ON COMPARING AND REDUCING MAGNETIC OBSERVATIONS. 



69 



posed to be the cause of the variations of the tension of vapour can also 

 produce the movements of the inclination needle ? 



It will be necessary to carefully verify this connection for different 

 places. I send you the diurnal curves for these two elements. 



. diurnalLva ft'/ a tionAoeLinclina tion. 



I I I l/8ffl?^/S67 Mill 



7 8 9/0 II ma. I z 3 • 



S 6 7 8 9 10 limlTL 



III. Extract of letter from Sir J. Henry Lefroy to Professor Steivart. 



I have virtually finished the comparison of Photographic Records of 

 Declination at Greenwich and Toronto for 1850, and have made up mean 

 curves for each month from the undisturbed days alone, generally 7 or 8 

 in number (the total number for Greenwich is 99). On plotting the 

 means they give, notwithstanding the small number of days in some of 

 them, very regular curves are obtained, and they present a feature which 

 is new to me. Sabine never compared Greenwich with his colonial 

 stations, and has not, that I remember, remarked it. It is that the N. end 

 of the magnet reaches the most westerly position of the 24 hours from 

 1 to 1^ hour earlier at Greenwich than it does at Toronto. This appears 

 in every month compared ; there are only seven ot them, as the whole 

 apparatus caught fire and was destroyed in June, and did not get to work 

 again before November ; but seven months are good for something. 



The detailed comparison of disturbed movements has not suggested 

 much beyond the fact that there is rarely any marked correspondence, and 

 that the movements are usually in contrary directions. I have transferred 

 all the Greenwich movements of any magnitude to my sheets, and this is 



