80 Royal Society. 



whence . ^ I=^^7^'^(h;+ K^ ^'^ " * ^ «* 

 If on the right we substitute foi: cos *I its value, then 

 ,T H«cosa ,/V\ , HV . , 



'As the possibility of error is inversely as the force, I multiply the 

 equation by K, to give to the different determinations equal weight, 

 thus 



If^T-H^cosa ,/V\^HV . , 



having determined the dips in the customary maimer with the ap- 

 proximately known values of «, I obtain the values d I, which serve 



to find d Itj); ('€. the correction of I. I possess now with my in- 

 strument six needles, which I Iiope to compare very accurately with 

 each other in the course of this year ; but some months must first 

 elapse, as I make all these determinations in the open air, and the 

 bad autumn we have had has interrupted me in the work. I have 

 had two of my needles fitted according to Fox's method, with wheels 

 on their axles ; two others have brass indexes, as was formerly pro- 

 posed by Bernoulli and Euler (Berlin Trans. 1755), and I can now 

 determine the absolute intensity with the inclinatorium. I know 

 Fox's method only from a short notice in the London and Edinburgh 

 Phil. Mag. ; if I do not mistake, he proposed also to determine the 

 declination by the same apparatus. With ordinary needles there 

 remains an uncertainty. If we load the S. end of the needle so that 

 the N. end is about 10^ above the horizon, the S. end sinks down; 

 and if we seek the azimuth in which the needle is perpendicular and 

 then observe at about half a degree of azimuth on either side, the 

 inclination alters so rapidly with the azimuth, that I have thus been 

 even able to follow the diurnal variations of the declination ; and 

 the magnetic meridian may thus be determined for the observations 

 of absolute declination whilst travelling. 



I will not trouble you further as my letter is already so long, and 

 will only add one request. The Phil. Trans, arrive here rather late, 

 and the last communications which I have seen of yours contain 

 Keely's determinations. All the observations of the Erebus and 

 Terror have not yet appeared ; in the Atlantic I know only the total 

 intensities but without inclinations or declinations, and yet I am very 

 anxious for some determinations that have been made between 10° 

 and 20° of longitude in the higher latitudes to compare my calcula- 

 tions witli them. If your time permits, I should be very much obliged 

 to you if you could communicate to me the inclination and force at 

 some points. In the mean time I will occupy myself with the dis- 

 cussion of the two horizontal forces ; unfortunately the number of 

 determinations serving for this purpose is much smaller. For North 

 America those recorded by Lament in Dove's ' Repertorium ' are 

 for the most part in comparatively low latitudes. 



