Hagstroem as to
what portion of a cloud should be observed. The latticework tube, the
cross wires in place of an object glass, and the vertical circle are
very obvious, while the horizontal circle is so much end on that it
can scarcely be recognized except by the tangent screw which is seen
near the lower telephone.
Two such instruments are placed at the opposite extremities of a
suitable base. The new base at Upsala has a length of 4,272 feet; the
old one was about half the length. The result of the change has been
that the mean error of a single determination of the highest clouds
has been reduced from 9 to a little more than 3 per cent. of the
actual height. At the same time the difficulty of identifying a
particular spot on a low cloud is considerably increased. A wire is
laid between the two ends of the base, and each observer is provided
with two telephones--one for speaking, the other for listening. When
an observation is to be taken, the conversation goes on somewhat as
follows: First observer, who takes the lead--"Do you see a patch of
cloud away down west?" "Yes.
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