40 Lovering and Bond on Magnetic Observations at Cambridge. 
be deduced from as large a number of days as can be conveniently 
observed; every day in the month would not be superfluous. This 
could easily be done at Cambridge if provision were made for con- 
ducting the two hourly observations on every day according to the 
English plan. But these daily observations of the Magnetometer 
have not been attempted and would be impossible with the present 
resources of the Observatory.* It is not surprising that what oc- 
curred once in six months’ observations on the temperature should 
have exhibited itself also in the diurnal curves of the Declination 
Magnetometer. This brings up the case of the September magnet- 
ic curve which we have excluded from any part in determining the 
mean quantities because it was calculated to injure the just average 
values which we are endeavouring to reach. ‘The curve in question 
may be traced out on Plate V. by following the directions there 
given and a single glance will show how the times of its extreme 
elongations differ from those of the other three mean curves which 
are on the same Plate. The general appearance of these three 
curves indicates a law to which the fourth curve must be a palpable 
exception and transgression. An attention to the separate diurnal 
curves for the five days in September from which the mean curve 
is reduced will afford an explanation of this violation of what we 
may regard a principle of the earth’s magnetism. On September 
20-1, (Magnetic day, commencing as in all cases at 10 P. M. Gott. 
M. T.) we have these three maxima: 
Readings of the Scale. 
Ist at 11° 36’ P. M. Cambridge M. T., September 20 — 82'.5, 
2d) S64. 56 AGM: fe s ee 21 — 79.5. 
3d “11 56 A.M. *$ ee ce 21 = 28820. 
* A Director with three Assistants all of whose time should be devoted to 
the work have been considered elsewhere as the full personnel of such an es- 
tablishment. 
