15'2 - PHILOSOPHICAL TRANSACTIONS. [aNNO 1764. 



Millionth parts. Specific gravity. 



Compression of spirit of wine .... 66 — 846 

 Oil of olives .... 48 — 9I8 

 Rain-water . . . . r .46 — 1000 



Sea- water 40 — 1028 ■' 



Mercury 3 — 13395 



These fluids are not only compressible, but also elastic: for if the weight by 

 which they are naturally compressed be diminished, they expand; and if that by 

 which they are compressed in the condenser be removed, they take up the same 

 room as at first. That this does not arise from the elasticity of any air the fluids 

 contain, is evident ; because their expansion, by removing the weight of the at- 

 mosphere, is not greater than their compression by an equal additional weight: 

 whereas air will expand twice as much by removing half the weight of the atmo- 

 sphere, as it will be compressed by adding the whole weight of the atmosphere. 

 It may also be worth observing, that the compressions of these fluids, by the 

 same weight, are not in the inverse ratio of their densities or specific gravities, as 

 might be supposed. The compression of spirit of wine, for instance, being com- 

 pared with that of rain-water, is greater than in this proportion, and the com- 

 pression of sea-water is less. The weight of 324- ^^t of sea-water is equal to the 

 mean weight of the atmosphere: and as far as trial has yet been made, every 

 additional weight equal to that of the atmosphere, compresses a quantity of sea- 

 water 40 millionth parts; now if this constantly holds, the sea, where it is two 

 miles deep, is compressed by its own weight 69 feet 2 inches; and the water at 

 the bottom is compressed 13 parts in 1000. 



XLFJII. Concise Rules for Computing the Effects of Refraction and Parallax 

 in Farying the Apparent Distance of the Moon from the Sun or a Star; also 

 an Easy Rule of Approximation for Computing the Distance of the Moon from 

 a Star, the Longitudes and Latitudes of both being given. By the Rev. Nevil 

 Mashelyne, A.M., F.R.S. p. 263. 



The following rules, excepting one, are the same which Mr. M. before com- 

 municated to the R. s., but without demonstration, in a letter from St. Helena, 

 containing the results of his observations of the distance of the moon from the 

 sun and fixed stars, taken in his voyage thither, for finding the longitude of tiie 

 ship from time to time; since printed in vol. Hi. of the Phil. Trans. The two 

 rules for the correction of refraction and parallax, he had also communicated to 

 the public in his British Mariner's Guide to the discovery of longitude from like 

 observations of the moon; and added in the preface a rule for computing a second 

 but smaller correction of parallax, necessarv on account of a small imperfection 



