M'etTjod to determine the Magni^mig Tower oj i t:Ufcopcs. ^7 

 vacuum. For this purpofe I caufed the phofphorus to afcend 

 through the quickfilvcr of a barometer, and it exhibited no 

 light*. In order that this experiment may completely i'uc- 

 ceed, you muft employ a barometer, the tube of which has 

 been well freed from atmofpherlc air, by boiling the quick- 

 filvcr in the tube. If you ufe a barometer the quickfilvcr of 

 which has not been boiled, the fmall quantity of air con- 

 tained in it will be fufficient to make the phofphorus lu- 

 minous ; but this appearance will be of fliort duration, be- 

 caufe the exhaufted fpace of fuch a barometer can contain 

 only a very fmall quantity of oxygen gas. 



XV. A Jimple Method of defertnining the magn'ifj-ing 

 Power of Telef copes. By Mr. S. Varlev. Conwiunl^ 

 cated hy the Author f, 



JL HOUGH the fubjeft on which I wifh to trouljlc voij 

 with a few obfervations muft be well underftood bv many of 



* It has been already inconteftably proved by experiments, that phofoho- 

 rus is not luminous in a vacvium. BefiJes thofe mentioned in the treatife 

 entitled Uijoer d.u Lcuchten des Phof/ibors im atmofjiirifihen S'ickgas, 

 Weimar 1795, P- 5^> ™*y ^^ mentioned the following: Hawkfljce ap- 

 proached very near to this difcovery, and to that alio, made by t!ie author, 

 of phofphorus inflaming in Vacuo. He drew feme lines with phofphorus 

 on a piece of paper, and fufpended it within the receiver of an air pump. 

 '♦ After long pumping,'' fays he, " the luminous appearance was appa- 

 renily brighter." See the Pbilofiphical Tianfatt^ions, vnl, xx'w, p. iS6^. 

 Lemery mentions experiments of the like kind made by Hpmberg, in conr- 

 jundlion with M. D'Alence, at Paris, and which corrcfpond exadtly wiih 

 thofe of Hawklhee. See hi<i Corns t^i- Chymic, Paris 16S7, p. 577. Lavni. 

 flcr found that phofphorus did not intlame, McmrArci dc I' Academh, an, 

 J 777. P- 65; and Profeirors Lempc and Lanipadius, that phofphorus was 

 not luminous in the Toirictllian vacuum. Sec Sthercr's Nacbiri:'/', 

 {>. 200. 



t A " Conftant Reader," in a letter of 23d Mav. but which did not 

 reach Ui in time for our laft Number, propofcd die following qucllion; 



^ f " W hit 



