My previous blog raised the issue of the myth of the two St Helena's. There seems to be two schools of thought - 1) that the depiction of two islands on the early charts was the inevitable result of inaccurate measurement of longitude. 2) that the island(s) charted were in fact large rock formations a few metres below sea level which at times would have been visible to early sailors.
I have no claims to expertise on this, but have been forwarded this reference which is I think worth perusing by anyone who is interested.
The origin of the myth of New St. Helena is unclear. Supposedly located east of St. Helena at approximately 16½°S 4°E, New St. Helena was coveted and sought for by the Dutch after they relinquished St. Helena for the Cape. New St. Helena is marked on the Universal Hydrographic Chart of Jean Guérard, 1634 and on Jansson's 1646 chart of the South Atlantic Mar di Æthiopia Vulgo Oceanus Æthiopicus (part of this chart is illustrated above) from Volume V of the Novus Atlas. Although New St. Helena continued to be marked on maps and charts to as late as 1803 (the map of Africa by Rochette), it appeared with decreasing frequency through the 1700s. The disappearance of New St. Helena could be very rapid; a 1713 map of Africa by Aa shows the island but it was removed from a subsequent map of Africa by Aa published just one year later.