H. G. FORDHAM-HERTFORDSHIRE MAPS. 
In the third period the Ordnance Survey becomes the basis of all 
maps, whether published by public authority or private effort, and 
the maps have thus a certain stereotyped excellence and uniformity 
of matter and detail which cannot he mistaken. 
But this classification, although sound, is somewhat vague in its 
limits in point of time, and I prefer one more scientific in its 
foundation, but which substantially, though more accurately, defines 
much the same periods. 
The early map-makers adopted as a basis either the meridian of 
the Azores or of the Canary Islands, which had come down to them 
from Ptolemy, but there was a good deal of doubt about the 
matter, and consequent variation in the meridian of longitude 
selected by different geographers. Blaeu, in the preface to the 
first part of his ‘ JYovus Atlas’ (1638, French edition) says:— 
“ Ptolomee a mis le premier Meridiem aux Isles Fortunees , qu’on 
nomme aujourd’huy Canaries ; et depuis les mariniers Espagnols 
l’ont mis aux Isles des Assores, et mesmes quelques-uns au milieu 
de VEspagne.” Camden also, in his address to the Header, which 
is the preface to his edition (Latin) of 1607, as translated by 
Philemon Holland (1610), deals with the then current difficulty 
thus:— 11 ‘ Mathematicians wil accuse me as though I had wholy 
missed the mark in the Cbsmographical dimentions of longitude, and 
latitude. Yet heare me I pray you. I have carefully conferred 
the Locall tables new and old, Manuscript, and printed, of Oxford 
and Cambridge, and King Henry the Fifth. In the latitude they 
doe not vary much from Ptolomy, but agree wel together, neither 
do I thereupon imagine with Stadius, that the globe of the earth 
is removed from his centre, therefore I have relied upon them. 
But in the Longitude there is no accord, no consent at all. What 
should I then doe ? When as therefore the moderne navigators 
have observed that there is no variation of the Compasse at the 
Isles of Asores,* I have thence begun with them, the account 
of Longitude as from the first Meridian, which yet I have not 
precisely measured.” 
Martin Cortes, in his hook on ‘ The Arte of Navigation,’ published 
at Seville in 1556,f lays down the rule that for a first meridian 
of longitude we should draw a vertical line 11 through the Azores, 
or nearer Spain, where the chart is less occupied.” In the black- 
letter pamphlet of 1594 entitled 1 The Seamens Secrets,’ the 
author, the celebrated navigator John Davis, states that the first 
meridian passed through St. Michael, because there was no variation 
at that place; * the meridian passing through the magnetic pole 
* The identity of the magnetic meridian with any particular meridian at 
a given time was, of course, an entirely unsound basis for fixing the first 
meridian of longitude, as the variation of the compass at any point on the 
earth’s surface is never constant. In 1581 the variation at London was 
11° 15' E. ; in 1657 there was no variation there, and it moved westerly until 
1815, when it was 24° 27' W., and is now returning eastwards. (See 
‘Encyclopedia Britannica ,’ 9th ed., vol. x (1879), article ‘Geography,’ 
at p. 187.) 
t English translation, London, 1561. 
