394 ilfr. cavendish’s Account of 
heberden, who has made obfervations of this kind for 
feveral years paft, that the annual alteration of the varia- 
tion has, in general, been not more than i o'; and in par- 
ticular, that the alteration in the laft year appears to be 
only iiy ; fo that the great difference obferved at the 
Society’s Houfe feems to be owing, not lolely to the real 
alteration in the variation, but partly to fome other caule ; 
though what that fhould be I cannot conceive, unlefs 
fome change was made in the iron work either of this 
or the adjoining houfes between the two periods ; but I 
do not find that any fuch change has been made. During 
the lall year, indeed, there have been two large magnets 
in the houfe, each confifting of feveral great bars joined 
together, being what the late Dr. knight ufed for making 
artificial magnets, and at the time of the obfervations in 
1774 there was only one; but their diftance from the 
compafs is above fifty feet: and I am well allured, that 
in the fituation in which they are actually placed, they 
cannot draw the needle afide more than 3', and not more 
than 1 5', when the line joining their poles is placed in 
fuch a direction as to acft with moft forces;. The fingle 
(c) The principle by which this was determined is, that if a magnet is 
placed near a variation compafs, with its poles equi-djftant from it, and lituated 
fo that each fhall aft equally oblique to the length of the needle, it can have no 
tendency to alter the variation; and that the fituation in which it alters it moft, 
except when placed nearly North or South of the compafs, is when the line 
joining its poles points almoft direftly towards the needle. This experiment I 
tried purpofely on the occafion, and found it anlwer; but, I believe, any one 
fkilled in magnetifm would have granted the truth of the pofition without that 
precaution. 
magnet 
