Lovering and Bond on Magnetic Observations at Cambridge. 43 
elongation from the astronomical meridian can be trusted. The 
mimimum is at 6" 26’ A. M. and the maximum at 0° 16’ P. M. 
Cambridge M. T. 
We have here ample and abundant explanation of that singular 
figure assumed by the mean daily curve for September. We look 
for the time when some method shall be devised for evading the 
errors which such extraordinary changes of the magnetic declination 
entail on the mean values of the regular variations. If hereafter 
the dependence of the magnetic declination upon the hour of the 
solar day shall be so accurately discovered as to be reduced to a 
Formula we may be able, by the help of that portion of the curve 
which is undisturbed, to calculate the remainder. At present this 
Formula must be an empirical one, derived from the faulty observa- 
tions themselves and in its defective state is available only in a 
partial degree for purifying these observations. Our chief resource 
now lies in levelling, as far as possible, the excessive excursions by 
the influence of undisturbed days with which they are combined ; 
though this can be done only by sacrificing in part the more per- 
fect observations. ‘The case in hand teaches us that this method 
will not always be effectual in bringing out approximate results. 
The irregularities may be so great as to overrule the regular law. 
This is less likely to happen in proportion to the number of days 
that can be observed in each month and hence again the necessity 
of deducing our means from as numerous observations as can be 
obtained. 
The dependence of the diurnal magnetic changes on solar 
time rests upon the evidence of a large number of observations 
collected from remote sources. But there is a difficulty in con- 
ceiving of the exact manner in which this connexion is sustained. 
Perhaps it will always be a hopeless task to attempt to trace the 
