298 On the Improvement of Time-Keepers, 



mosaic represents a hunting party. Observations sur la Mo- 

 smque des Ancietis, a V Occasion de quelques Tableaux e^i 

 Mosdique qui se trouveiit a la Galerie des Peintres de V EUc- 

 teur Palatin, par M. VAhh6 Htefflin, in the Comment. Hist or. 

 Academics Theodora PalatincB, Vol, W . No. 3^ p. 89: Man- 

 helm 17S3; 4to. M. Haefflin compares mosaic in glass and 

 in stone with the paintings executed by means of the feathers 

 of birds by the Americans : Explicatio7i de la Mosdique de 

 Palceslrtne ; par M. I'Abbe Barthelemy: Paris 1760; 4to. 

 And in the Memoirs of the Academy of huscriptions, Vol. 

 XXX. Kircher in his Latium, and Montfaucon in the fourth 

 volume of his Supplement de V Antiq^dte Expliquee, had be- 

 fore attempted to give an explanation of this mosaic : Osser- 

 4}ationi di Ennio-Quirino Visconti, su due Musdichi antichi 

 istoriali : Parm. 1788; 4to. These two mosaics, which, ac- 

 cording to Visconti, relate to pyromancy, were found, in 1 788, 

 in the Campagna di Roma. They are now in the collection 

 of the chevalier Azzara ; Description de la Mosdique trouvte 

 a Seville, et pulliee par Ordre du Roi d'Espagne. Alexan- 

 der Laborde is about to publish a description of nine mosaics 

 found in Spain. 



XXXVII. On the Improvement of Time- Keepers, By David 

 RiTTENHOusE, LL,D, President of the American Philo- 

 sophical SccietT/*, 



J. HE invention and construction of time-keepers may be 

 reckoned among the most successful exertions of human 

 genius. Pendulum clocks especially have been made to 

 measure time with astonishing accuracy ; and if there are 

 still some causes of inequality in their motions, the united 

 efforts of mechanism, philosophy, and mathematics, will pro- 

 bably in time remove them. 



The last and least of those causes, which perhaps may be 

 worthy of notice when all others of more importance are re- 

 moved, is that rising from unequal density of the air, which 

 bv varying the actual weight of the pendulum will accelerate 



• Fioro the Trantactiom of the American Philofophical Society. 



or 



