Figure of the Earth. — Harvey on Chronometers. 311 



ANOMALY IN THE FIGURE OF THE EARTH. 



So many ships touch at Madeira, and take a new departure 

 from it, that the longitude of the island is a matter of consi- 

 derable importance. Dr. Tiarks was therefore sent out by the 

 Board of Longitude to ascertain it, with sixteen watches, in 

 the summer of 1 822 ; and a remarkable circumstance occurred, 

 which was not within the object of his original mission. For, 

 in going from Greenwich to Falmouth, a difference of lon- 

 gitude was found equal to 20' 11" # 49; and in returning from 

 Falmouth to Greenwich, a difference of 20' 11"*13. Now, 

 the difference, as determined from the Trigonometrical Survey 

 (given in the third edition of the requisite tables), is only 

 20' 6" - 9 ; and this variation made it expedient to engage Dr. 

 Tiarks to verify his observations in the Channel. He was 

 furnished with twenty-nine chronometers, and was employed 

 from the latter end of last July till the middle of September 

 in sailing between Dover and Falmouth. His results are as 

 follows : 



Longitude of Dover Station, . . . h 5' 17"*54 E. 

 Portsmouth Observatory, 4 24 '77 W. 

 Pendennis Castle, . . 20 10 '85 W. 

 Madeira, 1 7 39 "08 W. 



From hence it is clear that the figure of the earth must be 

 somewhat different from that assumed for determining the 

 longitudes from the Trigonometrical Survey, and that about 

 5" must be added, in the latitude of the Channel, for every 

 20' of longitude which is deduced from it. 



THE RATE OF A CHRONOMETER VARIES WITH THE DENSITY OF 

 THE MEDIUM IN WHICH IT IS PLACED. 



Mr. Harvey, F.R.S. E., has lately discovered that the den- 

 sity of the medium in which a chronometer is placed has a 

 sensible influence on its rate, in most cases, producing an ac- 

 celeration when the density is diminished, or a retardation 

 when the density is increased. In a few time-keepers he has 

 found the reverse to take place, viz. a decrease of rate from 

 diminished density, and an increase from increased density; 

 but the former appears to be the most general effect. Mr. 

 Harvey has proved this to be the case by an extensive course 

 of experiments, and in which he has subjected many chrono- 

 meters to pressures, from half an inch of mercury to 75 inches; 

 and in all cases has found, that if a time-keeper gained by 

 increasing the density, it lost by diminishing it, and vice versa. 

 A difference of density denoted by an inch of quicksilver, is 

 sufficient to produce, in many chronometers, a visible altera- 

 tion of rate. 



The 



