Proceedings of the Royal Society. 273 
the effect of the variable density of the medium in which the ba- 
lance vibrates has been overlooked, the author, therefore, pro- 
poses to investigate the effects of diminished and increased pres= 
sure, of transference from one to the other, and of the ordinary va- 
riations of atmospheric density upon the rates of chronometers. 
In respect to diminished pressure he found that chronometers 
gained by being placed in air of less density than that of the ordi~ 
nary state of the atmosphere, and that on the other hand they 
lost when subjected to air of greater than ordinary density. These 
experiments were made with a variety of chronometers, placed in 
the receiver of an air pump, or in that of a condensing apparatus. 
In respect to the influence of ordinary changes in the density 
of the air, the author remarks that pocket chronometers are more 
readily affected than box chronometers, but that they all exhibit 
an increased rate under diminished density, and vice versd. 
The author shews that these changes in the rates, as observed 
in the air-pump and condensing apparatus, are independent of the 
changes of temperature resulting from changes in the density of 
the air thus rapidly effected, and therefore proceeds to inquire 
into the actual cause of the changes which his experiments indi- 
cate; he refers them to an increase in the arc of vibration when 
the density is diminished, and to a diminution in the arc, under — 
increased density. 
Thursday, May 20. 
The Rey. Baden Powell was admitted a Fellow. 
A Letter was read 
To the President from Professor Berzelius, dated Stockholm, 
April 21, 1824, 
In this letter Professor B. announces his discovery of a combi- 
nation in a mineral, of chloride and oxide of lead, in the pro- 
portion of one atom of the former to two of the latter. He states 
that he has verified Mr. Phillips’s researches on uranite, and gives 
a somewhat detailed account of his experiments on fluoric acid, 
and of the properties of the base of silica, which he procured by 
