YOL. LXXXVII.] PHILOSOPHICAL TRANSACTION*. gy 



— fy, ; but p cannot exceed 57 /; , and the horizontal parallax cannot differ from the 

 mean by more than T V part of the whole; so that the error arising from thence 

 cannot exceed 3 ff or A". This small error however may be diminished by giving the 

 quantity c for more than one horizontal parallax." 



Addition to the foregoing Letter. — " I have procured tables of the above-men- 

 tioned kind to be computed, which are intended to be inserted in a work now print- 

 ing by Mr. Mendoza y Rios. Allowance is made in them for the alteration of the 

 refractive power of the atmosphere, which is done by 2 new tables, one giving the 

 correction of the logarithms m and n, and the other the sum of the corrections of 

 tji and 3v. Now* it must be observed, that the quantities ^ and v vary only from 

 57" to 51 /; ; and therefore the corrections of tp and Sv, may, without any material 

 error, be considered as the same at all altitudes; and therefore the sum of the cor- 

 rections may be comprehended in a table, under a double argument, namely, the 

 refractive power of the atmosphere and the apparent distance. 



" In order to avoid as much as possible the inconvenience arising from using 

 negative quantities, or giving different cases, the table d is continued to 125° of 

 apparent distance, and the numbers in the table a are increased by O.G003, so as 

 to make them always positive; and to compensate this, the numbers in d are in- 

 creased by 0.0002, and those in the correction of fyt -f" h by 0.0001 . It was found 

 proper also to give the table c for 4 different values of horizontal parallax. The 

 above tables are short, and do not require proportional parts to be taken. The 

 only part of the work in which this is wanted, is in finding the angle answering to 

 the natural cosine of the true distance. In finding the natural cosine of the appa- 

 rent distance this is avoided, by neglecting the odd seconds in working the problem, 

 and adding them to the result." 



IF. On the Nature of the Diamond. By Smithson Tennant, Esq. F.R.S. p. 123. 



Sir Isaac Newton having observed that inflammable bodies had a greater refrac- 

 tion, in proportion to their density, than other bodies, and that the diamond re- 

 sembled them in this property, was induced to conjecture that the diamond itself 

 was of an inflammable nature. The inflammable substances which he employed 

 were camphire, oil of turpentine, oil of olives, and amber ; these he called " fat, 

 sulphureous, unctuous bodies ;" and using the same expression respecting the dia- 

 mond, he says, it is probably " an unctuous body coagulated." This remarkable 

 conjecture of Sir Isaac Newton has been since confirmed by repeated experiments. 

 It was found, that though the diamond was capable of resisting the effects of a 

 violent heat when the air was carefully excluded, yet that on being exposed to the 

 action of heat and air, it might be entirely consumed. But as the sole object of 

 these experiments was to ascertain the inflammable nature of the diamond, no at- 

 tention was paid to the products afforded by its combustion ; and it still therefore 

 remained to be determined whether the diamond was a distinct substance, or one of 

 the known inflammable bodies. Nor was any attempt made to decide this question 

 yoL. xviii. O 



