APPENDIX. 155 
daily, as a matter of regular duty at every station, as well 
primary as secondary. At primary stations also the baro- 
meter and thermometer should be observed at regular inter- 
vals, and the magnetic variation ascertained by taking the 
sun’s azimuth immediately before and after the observation 
for time (noting the exact moments, and thus obtaining data 
for interpoiating to the time of observation). At such stations, 
likewise, a careful investigation of the index errors of Sex- 
tants should be made, the zero points, or index corrections 
of the sympiesometer should be determined by a careful 
comparison with the mountain barometer, (giving time for 
the instruments to attain the same temperature) and the 
difference noted in the observation books. ‘The necessity of 
frequent comparisons of these instruments will be apparent, 
if it be considered that in the event of fracture of the baro- 
meter tube, no other means will exist by which the zero 
point of a new one can be determined. Occultations of 
stars by the moon, and, if possible, eclipses of the satellites 
of Jupiter, should be observed whenever an opportunity may 
occur. The former especially, affording the best known 
method of ascertaining the longitude by a single observation, 
should be constantly borne in mind, and the Almanack con- 
sulted several days in advance, so that no occultation of a 
large star, certainly identifiable, should be allowed to escape 
through inadvertence. 
The Committee especially recommend that every obser- 
vation made should be registered in a book devoted to that 
purpose, and preserved in the exact terms of the readings 
of the instruments and chronometer, and kept rigorously 
separate in its statement from any calculation thereon 
grounded ; and that the observed or presumed index or zero 
corrections, whether of chronometer, sextant, barometer, or 
other instrument, should be stated separately in every case, 
