FOR DETERMINING THE MEAN DENSITY OF THE EARTH. 
331 
probable cause is some trifling error or unsteadiness in the manner of fixing the 
agate-plane-frames upon the iron stands. It is not a change in either of the pendulums 
after the Second Series, inasmuch as the value of the mean is altered the same way 
under the alternation of position of the pendulums. Whatever the cause may have 
been, the effect is extremely small. There appears to be no reason for altering the 
concluded ratio of Gravity below to Gravity above. The probable error stated in 
the last paragraph may be doubled, but I think that there is no sufficient ground for 
trebling it. Hie amount of the uncertainty, so increased, is insignificant for the 
purposes of this experiment. 
47 . There remains, however, a serious question whether there may have been any 
difference in the circumstances of the upper and lower pendulums, not included in 
the corrections applied, which can produce an effect similar to that of a change of 
gravity. The first point to be considered is, the instability of the mountings. The 
importance of a very firm foundation was perfectly understood by the able practical 
men by whom the ground-work was arranged, and particularly by Mr. Arkley ; and 
I conceive all was done which it was possible to do, to make the floors solid. The 
form of the iron stands is particularly well adapted to firmness. Any tendency to 
lateral or other movement is counteracted by the endwise resistance of strong straight 
iron bars. I had at first intended to interchange the iron stands in the middle of 
the operation, but upon contemplating the mechanical firmness ensured by the plan 
of their construction, and the exact similarity of the two stands in every respect, I 
gave up this design ; being fully convinced that there might be risk of instability in 
a change, but that there could scarcely be any sensible absolute instability and (as I 
believe) no sensible relative instability, in the stands as they were planted. The 
stands were supported in the same manner at both stations. 
The mere determination of the relative rates of the detached pendulum and the 
clock pendulum, by the method of coincidences, is accurate almost beyond conception. 
I do not see how it is possible that an error of 0®‘01 per day can enter from this 
cause. 
The observers were so evenly interchanged in the upper and lower stations, that 
no personal peculiarity in the method of reading the thermometers or of taking any 
other observations can have produced a sensible effect. The following Table shows 
the aggregate number of turns taken by each : — 
Initials of observer’s name C D E P R S. 
Number of turns above 5 5 4 4 5 5. 
Number of turns below 5 2 5 6 5 5. 
48. I will now point out the only cause from which, in iny judgment, any percep- 
tible error can arise. It was my intention that the temperature of the upper station 
should be brought, as near as could practically be done, to that of the lower station. 
In the first week, however, Mr. Dunkin was seized with a sudden and severe illness, 
2x2 
