270 Df Moll dn the [Oct. 



thh other eiid, and thii^ the medhim of these tv^^o velocities will 



five the velocity in tranqtiil air. This methbd wa^ not adopted 

 y thd French Acaderiirciahs bF'1738, 'iti'^theiV 'expdimcnts 

 betw^tin Monthlery and Moiitnilartfei' '^(Jiirtbii xi'is' fired at orie 

 of thfe^B stations, v^^hilst the obsfet-vets were at the oth<^i^," arid 

 thiisthe results remained affected by the whole effect of tt^ 

 ivind. It was found expedient therefore to repeat these' e:k^6ln- 

 ments with more accuracy, and this was executed" with'^t^t 

 precaution on Mr. Laplace's proposal, by Messrs. Afago^ Prdhyi 

 Mathieu, Bouvard, Humboldt, and Gay Lussac. ' Tfe'^btb^r?- 

 Bttents were made in 1822, on the basis of Monthlery and vill^- 

 juif. In two successive days, the 21st and 22d of June, 1822| 

 seven shots were fired on both stations, and observed on the 

 other; the diflference of time in vvhich the corresponding shbts 

 were fired at both stations not exceeding five minutes, sind 

 from these seven corresponding shots the result was deduced". 

 *^' These experiments having never been made in this toiXtiii/f 

 #i"th any thing like sufficient accuracy, His Royal llighhej^fe 

 Prince Frederick, second son of His Majesty the King of the 

 Netherlands, and Master General of the Ordnance, was pleased 

 to approve of our proposal of repeating the same, and to autho- 

 rise Lieutenant-Colonel Kuytenbrouwer, and the officers and 

 .men of the battalion of Artillery under his command, to give u$ 

 tfVery assistance in their power, and to take an actual part" tifr 

 ^ese experiments. 



"^As fitted places to iti&kfe these experinlents, two elevated spot:^ 

 were selected on the extended heaths of the Province of 

 Utrecht. One of these is a small hill between the town of 

 Naarden and the village of Blaricum, and called the Kooltjesher'gj 

 the other is somewhat higher, and situated on the right of th^' 

 road from Utrecht to Amersfoort, and very near the last town. 

 Both places were distinctly visible from one another, and the 

 distance was between 17000 and 18000 metres (9664 fathoms). 

 Our time was kept by two time-keepers, which the Minister of 

 Marine had kindly furnished us with ; one made by Arnold, the 

 other by our countryman Mr. Knebel. But the exact interval 

 between the observation of light, and the perception of sound, 

 and consequently the velocity of sound, was measured with 

 stiiall clocks with conical pendulums. They are made at Wesel 

 by Mr. Wilhelm Pfaffius, and proved remarkably well adapted 

 foi this purpose. It is well known that Huigens laid down the 

 properties of the conical, or centrifugal pendulum, but if we are 

 n6t mistaken, they were employed for similar purposes for th^' 

 fii^t! time by the German philosopher Benzenberg.* TheS^' 

 ciy^lils'Witfe'a cionical pendmum divide the 24 hours of th^ '<JafV' 



Vd "'^ -^ nOliL^H 



^. Sppae account of these qbcks is given in (5ilb<?rt's Annalen d. Pbysik, lS04ij,g. yfj^j 

 p. 4^4 : and New Series, vol. v. p. Ssi. * 4 r 



