8 INSTRUCTIONS FOR MAGNETIC OBSERVATIONS. 



brought along. Sketches of geological formations, rock exposures, &c., may frequently be 

 made to great advantage. 



When possible, the subject of the sketch should always be brought along, and a corresponding 

 number made to indicate the relation of the two. 



2. Sketches of Indians should be made and colored from life, with care to fidelity in com 

 plexion as well as feature. 



In their games and ceremonies, it is only necessary to give their characteristic attitudes, with 

 drawings of the implements and weapons used, and notes in detail of each ceremony represented. 

 It is desirable that drawings of their lodges, with their historical devices, carving, &c., be made 

 with care. 



INSTRUCTIONS FOR MAGNETIC OBSERVATIONS. 



BY J. E. HILGARD, ASSISTANT U. S. COAST SURVEY. 



A. Observations with azimuth compass. 



These observations will be made by reading the magnetic azimuth of the sun, near rising or 

 setting, and noting the time. An observation should be taken on each limb of the sun, and the 

 mean used. The error of the chronometer must be known by a suitable observation for time. 

 The sun s true azimuth and resulting magnetic declination will be computed according to the 

 printed form. 



B. Observations ivith the magnetometer and dip circle. 



1. Observations of dip, declination, and relative horizontal intensity will be made daily, if 

 practicable. 



2. One set of observations of dip, without reversal of poles, will be made with each needle 

 whenever time permits, and will be recorded in the printed form. At convenient stations, at 

 intervals of four or five weeks, the connection depending on the reversal of poles will be deter 

 mined by four sets of observations with reversal, with each needle. The dip circle may be 

 placed in the magnetic meridian by means of the magnetic bearing of some object, determined 

 by the azimuth compass. 



3. Observations of declinations will be made and computed according to the form prepared 

 for that purpose. The zero of the collimator magnet will be determined once a week by inversals. 

 The observation of true azimuth may be made on the sun,, or any star near rising or setting. 



4. The observations of relative horizontal intensity will be made by vibrating one or both of 

 the larger magnets in the vibration apparatus, and observing the time of 200 vibrations. 



The largest arc of vibration should not exceed 2; observations of deflection, for attaining 

 the magnetic moment of the vibrating magnets, will be made once a month, in connexion with 

 the observations of vibration by deflecting one of the shorter magnets with the magnets used in 

 vibrating ; the observations being made and recorded strictly according to the printed form. 

 These sets of deflections should be observed with each magnet, at distances no less than 1.5 feet 

 from the deflected magnet. 



5. The observations of horizontal intensity cannot be reduced to absolute measure, until the 

 moments of inertia of the vibrating magnets, (with their stirrups and suspension,) designated 

 by K in the formula, are determined. This is done by vibrating them loaded with a ring, the 

 moment of inertia of which K is known by its dimension and weight. Both inertia rings- should 

 be used for this purpose, and at least five series of vibrations, with and without rings, should 

 be observed with each magnet. 



These observations, as well as those for the temperature, co-efficient q, may be made during 

 the ensuing winter. They may be made in a room, and are not influenced by local attraction. 



