The Zeno imposture was first studied by Professor Storm, in the light of the Zamoiski and Olaus Magnus maps, and he exposed the falsities of the narrative, and the imposture of the map. The whole subject was discussed in an exhaustive work by Mr F. W. Lucas, from which the above details have been taken[26]. The mischief done by the Zeno forgery, while it lasted, was very serious; causing confusion in the work of cartographers as well as mistakes in the reports of navigators. * * * * * In the period of the beginning of English Arctic exploration, the instrument mainly used for finding the latitude was the astrolabe. The cross-staff had been invented, but was not in general use, nor was the quadrant with a plumb-line, though it had been used by Columbus. The astrolabe was a circular metal ring with inlet plates and discs. These plates were fitted to drop into an inner depression of the ring, the principal one being called the _rete_. It consisted of a circular plate marked with zodiacs sub-divided into degrees, with narrow branching limbs having smaller tongues terminating in points, each denoting the position of a star. The plates, or “tables” as Chaucer calls them, were differently marked for places having different latitudes. Within all these scales of _Umbra recta_ and _Umbra versa_ there is a division into 12 parts for taking and computing heights and distances by an approximate method. The _alidada_ is a straight-edge across the ring moveable with two sights, and a pin ties them all together. [Illustration: Astrolabe in Gonville and Caius College, Cambridge (_early 14th century_)] The _alidada_ is for taking the altitude of the sun, and the _rete_ adjusted to this altitude shows the hour of the day. To take an observation the right thumb is put into the ring of the astrolabe, and the left side is turned against the light of the sun. The _alidada_ or rule is moved up and down until the rays of the sun shine through both sights. Then the number of degrees the _alidada_ is raised from the little cross placed to show the east line is the altitude of the sun, read off on the outer ring. The Spaniards constructed their astrolabes small and heavy, to prevent them from being blown about, not much over five inches in diameter yet weighing 4 lbs. The diameter of the English astrolabes was six or seven inches, sometimes more. This instrument, invented by Hipparchus and developed by Ptolemy, was in use until the days of Elizabeth. It has a peculiar interest for those who are fond of studying the history of maritime discovery, but it is by no means simple in construction and it is necessary to examine the astrolabe itself to understand it and its uses[27].