[JACKSON] MAGNETIC OBSERVATIONS 5 



pattern, which were used for the determination of Inclination and 

 Total Force. These instruments were compared with the standard 

 instruments at the Agincourt observatory both before and after the 

 cruise, and their index corrections and constants determined. 



The method of observation with the land instruments was given 

 in detail in my paper in Vol. V, series 111, of the Transactions of the 

 Royal Society of Canada. 



The sea magnetic observations consisted wholly in the determination 

 of declinations or variation of the compass. The manner in which these 

 observations were made is given below for a single heading of the ship. 



Whenever weather conditions would permit the observations were 

 made with the ship's head in as many of the principal compass directions 

 as possible, and when observations were made on the 16 points with 

 both port and starboard swings, the results were used to determine and 

 control the ship's deviation. These results were reduced according to 

 the method given in the Admiralty Manual for deviations of the compass. 



It is there shewn that the Deviation curve may be expressed in 

 the form 



Deviation=A +B sin 3 +C cos 3 + D sin 2 3 + E cos 2 3 



when the deviation does not exceed 20 degrees on any point and where 

 A is the constant part of the Deviation and is independent of the ship's 

 head and is due to the lubber line of the compass not being coincident 

 with the ship's fore and aft amidship line. B, C, D, and E are constants 

 dependent on the strength and location of the permanent and sub- 

 permanent magnetism of the ship. D and E remain constant for change 

 of magnetic latitude, while B and C vary with change of Inclination 

 and Horizontal Force. 



These changes in B and C are given by the formulœ — 



B = x tan I + y i ; C = x' tan I + y' g. 

 where x, y, x' and y' are constants dependent on the distribution ot the 

 ship's magnetism, I is the magnetic Inclination and h the Horizontal 

 Force ot the eaith's magnetic field. 



The constant A was determined at Rimouski for the Ritchie 

 compass to be — 1.93 by the method of reciprocal bearings, but the 

 values for the other constants did not prove to be satisfactory and as 

 these could be determined at sea quite as readily, and in fact, the con- 

 ditions would then be similar to the conditions under which sea 

 observations would be made, it was considered inadvisable to remain 

 longer at Rimouski for their determination and also owing to the 

 lateness of the season, it being then June 30, and our instructions being 

 to find out if possible the earliest date that navigation in Hudson Strait 

 would be possible. 



