridian on which the needle is observed. In the 

 northern hemisphere, the northern end of the needle, four to five 

 hours before mid-day, has its extreme easterly position ; hence it begins 

 to swing with an increasing velocity, which attains its maximum nearly 

 at the moment when the sun crosses the magnetic meridian of the 

 station. One or two hours after the needle comes to rest, and soon 

 after begins its eastward spring, and comes, with a slight secondary 

 vibration, to its first position, about sunrise. The arc of the daily 

 oscillation is small, only from 5' to 25', and its extent changes with 

 the seasons, being nearly proportional to the arc described in the 

 visible path of the sun. Thus, from Dr. Bache's observations at 

 Philadelphia, the mean daily arc of vibration for the year is 1\' . 

 For summer it is 10^', and for winter it is 5|-'. This daily vibration 

 also increases from the magnetic equator to its poles. On the equa- 

 tor it is only 3', while Dr. Kane found over 60' at Rensselaer Harbor, 

 north latitude 78° 37'. 



Yet another cause, apparently removed from the earth itself, affects 

 the magnitude of the needle's daily swing ; which, strange to say, 



