220 



Hence Dr. Tiarks thinks it fair to conclude that the diameter of 

 the parallel circle, in which the longitude is measured, has in the 

 survey been taken somewhat too great, and consequently the earth's 

 ellipticity greater than the truth. He remarks that the measurement 

 of the spheroidal triangle concerned, determines only the actual flat- 

 ness of the part of the earth's surface on which it is situated, and 

 not the actual magnitude of the whole parallel, unless its curvature 

 be supposed perfectly uniform, which we cannot assume with confi- 

 dence ; while on the other hand, if we compute the ellipticity from 

 the result of the chronometrical determination, it becomes one 314th 

 instead of one 150th, and agrees with the most accurate measure- 

 ments obtained from different principles. The longitude of Falmouth 

 is finally determined to be 20 m ll s 'l of time, and that of the British 

 Consul's garden at Funchal, l h 7 m 39 s W. of Greenwich. 



Of the Effects of the Density of Air on the Rates of Chronometers. 

 By George Harvey, F.R.S.E. $c. Communicated by Davies Gil- 

 bert, Esq. V.P.R.S. Read May 13, 1824. [Phil. Trans. 1824, 

 p. 372.] 



Among the sources of error to which chronometers are liable, the 

 effect of the variable density of the medium in which the balance vi- 

 brates has been overlooked ; the author therefore proposes to inves- 

 tigate the effects of diminished and increased pressure of transference 

 from one to the other, and of the ordinary variations 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 ordinary 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 

 shows that these changes in the rates, as observed in the air-pump 

 and condensing apparatus, are independent of the changes of tem- 

 perature, 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 indicate ; 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. 



