8 
GLEANINGS IN SCIENCE. 
nroDOrtion will require'to be modified, by taking into account tlie attraction of the 
elevated materials, interposed between the general surface and the place of obser. 
V!, When pendulums are employed in different latitudes, to obtain the ratio of 
gravitation between the equator and the pole, for the purpose ol deducing the 
ellieticitr of the earth, all the places of observation, being on land, are more or less 
derated above the sea ; inland stations, in particular, are sometimes at considera- 
ble elevations : to render these results comparable one with another. It is necessary 
to reduce each result to what it would have been, had it been made at some level 
common to all the experiments ; and the surface of the sea lias hitherto been taken 
as that common level. Previous to the publication of a paper of Dr. Young’s in 
the Philosophical Transactions for 1819; the consideration which we have men- 
tioned, that of the attraction of the matter interposed between the place of observa- 
tion and the level of the sea, was generally unheeded in estimat ing the allowance 
to be made for the reduction of different heights to the common level : in that 
paper however, Dr. Young took occasion to point out the probable effect of the 
interposed matter in modifying considerably the usual allowance ; that supposing 
its density to be about half the mean density of the earth, the effect of an hemi- 
spherical hill of such matter, on the summit of which the pendulum should be placed, 
would be to diminish the correction, deduced from the duplicate proportion from 
the earth’s centre, about |th ; that, in like manner, a tract of table-land, consi- 
dered as an extensive flat surface, of the same relative density, would diminish the 
correction about |ths : and that, accordingly, in almost any country that could be 
chosen for the experiment, the proper correction for the height would vary, ac- 
cording to the form and density of the interposed materials, from rather more than 
a half to rather less than three-quarters of the usual allowance*. This view has 
been subsequently acted upon by the English pendulum experimenters, in reducing 
their observations ; but it has not beeu yet adopted by the F rench. The experi- 
ments of Professor Carlini were calculated to afford a practical illustration of the 
correctness of Dr. Young’s reasoning. 
Professor Carlini was engaged, in t he summer of 1821, in concert with Professor 
Plana, in determining the amplitude ol' the celestial arc, between the Hospice on 
Mont Cents and the Observatory at Milan,' by means of fire-signals made on the 
Roche Melon, and observed simultaneously at Milan, and at a temporary observa- 
tory established at the Hospice. Whilst thus engaged, Professor Carlini, being 
stationary for several days on Mont Cetus, and obliged to have time very accurately 
determined, for the purpose of comparing with the observatory at Milan, availed 
himself of the Opportunity to employ a pendulum apparatus of the same general 
nature as that used by M. Biot at Paris, which had been prepared at Milan some 
years before, under the direction of a commission of weights and measures, with 
the view of determining the value of the divisions of the national linear scale. 
As this apparatus differed in some few particulars from the original employed iii 
France, we shall briefiy notice the differences, presuming our ^readers to be ac- 
quainted with the apparatus of M.M. Borda and Biot. 
}■ In t1le y u » n apparatus, by means of two microscopes furnished with wire 
micrometers, the length of the pendulum may be measured without touching it; 
without approaching It ; without even opening the case which contains it. The 
measure is obtained by bringing the wires in contact with the images of the'knife- 
edge suspension, and of the upper and lower borders alternately of the platinum 
disk suspended to the thread: thus preventing the risk of deranging the equilibri- 
d“,\^e 8C * Wh,Ch the heat ° f the bod >' might - the very 
and lowwi^T ( n!. t a e v di8ta, ^ B i tak J eU between the suspension, and the upper 
^ the disk, give, the distance of the centre of the disk itself. 
diameter with a compass, an operation exceedingly difficult 
length mav^ he pessary precision, liy this apparatus of microscopes, the 
being attached mTt, 'll** plea * ur ?' ev™ during the time of oscillation; and 
.....fi. „„5 3sz&~~i “s —••>»«<-<* — «► 
separated. The coincidences of the oscillations were Obs^Ce, ' 1 ■ W 
image of the pendulum of the clock, reflected by mean, L bnn ? m S the 
*» - - - —4 
