2 co JVUfcellanea Curio fa. 



by the proportional Differences always ob- 

 ferv'd between the Sines of the Angles of In- 

 cidence and Emerllon. 



This Property is not always propor- 

 tional to the Denfity (at leaft not to the 

 Gravity) of the Refracting Medium. For 

 the Refra&ive Power of Glafs to that 

 of Water, is as 55 to 34, whereas its Gra- 

 vity, is as 87 to 34-, that is, the Squares of 

 their Refractive Powers are (very near) as 

 their refpective Gravities. And there are 

 fome Fluids which though lighter than Wa-. 

 ter yet have a greater power of Refraction ; 

 thus the Refractive Power of Spirit of Wine 

 (according to Dr. Hookas Experiments, Mi- 

 cro* . p. 220.) is to that of Water, as 36 to 

 33, and its Gravity reciprocally, as 33 to 

 36, or 36i. But the Refractive Powers of 

 Air and Water fecm to obferve the fimple 

 Proportion of their Gravities, direftiy ; as I 

 have compar'd them in the following Table. 

 The lumbers there exprefling the Refra- 

 ction of Water are taken from the Mean of 

 * Nine Obfervations at fo many feveral An- 

 gles of Incidence, made Jan. 25. 164?-. by 

 Mr. Gafcoigne the ingeniuos Firft Inventor of 

 the Micrometer, and the ways of meafuring 

 Angles by Telefcopes, and thofe of Air are 

 produc'd by the Experiment above related. 



* I am indebted for them to Mr. Flanrfteed, who had 

 cover* d them whh % his Obfervations y and feveral Taffages 

 .relating to them, from.his Letters to Mr. Crabtree, which 

 were happily pr'efervd in the lime of our Civil War by 

 Sir Jonas Moor, and Mr. Chriffopher Towneley ; and 

 are novo in the Hands of Mr. Richard Towneiey of 

 Towneley m Lancafhire, by whom they were imparted to 

 Urn. - The 



