438 Different “ Rates” of Pennington’s Astronomical Clock, 
ulowance is made for this circumstance, as well as for the pro- 
bable errors of observation, there will still remain such diversity 
in the results from any law which has yet been assigned, as will 
induce the investigator to seek for some general cause distinct 
from imperfections in the apparatus, and from inevitable aberra- 
tions in their use. 
Laplace, when reasoning on this subject from the excmination 
of results obtained at only fifteen different places, says, ‘‘ Nous 
remarquerons ici que les mémes anomalies que présentent les di- 
vers degrés mesurés depuis Dunkerque jusqu’a Barcelonne, et 
dont la cause est sans doute Pirrégularité des parties de la terre, 
se retrouvent dans les longueurs observées du pendule ;_ car Gris- 
chow a observé a Pétersbourg et & Arengsberg, sous des latitudes 
tres-peu différentes entre eiles, des variations dans ces longueurs, 
sensiblement plus grandes que celles qui résultent de la loi pré- 
cédente de la variation du pendule de |’équateur aux pdles.”— 
Mec. Céleste, tome ii. p. 151. 
Another philosopher, who (though he thinks fit to give his opi- 
nion on this subject anonymously) is so clearly seen through the 
veil that he may be appealed to as an authority, observes—“ It 
must not be supposed that, with the pendulum carried to its pre- 
sent state of sensibility and precision, the results will be free from 
inconsistency, or beyond the influence of the local irregularities 
that may exist immediately under the surface of the earth. Were 
the pendulum the same inaccurate instrument that it was a few 
years ago, it might not feel the influence of such causes as only 
increase or diminish the intensity of gravity by a very small part 
of the whole. But when the length of the pendulum can be de- 
termined to the ten-thousandth of an inch, or to 73453, Of its 
whole length, the force of gravity is measured with the same pre- 
cision, and one part out of 134959 is rendered sensible. Now, 
it seems to us probable, that the variation in the density of the 
strata immediately under the surface, may produce a change in 
the intensity of gravitation, much more considerable than one 
part in 134959; the pendulum will not fail to be affected by this 
irregularity, and to give information of it. The force with which 
Schehalien disturbed the plumb-line was about 573-5 of gravity, 
or nearly four parts in 134959. We think that, without any ex- 
aggerated suppositions, by the presence of an extensive stratum 
of gneiss, or of hornblende schistus, or of any great body of gra- 
nite, immediately under the surface at one place, and of chalk, 
common sandstone, or limestone at another, a difference in the 
intensity of gravity, even greater than the preceding, may be 
readily produced.” — Edin. Rev. vol. xxx. p. 421. 
On the whole, if I thought the opinion of so humble an indi- 
vidual as myself would have any weight in this interesting in- 
quiry, 
ee 
as 
