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, carying, &c., be made 
with care. 
INSTRUCTIONS FOR MAGNETIC OBSERVATIONS. 
BY J. H. HILGARD, ASSISTANT U. 8. 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 with the magnetoneter 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 are 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-eflicient gy, may be made during 
the ensuing winter. They may be made in a room, and are not influenced by local attraction. 
