CHR 
lOM, and is reckoned from the year B. C. 
312. It is generally supposed to have be- 
gun in the spring. It was used in a large 
district of Asia, and adopted by the Jews. 
The Spanish aera, founded on a division of 
the Roman provinces among the Triumviri, 
was long in use in Spain and Africa, and was 
adopted in the dates of the principal councils 
and synods held in those countries. It is 
reckoned from the first of January B. C. 38. 
This was afterwards superseded by 
The Christian aera. Learned men have dif- 
fered in opinion with respect to the exact 
time of the birth of Christ, some placing it 
four, others seven, years earlier than the first 
year of the Christian aera. The uncertainty 
which exists upon this point arises from 
the mra not having been used until so many 
centuries had elapsed, that it was im- 
possible to fix the date with accuracy. 
This is, however, of very little consequence 
in the application of this aera to chronologi- 
cal purposes, for all are agreed as to the 
numerical date of every year, the year 1808 
for instance being universally received as 
the year 1808 of the Christian aera, al- 
though probably not the exact measure of 
the time which has elapsed from the birth 
of Christ. This ®ra was invented about 
the year 527 by Dionysius, a Roman 
abbot, who reckoned the first year of it to 
correspond with the 4714th of the Julian 
period. It may be useful to give the reader 
a view of the years of the other principal 
teras which correspond to the first of this : 
according to Playfair, (who, it is to be ob- 
served, differs in many respects from other 
chronologers, but is nevertheless a most 
respectable authority) these are the 4008th 
year of the world, the first year of the 195th 
Olympiad, the 754th year of Rome, the 
749th of the Nabonassarean aera, the 313th 
of the Seleucidae, the 46th Julian year, and 
the 39th of the Spanish aera. 
The aera of Dioclesian was used pretty 
generally by the Christians previous to the 
invention of the Christian aera. It is dated 
from the year A. D. 284, and probably 
took its rise from the persecution under 
tliat Emperor, although its date is com- 
puted from the first year of his reign. 
The Hegira, which may be called the 
Mohammedan aera, is founded upon the 
flight of Mohammed from Mecca to Me- 
dina, to escape the pemecution of his ene- 
mies, and is computed by his followers 
from A. D. 622. The beginning of their 
year is however made to correspond with 
the 16th day of July. In comparing any 
CHR 
year of this sera, therefore, with the cor- 
responding year of the Christian aera, it will 
be necessary to bear this in mind before it 
can be done with accuracy. The same 
may also be observed with regard to some 
of the other aeras, the beginnings of the 
years of which do not exactly correspond 
with that of the Julian year. 
The Persian aera, or the aera of Y ezdejerd, 
is the last we shall notice. Yezdejerd was the 
last of the Persian monarclis who was sub- 
dued by the Saracens. According to the 
opinion of the most reputable modern chro- 
nologere, this aera commenced in June A, D. 
632, corresponding with the beginning of the 
eleventh year- of the Hegira, and with the 
first year of the reign of Yezdejerd. Tlie 
years of this aera, like the Nabonassarean, 
consist of 1 2 months of 30 days, with an ad- 
dition of 5 intercalary days at the end, 
making in all 365 days. 
The limits of our plan will not allow us 
to enter more minutely into the details of 
this important science. For these we must 
refer to separate treatises on the subject. 
The abstract which is here given will, how- 
ever, be found sufficient for all the general 
purposes of the historical student. We have 
purposely refrained from giving a chrono- 
logical table of remarkable events, as such 
tables are to be procured with very little 
trouble. Various ingenious methods have 
been invented of associating the name of 
some remarkable event with the date of its 
occurrence, with the view of impressing it 
on the memory ; for some account of these 
we must refer to the article Memory, Ar- 
tificial. 
CHRONOMETER, an instrument or 
machine for measuring time. The word is 
more particularly used by workmen and 
navigators to denote a watch or portable 
machine, in which, by the nature of the es- 
capement and the compensations for heat 
and cold, mean time is or ought to be kept 
with sufficient accuracy to detemine the 
longitude at sea. 
The relation between time and longi- 
tude will be fully explained hereafter : it 
will therefore be sufficient in this place to 
remind the reader, that the rotation of the 
earth upon its axis brings the several places 
upon its surface, in succession, opposite the 
sun, causing day and night ; so tliat the ab- 
solute instant of noon or of any other de- 
terminate apparent time of the day at each 
place must be earlier, at a place which lies 
to the eastward of another, with which 
that place may be compared. From this 
