219 



These experiments were repeated with bars of other dimensions, 

 which were loaded till they broke ; and from them the author also 

 infers that the elastic force of steel is not altered by temper, and 

 that the force which produces permanent alteration is to that which 

 causes fracture in hard steel, as 1 : 1'66 ; and in the same steel of a 

 straw yellow temper, as 1 : 2'56. From comparisons of the strain 

 required to cause permanent alteration in different kinds of steel, the 

 author concludes, that in the process of hardening, the particles are 

 put into a state of tension among themselves, which lessens their 

 power to resist extraneous force ; and the phenomena of hardening 

 may be referred to the more rapid abstraction of heat from the sur- 

 face of the metal than can be supplied from the internal parts, 

 whence a contraction of the superficial parts round the expanded 

 central ones, and a subsequent shrinking of the latter, by which the 

 state of tension is produced. 



A short Account of some Observations made with Chronometers, in two 

 Expeditions sent out by the Admiralty, at the recommendation of the 

 Board of Longitude, for ascertaining the Longitude of Madeira and 

 of Falmouth. In a Letter to Thomas Young, M.D. For. Sec. R.S. 

 and Secretary to the Board of Longitude. By Dr. John Lewis 

 Tiarks. Read April 29, 1824. [Phil. Trans. 1824,^. 360.] 



Dr. Tiarks was sent to Madeira in the year 1822 with 15 chro- 

 nometers, of which the rates had principally been ascertained in the 

 Royal Observatory of Greenwich ; he touched at Falmouth both in 

 going out and returning ; and having again ascertained the rates of 

 his time-keepers, he was thus enabled to obtain two distinct deter- 

 minations of the longitude of Falmouth, which differed about four 

 seconds of time from that which had been inferred from the Trigono- 

 metrical Survey of Great Britain. It became therefore desirable that 

 some further operations should be undertaken for the removal or 

 elucidation of this discordance ; and the following year a similar 

 method was adopted with 25 chronometers, for determining the dif- 

 ference of longitude between Falmouth and Dover ; this latter station 

 having been chosen as easy of access, and as being perfectly deter- 

 mined ; and the computations were made by interpolation, without 

 employing any other rates for the chronometers than those which 

 were observed in the different trips while they were actually on 

 board the ship ; and latterly, when Dover Roads became unsafe, the 

 operations were limited to the distance from Portsmouth to Fal- 

 mouth : thus, between the months of July and September, the ob- 

 servations were made three times at Dover, four times at Falmouth, 

 and three times at Portsmouth ; and the comparison of their results 

 affords a correction of five seconds of time for the difference of lon- 

 gitude of Dover and Falmouth, and of three for the difference of 

 Falmouth and Portsmouth, agreeing completely with the error of four 

 seconds, attributed from the observations of the preceding year to the 

 difference of longitude of Falmouth and Greenwich. 



