16 



LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 



I have allowed, therefore, a loss of magnetism in this needle equivalent to 5^*4 in 556 

 seconds at all the stations subsequent to Mazatlan, and have accordingly deducted 

 •00848 from the logarithms of the squares of the times of vibration of No. 7- at those 

 stations. 



We now proceed to a similar general intercomparison of the needles at Martins 

 Island. 



Here, with the exception of No. 5, which appears to have sustained a slight loss 

 of magnetism, the agreement of the quotients shows the general steadiness of the 

 magnetic condition of the needles. In the case of No. 5. the difference does not ex- 

 ceed the limits of occasional error of observation ; the results with this needle sub- 

 sequently to San Bias may not deserve to be regarded asfulli/ equal in value to those 

 of the other needles ; but the amount of error in the final determinations hazarded 

 by retaining its results independent of correction is insignificant. 



As this Table includes the whole interval between the observations at Panama in 

 1839, and tliose at Martins Island in 1840, we may regard it as substantiating the 

 general steady magnetic condition of the needles in the whole of that interval, with 

 the exception of the changes already noticed in Nos. 7- and 8, which have been traced 

 to the period of their occurrence, and their amount examined and allowed for. 



The times of vibration at all the stations visited subsequently to March 1839 were 

 taken in arcs commencing with 20° : the time of the chronometer was noted at every 

 10th vibration during 300, and the mean time of 200 vibrations derived from ten 

 partial results, i. e. from the 0th and 200th, the 10th and 210th, the 20th and 220th. 



No satisfactory experiments having been made to determine the individual coefii- 

 cients in the correction for temperature of these needles, I have taken an arbitrary 

 coefficient for that purpose, being the arithmetical mean of the coefficients experi- 

 mentally ascertained for the twenty-nine needles specified in the following list : — 



Needle 

 Needle 

 Needle 

 Needle 

 Needle 

 Needle 

 Needle 



Phil. Trans., 1828. Art. I. 



n 



5 M6m. de 1' Acad. Imp. de 



2 



4 



L(4) 

 L(a) 





St. Petersbourg, 1824. 



Trans. R. I. A., vol. xvii. 

 VBrit. Assoc. Report, 1835. 



