Lovering and Bond on Magnetic Observations at Cambridge. 49 
form must vary with the observations. Hence the difference be- 
tween the constants in the formule for the four months. They are 
no greater than might be expected from the known change of ab- 
solute declination from day to day, the limits of the times of maxima 
and minima and the longer and more irregular derangements which 
beset the diurnal movement. The mean curves of many months, 
drawn from the most abundant materials, are requisite for investigat- 
ing the law by which these constants vary and rendering them 
available for calculating the secular periods of the earth’s magnet- 
ism. We think it is apparent from all that has been adduced, that 
the diurnal magnetic curve is as clearly a function of solar time as 
the daily thermometric curve. We are not to expect any greater 
uniformity in the effect than in the cause. If the thermometric 
curve is sometimes imperfectly formed, the same thing may happen 
to the magnetic curve without destroying our belief in its connexion 
with the sun. The change of constants in one class of formule 
appears likewise in the other, as the three following thermometric 
formule make manifest: 
August, 5 days. Temperature 
= 67°,648°.8 sin (¢—15" 57” 28:)-4.0°.9 sin 2(t—5h 34m 56*)419.1 sin 3(t40n 440 56s), 
September, 5 days. Temperature 
= 50°,2—10°.0 sin(¢—13)33"54*)—3°,2 sin2(t—931™4*)—0°,3 sin3(¢—2") 0° Ssind (t1"3948°), 
October, 5 days. Temperature 
= 47°,7—3°,8sin(t—14>2144°) —0° 8sin2(t—917"36*)—0°,4sin3 (t-6"35™25*) —0° sind (t-038"12s). 
It still remains to discuss briefly those disturbances of the mag- 
netic meridian which have no apparent law. We have occasionally 
alluded to them as irregular perturbations which produce perplexity 
in ascertaining the true diurnal curve. We are to inquire whether 
even they must be regarded as wholly inexplicable or whether they 
cannot be connected in coincidence of time at least with other well 
known phenomena of nature. There are few days in the year 
q 
