324 Analym of Books. [Sept. 



has been raised to a proper temperature, a cooling fluid is 

 applied capable of abstracting heat more rapidly from the sur- 

 face than it can be supplied from the internal parts of the steel. 

 Whence the contraction of the superficial parts round the central 

 ones which are expanded by heat ; and the contraction of the 

 central parts in cooling, while they are extended into a larger 

 space than they require at a lower temperature, produces that 

 uniform state of tension, which diminishes so much the cohesive 

 force in hard steel. The increase of bulk by hardening agrees 

 with this explanation ; and it leads one to expect, that any other 

 metal might be hardened if we could find a means of abstracting 

 heat with greater velocity than its conducting power." 



XIX. A short Account of some Observations madeivith Chrono^ 

 meters, in two Expeditious sent out by the Admiralfj/, at the 

 Recommendation of the Board of Longitude^ for ascertaining the 

 Longitude of Madeira and of Falmouth. In a Letter to Thomas 

 Young, MD. For. Sec. RS. and Secretary to the Board of Lon- 

 gitude. By Dr. John Lewis Tiarks. 



Dr. Tiarks terminates this article with the following summary 

 of the ultimate results he has obtained : — 



" From the foregoing observations we may now conclude, 

 that the longitudes laid down in the account of the Survey will 

 deviate from the truth, in the same proportion in which the paral- 

 els of latitude on a spheroid, having the degree of the meridian 

 in latitude 50° 4V equal to that of the earth, and the ellipticity 

 -^-f^ differ from thoFe of the terrestrial spheroid, the compression 

 of which is nearly -j-f^. The following comparisons will further 

 illustrate the subject. If the radius of the Equator be =3486908 

 fathoms, and the semi-axis of the earth = 3475550 fathoms, 

 which is nearly the result of the measurements in France, and 

 Bouguer's in Peru, and corresponds to the compression -j-^, the 

 length of the degree perpendicular to the meridian in latitude 

 50° 41'' will be 60975*7 fathoms. For the spheroid adopted in 

 the Survey, that degree is found 61,182 fathoms. The ratio of 

 these numbers is 296 : 297, and the correction of the longitudes 

 would be ^75-; the same correction is, by the clironometrical 

 observation, rj-^. The length of the geodetical line BD, sup- 

 posing the difference of longitude as determined in the account 

 of the Survey, viz. 1° 26' 47^-93", would be 33S231 feet; whereas 

 it was found to be 339397*6 feet; but if the longitude be 

 increased in the ratio determined by the chronometers, the line 

 will be 339334 feet, which is only ()S'6 feet short of the measure- 

 ment. The spheroid resulting from the compression which 

 would make the difference of longitude of B and D= 1° 27' 4*75?' 

 (as it ought to be according to the results of the chronometers), 

 and from the degree of the meridian in latitude 50° 4 1', viz. 

 60851 fathoms, would have these dimensions : radius of the 

 JEquator = 3487907 fathoi^isj semi-axis ::;= 3476687 fathoms; 



