135 



the cylinder being flattened so as to yield to the expansion of the 

 water forced in. By weighing this instrument full of water, before 

 and after it had been submitted to the pressure, the quantity of water 

 forced in was ascertained; whence it appeared that under a pressure 

 of 326 atmospheres, the water had sustained an increase of 3^ per 

 cent. In a future communication the author proposes to detail the 

 results of a new set of experiments on the compressibility of water, 

 which he hopes will be susceptible of greater precision. 



Astronomical Observations. By Stephen Groombridge, Esq. F.R.S. 

 Read June 29, 1820. [Phil. Trans. 1820, p. 330.] 



In the present improved state of astronomical observations, it is 

 material to possess the readiest and most accurate means of finding 

 the apparent time. The right ascension of certain fixed stars having 

 been precisely obtained relatively with each other, and with the equi- 

 noctial points during the course of many years, affords the direct 

 method of ascertaining the right ascension of the mid-heaven : hence 

 the convenience of having the corrections of these stars in the form 

 of tables, that the same may be taken out at one view with the ar- 

 guments of the sun's longitude, and of the moon's node. For this 

 purpose the mean diurnal motion is adapted to the longitude of the 

 sun, as found in the Nautical Almanac, at the time the star passes the 

 meridian. The mean epoch is reduced to the vernal equinox less 

 four seconds, in order to render the corrections additive ; which, being 

 an universal period, the same applies to all parts of the world. To 

 these tables Mr. Groombridge has subjoined some observations of 

 the planets at and near the oppositions ; also of the solstices of the 

 last two years, and of the comet of 1819. 



On the Black Rete Mucosum of the Negro, being a Defence against the 

 Scorching Effect of the Sun's Rays. By Sir Everard Home, Bart. 

 F.R.S. Read November 9, 1820. [Phil. Trans. 1821, p. 1.] 



The use of the black rete mucosum of the negro is a subject which 

 has fruitlessly engaged the attention of the physiologist. The author's 

 mind was directed to this inquiry by the circumstance of a silver fish 

 having its back scorched in consequence of the removal of some trees 

 which shaded the pond in which it lived; this recalled to Sir Everard's 

 recollection the circumstance of his having suffered severely from the 

 scorching sun of the tropic, upon parts of the body protected from 

 the direct rays of the sun by thin white linen, and led him to suspect 

 that the noxious effects were derived not, as has commonly been sup- 

 posed, from the mere heating power of the sun's rays, but from the 

 joint agency of heat and light : he therefore made certain experi- 

 ments, detailed in this paper, which show that the face and hands 

 may be exposed to a temperature of 100 to 120, without producing 

 pain, provided light be excluded ; but that if the same, or even an 

 inferior degree of heat, be produced by the direct light of the sun, it 



