Pendulum. — Difcover^ if the Tele/cope. SoJ 



pur|K)fe of compenfation for temperature will be obtained, as nearly as obfervatlon can indi- 

 cate its changes, by making the lengths of the bars inverfely as their expanfions, as known 

 from genefal experiment, or from tables. Thus, for example, if brafs and ijeel were ufed, 

 and their expanfions taken as 113 to 68, and the whole length of the brafs were 10 inches, 

 we may conclude that the changes of temperature would not alter the pendulum, provided 

 the above p/oportions were accurately kept, namely, that the length of the fteel were 16.62 

 inches. But if the proportions either of expanfion or length {hould be fo far miftaken or 

 neglefted, as that the brafs Ihould be made one-tenth of an inch too fliort, = 9.9 inches, 

 this piece would compenfate only 16.56 inches of fteel, and confequently (the fteel being 

 lengthened o.i inch) there would remain 0.16 inch of fteel uncompenfated ; or O.016 part 

 of the whole pendulum, which would produce a difference of lefs then one-tenth of a fecond 

 between the mean daily rates in fummer arid winter. 



The adjuftment for time is made by a fcrew adapted to the fprings of fufpenfion, or by 

 aity other of the ufual methods ; of which I do not mean at prefent to confider the advan- 

 tages and defedls. 



VI. 



On the Epocha of the Difcovery of the Telefcope, and the Opinion of Boyle, that Plants derive 

 their Nourijhinent from Water only. By Citizen £oiSSON/W£ *. 



Wi 



E fometimes find in books which are little known and read, particulars of information 

 of a very interefting nature. I have found in the Probabilia of the learned WefTeling, edi- 

 tor of Diodorus Siculus, a chapter (the eleventh) on the antiquity of aftronomical telefcopes, 

 of which 1 fend you a tranflation. The refearches it contains appear fufficiently curious 

 to deferve to be better known. 



The aftronomical telefcope f, by means of which the moderns have made difcoveries in 

 the heavens, which efcaped the penetration of the ancients, is, with juftice, placed among the 

 moft memorable inventions. It has often been enquired whether they were known to the 

 ancient aftronomers, and feveral writers have decided for the affirmative. They ground their 

 opinion on an ancient drawing in the fcholaftic hiftory of Peter Comeftor, in which Conrad, 

 a monk of the thirteenth century, had reprefented aftronomy J. " On the right hand of the 

 ftgure," according to father Mabillon, who had feen the manufcript, and mentions it in his 

 travels in Germany, there was " a figure of Ptolemy obferving the ftars, with a long In- 

 ftrument refembling an aftronomical telefcope with four fliding tubes §." This conje6ture 



* In a letter to citizen Millin, inferted in the Magafin Encyclop^dique, v. 466. 



+ i thus tranflate the words tubi optici. Note of the writer.— His objeft is to avoid the ambiguity of the 

 French word lunelles, which C\ga\i\t% fpeilacUi as well as telefcopes. I have not the original work. N. 



X The Latin of Mr. Weffeling is not clear in this paffage, for the undcrftanding of which, it is neceffary 

 to refer to the work of P. Mabillon, from which he quotes only a few lines. 



§ Ope inftrumenti longioris quod inftar tubi optici, quatuor du(5tus habentis, coEC'.nnaium eft. 



E e 2 * was 



