114 National Geographic Magazine. 
lunar tables being so inaccurate that an error of several hundred — 
miles was quite possible and an accurate determination would 
depend upon the respective errors of instrument, observation and 
the lunar tables happening to balance one another. Halley ven- 
tured to express the hope that the tables may be so amended that 
an error may scarce ever exceed three minutes, which would cor- 
respond to a degree and a half of longitude, amounting at the 
equator to a distance of a little less than one hundred miles. 
Messerschmidt, who preceded Bering as an explorer of Hastern 
Siberia, was according to Middendorf (Sib. Reise, iv. 1, p. 56) 
thirty-two degress out in his determination of the longitude, and 
the eastward extent of Asia in this region was underrated by that 
amount or thereabouts, on many maps. 
One other means of approximating to the meridian remained, | 
in the observation of eclipses. This from the comparative rarity 
of these occurrences in the case of the sun and moon, could 
with the imperfect instruments of those days be available but sel- 
dom. Owing to the difficulty of determining the exact time of 
the first and last contacts the longitudes computed by these obser- 
vations were liable to quite as great inaccuracy as those computed 
from the lunar tables. Still an ordinary spyglass would enable 
an observer to note the time within a minute or two, and, if he 
was possessed of the local time, a simple comparison with the ob- 
served time of the eclipse in some locality where the longitude 
was known would give a fairly good determination, considering 
the instruments and methods of those days. Of the four eclipses 
of the moon occurring in 1728-9 two might have been observed 
without difficulty by Bering, one would have been invisible to 
him, and one might barely have been noted, but in all probability 
was not observed by him. In none of the published reports of 
the expedition is any mention made by Bering or his officers of 
the occurrence or observation of an eclipse, which seems very sin- 
gular if by such an observation he was enabled to correct an error 
of 30° in the longitude of northeastern Siberia. However, Mid- 
dendorf states (Sib. Reise, iv. 1, p. 56) that “ Bering and his lieu- 
tenant in the years 1728 and 1729 observed in Kamchatka* two 
eclipses of the moon,” by which they corrected the longitude. 
He gives no authority for this statement. 
* It is possible that an eclipse observed at Ilimsk in Middle Siberia by 
Chirikoff is thus erroneously referred to. ; 
