370 EXPEDITION TOWARD THE NORTH POLE. 



tensity are practicable. Ou the sledge-journey tlie dip-circle may be 

 carried, and when baits are made longer tban necessary to determine 

 the place by astronomical observations, the dip and relative intensity, 

 according to Lloyd's method, should be ascertained. 



At winter quarters, in addition to the above-mentioned observations, 

 those of absolute horizontahintensity should be made with the theodolite 

 magnetometer, including the deter uiination of moment of inertia. Also 

 with the same instrument the absolute declination should be deter- 

 mined. 



The least that the observer should be satisfied with is the complete 

 determination of the three magnetic elements, namely, declination, dip, 

 and horizontal intensity. At one period, say within one week, three 

 determinations of each should be made. 



It is advisable that the same observations be repeated on three suc- 

 cessive days of each month during the stay at one place ; and that on 

 three days of each mouth, as the 1st, 11th, and 21st, or any other da^-s, 

 the variation of the declination-magnet be read every half hour during 

 the twenty-four hours; also that the magnetometer, or at least a theo- 

 dolite with compass, remain mounted at all times, that the variation of 

 the needle may be observed as often as practicable, and especially when 

 unusual displays of aurora horcalis take place. 



In all cases the time, which forms au essential part of the record, should 

 be carefully noted, 



iNfot long before starting on a sledge journey from a wintev station, 

 and soon after returning, the observations with the loaded dipping 

 needles for relative intensity should be repeated, in order to have a 

 trustworthy comparison for the observations which have been made ou 

 the journey. 



POECE OF GRAVITY. 



As the long winter affords ample leisure, pendulum experiments may 

 be made to determine the force of gravity, in comparison with that at 

 Washington, where observations have been made with. the Hayes pen- 

 dulum lent to the expedition. The record of the Washington observa- 

 tions, a copy of which is furnished, will serve as a guide in making the 

 observations. Special care should be taken while they are in progress 

 to determine the rate of the chronometer with great precision, by obser- 

 vations of numerous stars with the astronomical transit instrument, the 

 pointing of which on a fixed mark should be frequently verified. 



OCEAN PHYSICS. 



Beiiths. — Soundings should be taken frequently, when in moderate 

 depths, at least sufficiently often to give some indication of the general 

 depth of the strait or sound in which the vessel is afloat at the time. If 

 an open sea be reached, it should be considered of the greatest import- 

 ance to get some measure of its depth, and sin<ie no bulky sounding ap- 



