Pendulum Experiments on Mont Cenis. 155 



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 hemispherical hill of such 

 matter, on the summit of which the pendulum should be placed, 

 would be to diminish the correction, deduced from the dupli- 

 cate proportion from the earth's centre, about ^th; that, in 

 like manner, a tract of table-land, considered 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, according 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 experimentors, in reducing their observations ; but 

 it has not been yet adopted by the French. The experiments 

 of Professor Carlini were calculated to afford a practical illus-- 

 tration of the correctness of Dr. Young's reasoning. 



Professor Carlini was engaged, in the summer of 1821, in 

 concert with Professor Plana, in determining the amplitude of 

 the celestial arc between the Hospice on Mont Cenis and the 

 Observatory at Milan, by means of fire-signals made on the 

 Roche Melon, and observed simultaneously at Milan and at a 

 temporary observatory established at the Hospice. Whilst 

 thus engaged, Professor Carlini, being stationary for several 

 days on Mont Cenis, 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 pen^ 

 dulum 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 Unear scale. As this apparatus dif- 

 fered in some few particulars from the original employed in 

 France^ we shall briefly notice the differences, presuming our 

 readers to be acquainted with the apparatus of MM. Borda 

 and Biot. KWiivwtijB -.jiii ^miJj. 



1. In the Milan apparatus, by Witsans'df twO' mTCrOscopes 

 (urnished with wire micrometers, the length of the pendulum. 



