I take leave to suggest for this end, that his Majesty the King, should cause a line of instrumental levelling, to be carried along the high road, from Rome to Naples, with the necessary care to insure accuracy, starting from the sill of the great door of St Peter's Church at Rome, as the nearest presumably fixed and invariable point of level, and ending at the Palace of Naples, or at some other determinate point, in or near the capital, and there, as well as at intermediate points, to be permanently marked upon blocks of stone. To this line, or to some of its intermediate points, the changeable levels of Serapis, and the levels of every other place in the kingdom could, by easy and known methods, be referred when required, and with absolute certainty. The operation itself, of effecting this, would be very inexpensive, and could be performed this present spring season, by either his Majesty's Corps de Genie, or by officers of the Ponte e' Stradi, if appointed for the purpose. Not more than six or eight weeks need be consumed in this work, which, when completed, would cause his Majesty's name to be renowned throughout Europe, for the promotion of science, by a work so important. This datum, once established, should be made to correspond, in his Majesty's kingdom, to the "Ordnance Datum" of our British national surveys.
In these suggestions, I know that I should be supported by the opinion, of all the greatest of European geologists and physicists, as also, I have no doubt, by those of his Majesty's kingdom.
Should his Majesty graciously sanction, the commencement of this work, I should be happy, before quitting Naples, to confer personally with the executive authorities, who may be intrusted with its execution, and to convey to them my views, as to the precautions and conditions necessary, to insure to the result its just value to science.
I have the honour to be,
Signor Commandatore,
Your most obedient Servant,
Robert Mallet, F.R.S.,
Mem. Inst. Civil Engineers, of England.
