Royal Society. 493 



servations have been made which do not include long intervals, and 

 from which, nevertheless, inferences are drawn in respect to secular 

 change. Such observations, when not those of a fixed observatory, 

 are usually made at some hour in the day-time, when it needs only 

 a glance at the plate to perceive that annual as well as diurnal 

 variation-corrections are required, unless the month as well as the 

 hour are the same in the earlier and later observations. A table of 

 corrections for every hour of the day to the mean value in each 

 month — corrections derived, as in the instances now before the So- 

 ciety, from a series of strictly comparable observations continued for 

 several years — should be considered, not merely as a desirable, but 

 as an almost indispensable provision, in countries where magnetic 

 surveys are conducted with the degree of perfection of which they 

 are now susceptible. 



" On Induced and other Magnetic Forces." By Sir W. Snow 

 Harris, F.R.S. &c. 



The question as to identity in the source of those several and 

 mysterious powers of nature by which'masses or particles are moved 

 either toward, or from each other, being a question of deep physical 

 interest, the author of this paper has been led to some further in- 

 vestigation of the nature and laws of magnetic force, in the course 

 of which several new facts have presented themselves which he 

 thinks not altogether unworthy of attention. 



Magnetic attraction as commonly observed being found to depend 

 on certain impressions made on the attracting bodies usually de- 

 signated by the general term induction, it appears essential to the 

 progress of any inquiry into the laws of those forces operating ex- 

 ternally to a magnet through space, to commence with a rigid exa- 

 mination of the nature and mode of action of those inductive forces 

 upon which the reciprocal force of attraction between the bodies 

 immediately depends. These forces of induction may be considered 

 as a series of successive or reverberating influences, operating be- 

 tween the near and opposed surfaces of the magnetic bodies. When, 

 for example, a magnet is opposed to a mass of soft iron, a direct 

 impression is first made on the iron by which the iron is rendered 

 temporarily magnetic ; this induced force operates in its turn by a 

 species of reverberation or reflexion upon the near pole of the mag- 

 net, and calls into play a portion of the magnetic force in the direc- 

 tion of the iron, which was previously operating toward the centre 

 of the magnet; this action being once set. up, may continue for a 

 series of waves reverberating between the opposed surfaces, until 

 the action sinks away as it were into rest. The author examines 

 experimentally, by means of instruments, the principles of which he 

 has already detailed in the Transactions of the Royal Society, this 

 peculiar kind of action, and arrives at the following deductions re- 

 lative to the laws of magnetic induction. 



A limit exists in respect of induced magnetic force, different for 

 different magnets, and varying with the magnetic conditions of the 



Phil Mag. S. 4. Vol. 2. No. 13. Dec. 1851. 2 L 



