Magnetism, Cohesion, Adhesion and Viscosity. 439 



being urged to it by a bar-magnet, its point will exactly touch 

 some portion that is smooth and vertical. These conditions 

 being strictly attended to, and the object under examination 

 being placed about a quarter of an inch or so fi'om the point 

 of the magnetic needle, the pole of the bar-magnet that is of 

 a contrary character to the point of the needle intended to 

 touch the object, is to be approached with extreme gentleness 

 and very slowly, so as to cause the least tremulous motion 

 possible in the needle. When the bar-magnet is brought so 

 as to cause the needle to press with a slight force against the 

 substance examined, the former is to be kept in the same 

 steady horizontal position, for at least ten or fifteen seconds 

 after it has been observed that all tremor has ceased in the 

 needle ; upon which the bar-magnet may be withdrawn as 

 slowly and as carefully as it was brought near. And this is 

 the place to observe, that one great advantage of having the 

 supporting pivot so short, and consequently the magnetic needle 

 so low down, is that it enables the observer to allow the mag- 

 net to lie upon the table, and to advance and draw it back at 

 pleasure, as well as to have its axis in the same horizontal 

 plane as that in which the needle revolves ; for should it be 

 higher or lower than the latter, it would cause an inclination, 

 and a consequent displacement of the needle, from gravitation 

 the moment the bar was withdrawn, that might interfere with 

 the success of the experiment. If all the conditions just de- 

 scribed have been attended to, the needle will be found to 

 have attached itself firmly to the glass, copper, or other body 

 operated upon, and will in many cases so remain any indefi- 

 nite time, if not disturbed. But it should be well-remarked 

 that ttJiie must generally be given to the magnetic needle to 

 settle, and thereby coerce the substance to which it is to attach 

 itself. This is an essential condition in most cases. The 

 slightest movement in the room, even the motion of the air 

 from the impulsion given by the breath of the observer, are 

 to be carefully avoided in the nicer experiments. Any vibra- 

 tion in the house also will, in most cases, from the extreme 

 delicacy of the operation, cause a failure. Some time, too, 

 will in all probability elapse before the experimenter will have 

 actjuircd the dexterity necessary to the success of the difficult 

 cases, and he must be careful that the point of the magnetic 

 needle does not catch itself against the object under examina- 

 tion when it is rough ; and there is also great danger of its 

 becoming jammed when it approaches an angle of 90^ with 

 the magnetic meridian. To prevent such a conse(juence, 

 the object should be so placed, when the amplitude is large, 

 that only the side of the point of the magnetic needle may rest 



