ch. vn] Medieval Nautical Instruments 57 
alidada is a straight-edge across the ring moveable with 
two sights, and a pin ties them all together. 
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 alti- 
tude 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 devel- 
oped 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 1 . 
Besides the astrolabe our earliest Arctic navigators 
were supplied with large blank globes on which they 
puzzled out the navigation problems, an armillary sphere, 
a great chart with all that was known or conjectured 
on it, smaller navigation charts, compasses and hour- 
glasses, and the regiment of Medina, translated from the 
Spanish at the instance of the Arctic navigator Burrough. 
With such slight and rather unreliable help our brave 
seamen of the 16th century, in great peril and difficulty, 
found their way over the trackless ocean, a way now 
made easy for their descendants. 
1 There are several astrolabes in the British Museum, one of 1280, 
another of 1342 ; one at King's College, Cambridge (1540) ; two at Gonville 
and Caius College, one of early 14th century date, the other, rather 
later, formerly belonged to Caius himself ; one at South Kensington 
( r 374); one at Oriel College, Oxford, in rather bad condition; three at 
Morton College, one of 1350, another 1571, and a third very heavy one. 
At Merton there is also a very old quadrant of 12-inch radius, and a small 
disc of brass with pointers. At the Bodleian there is a Persian astrolabe. 
Mr Hyett's astrolabe at Painswick House has 21 stars marked and one 
ring at the back ; 36 festivals are marked. The number of English Saints 
shows it to be English. The interesting astrolabe which belonged to 
Sir Francis Drake is at Greenwich, its date is 1572. It belonged to the 
Earl of Chesterfield, who gave it to the Rev. T. Bigsby in 1783. Mr Bigsby 
gave it to King William IV, who presented it to Greenwich in 1833. 
