308 On a Specific-gravity Bottle for Liquids Inflammable in Air. 



for as the values of the two librations differ in amount, the circle 

 WENS is not a true representation of the excursions of the 

 intersecting point E and W, N and S ; so when the greatest 

 deviation from mean libration occurs, the real path of the inter- 

 secting point on the apparent disk is a wide ellipse, which gra- 

 dually contracts to a narrow ellipse as the epoch of mean libra- 

 tion is approached. This will be the case proportionally with 

 every point on the apparent disk, and the displacement will be 

 in every possible direction and at every conceivable angle with 

 the centre of the apparent disk. This suggests that by far the 

 most effective mode of determining positions on the moon's surface 

 is by measures for points of the first order ; for let x' y' represent 

 the measures in right ascension and declination from the east and 

 north limbs of the point E, x and y will be the corresponding 

 rectangular coordinates necessary to determine the selenogra- 

 phical position when the librations of the centre and the other 

 elements are ascertained/' 



There are some other defects which I have noticed. The in- 

 clination of the moon's equator to the ecliptic is given as 

 1° 30' 11", the text-book's value. Mr. Proctor, who finds great 

 fault, and properly so, with text-book repetitions, seems to be 

 unaware that Wichmann's determination, 1° 32' 9" (Ast. Nach. 

 No. 631), is used in the Nautical- Almanac Office. 



Mr. Proctor mentions the lunar seasons ; but he omits, if I 

 mistake not, the point at which each commences. Thus in 

 the northern hemisphere spring commences when © — & =0°, 

 summer when © — £ =90°, autumn when © — 8 =180°, and 

 winter when ©—S3 =270°. W. R. Birt. 



XXXVII. Specific-gravity Bottle for Liquids spontaneously In- 

 flammable in contact with Air. By Alfred Tribe, Esq.* 



THE bottle usually employed for specific-gravity determina- 

 tions of liquids consists, essentially, of a light flask pro- 

 vided with a perforated stopper. By means of this arrangement 

 sufficiently accurate results can be readily obtained when the 

 liquid is not very volatile or violently acted upon by the air. 



To meet the requirements of volatile liquids, Regnault em- 

 ployed a flask having a solid in place of a perforated stopper, 

 and a neck somewhat longer and narrower than the old form. 

 The liquid is poured up to a mark on the neck; and the water- 

 value of this being known, the specific gravity is found. It is 

 obvious that by this method loss by evaporation or expansion 

 during weighing is prevented. 



* Communicated by the Author, having been read at the Meeting of 

 the British, Association held at Bradford, September 18/3. 



