CONTENTS. xi 



JIT. A Memoir upon the Difcovery of America. By Mr. Otto. (Concluded 

 from page 77.) _ _ _ _ p. 107 



Deatli of Chevalier Bchem — Critical remarks and obfervatlons to prove that he was the difcoverer 

 of America. 



IV. Defcription of a Gravimeter, or Inftrument for meafiiring the Specific 

 Gravity of Solids and Fluids. By Citizen Guyton — . — p. 110 



On fpecific gravities — and hydrometers. Obfervations of the hydrometer of Nicholfon. Defcrip- 

 tion of an improved inftrument — Additional remarks. Formula- for finding fpecific gravitiej 

 witliout diftilled water, or the thermometer or barometer. Ufeful application of the inftrument. 

 Table of fpecific gravities of alcohol. Specific gravities of alloys of tin and lead. Account of 

 Gilpin's tables of ardent fpirit. 



V. Defcription of the Improved Air-Pumps of Prince and Cuthbertfon. With 

 Obfervations — — — — — P-^i9 



Familiar elucidation of the principles of the air-pump. Air-pumps with (lop-cocks and with valves. 

 Smeaton's pump. Brooks's experiments. Defcription of Prince's air-pump, which has no lower 

 valve or ftop-cock. Advantages. Remarks on condenfing engines. Defcription of Cuthbertfon's 

 air-pump, in which the valves are all metallic — Its great power. Comparifon between thefe 

 two pumps, Projedl for a pump which may unite the advantages of both. 



VI. Ufeful Notices refpefting various Objeds. — A Method of preventing 

 Heat in grinding. Concerning Gold, Silver, and other Metals reduced 

 into very thin Leaves by the Hammer. Globules for Microfcopes. On the 

 Plumb-Line and Spirit-Level — — — P* 'S'' 



Heat developed in grinding. A German grindftone which does not heat. Experiments to invefti- 

 gate this fubjeft. Succefs and advantages. Thicknefs of leaf gold, filver, brafs or Dutch gold, 

 tin or Dutch filver, and tin-foil. Method of forming very clear fpherules of glafs. Breadth of 

 the fined wire in fcconds of meafure. Spirit-level defcribed. Its adjuftment and degree of ac- 

 curacy. How to procure good tubes for levels. Method of grinding the infide of the tube. Im- 

 perfeftions of the fpirit. level. 



Mathematical Correfpondence — — — P- ^37 



Scientific News — — — — — P- 141 



Mcmoirtof the Polytechnic School. 



JULY 1797. 



1. Obfervations on Horizontal Refraftions which affcft the Appearance of 

 Terreftrial Objedls, and the Dip or Depreflion of the Horizon of the Sea. 

 By Jofeph Huddart, Efq. F. R. S. — — — p. 145 



Elevation of points of land by refraftion. Theory. Inverted appearance of a veffel at Its. Ex- 

 planation. Remarkable difference in the intenfity of Portland Lights dependent on th't theory. 

 The dip of the vifible horizoa i« rendered uncertain by this rcfraftion — Remedy. Geneial 

 annotation. 



in. Re. 



