Hiflory of AJlronomy for 1798. 38* 



tranfparcnt. This arofe from the thin vapours which at 

 that time were floating in it. 



It is of great importance to afcertain the laws according 

 to which the diftinanefs of the vifibility of an objea de- 

 creafes, either when the tranfparency of the medium through 

 which the objea is viewed is leffened, or when the thicknefs 

 of the ftratiim of the medium is increafed. Lambert, in his 

 Photometry, has given an account of many ingenious experi- 

 ments which he mnde on the decreafe of the quantity of 

 light, by its propagation through a medium imperfe&ly 

 tranfparent ; but M. de Sauffure is the firft who has con- 

 fidered the decreafe of the diftinanefs with which an objea 

 is feen through fuch a medium at different diftances. 



VIII. il'ijhry of AJlronomy for the Year 1798. Read in the 

 College de France, Nov. 20. By Jerome Lalande, 

 Infpettor and Dean of the College, and formerly Direclor of 

 the Ohferi'atory. 



JlJEING permitted for the tenth time to entertain the pub- 

 lic with the progrefs of a fcience which has engaged my at- 

 tention fifty years, I am happy in being able to announce 

 things ftill more interefting than on the laft occafion ; and, 

 in the firft place, the conclufion of that grand operation the 

 meafurement of the earth, or of 9* degrees of the meridian 

 from Dunkirk to Barcelona. 



About the middle of January, Delambre, impatient to be- 

 gin liis fatiguing labours, proceeded, notwithftanding the cold 

 and the rain, to prepare a bafc from Lieurfaint to Melun ; to 

 complete the wooden pyramids, which were feventy feet in 

 height, and to mcafure the angles. 



On the 24th of February he had already finifhed feven fia- 

 ti-ms for the angles at the bafc : three men had been em- 

 ployed for fi\ weeks to cut down and remove from the hi'.n- 



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