Lovering and Bond on Magnetic Observations at Cambridge. 37 
occasional tempests of heat and cold occur they cannot compete with 
the irregular disturbances of the magnetic force, where a change of 
20’ occurs in half an hour while the greatest range from minimum to 
maximum on any of these days is only 11’. There is safety then 
in affirming that the changes in the declination of a magnetized bar 
would be a better index of solar time than a standard thermometer ; 
or again, that starting from a known hour of the day with a given 
declination we might venture a closer prediction, founded on calcu- 
lation, as to the position of the same bar 6 hours afterwards than 
would be safe with a thermometer under like conditions ; care being 
taken of course to select that portion of the day which is most free 
from extraordinary magnetic and thermometric changes. 
Let us now see how this conclusion is sustained by more exten- 
sive observations. To this end Tables are presented of the Month- 
ly Means, comprising the mean periods of magnetic and thermo- 
metric minima and maxima since this kind of observation was 
instituted at the Cambridge Observatory. 
Thermometric Table. 
Number Time of the Value of the Timo of the Value of the Mean 
Months. of days ob-| Mean Minimum Mean Maximum. 
served. |Cambridge M. T'.| Mean Minimum. |Cambridge M, T.|Mean Maximum. Daily Range. 
August 5 4» 56’ A. M. 58° 36/ 24 36’ P. M. 77° 48! 19° 12) 
September 5 By a 41 cl Gnas 63 48 22 48 
October 5 6 16 se 43 2) OGnras 52 06 9 06 
November 3 5 36 ts 95 48 2 56 © 35 20 9 32 
December’ 3 16 40 OSG) at 29 50 13 10 
January 3 6 36 ee 24 20 ana, 33 30 9 10 
February 3 5 56 ce 20 20 po iy 0 39 20 19 00 
Means 5 53 RE 32 49 ee IPT 3s 47 23 14 34 
* Time of minimum for this month is 6" 26’ P. M. Cambridge M. T. From 
this time the thermometer rises during 18 hours till the period of maximum. 
