Much of Close's material is incorporated in the web page REubique[2] (de Santis 2002) along with details of his military career and further information communicated by one of Clarke's living descendants.

It was from this London address that Alexander Clarke, at the age of seventeen in 1846, applied to the Royal Military Academy, Woolwich as a Gentleman Cadet.

[12] Clarke was very unprepared for the entrance examination, having left a space of only three weeks for revision, and he was placed bottom of the list of candidates.

[14] Before Clarke could make progress on the calculation of the triangulation, the War Office intervened and abruptly dispatched him to military service in Canada.

Clarke met, and married,[15] Frances Maria, youngest daughter of Colonel Matthew C. Dixon, his commanding officer.

Superintendent Hall retired in 1854 and Yolland, the most competent member of the Survey, was passed over in favour of Colonel Henry James.

His endeavour was rewarded by his military promotion to 2nd Captain in 1855 and by being appointed head of the Trigonometrical and Levelling Departments of the Ordnance Survey in 1856.

The Clarke family lived in Southampton from 1854 to 1881, the last twenty years in a fashionable Georgian property at 21, Carlton Crescent.

He died suddenly in 1878 and after his death Clarke was made acting head of the Survey until the appointment of Colonel Anthony C.

Clarke, a man of some-what hasty temper (Close), strongly resented this summary order which involved such a complete break with his life's work and he promptly sent in his resignation from the Army.

"Clarke's retirement was a veritable disaster for the Survey, and his departure lowered the whole tone and scientific status of the department for many a long year."

[25] The text of the citation is as follows: "The medal which, in accordance with the usual rule has been devoted to mathematics and physics, has this year been awarded to Colonel A. Clarke for his comparison of standards of length, and determination of the figure of the earth.

His laborious comparison of the standards of length, carried out under General Sir Henry James, R.E., are universally regarded as models of scientific precision.

His determination of the ellipticity and dimensions of the earth from the great arcs of meridian and longitude involved a very high mathematical ability and an enormous amount of labour.

The conclusion at which he arrived removed an apparent discrepancy between the results of pendulum experiments and those derived from geodesy, and is generally accepted as the best approximation hitherto attained as to the figure of the earth.

With only his army pension to support the family he was in straitened circumstances and he was forced to cancel his subscriptions to London clubs and the learned societies.

[28] The Principal Triangulation of Great Britain[29] was initiated by the Board of Ordnance in 1791 and carried out under the direction of William Mudge and Thomas Frederick Colby.

Once the triangles had been fixed it was then possible to calculate all the sides of the mesh in terms of the length of either of the bases, one by Lough Foyle in Ireland and the other on Salisbury plain.

The final step was to use the distances and angles to work out the latitude and longitude of each triangulation point on the Airy ellipsoid.

The latitudes and longitudes of the triangulation were calculated on the Airy ellipsoid for which the semimajor axis (a) and inverse flattening (c=1/f with f=1-b/a) are In the penultimate section of the Principal Triangulation Clarke compared the calculated latitudes with the actual observed values and he adjusted the ellipsoid parameters so that the differences were minimized in a least squares fit.

In the final section of the report Clarke combined the British data with that for the meridian arcs of France, Russia, India, Prussia, Peru, Hanover and Denmark.

The result was In Clarke (1861) he notes that General T. F. de Schubert had published a paper in which he claimed that the meridian arc data established that the equator of the Earth was elliptical in form.

Clarke was prompted to analyze a larger data set from which he deduced that if the Earth was indeed a tri-axial ellipsoid with a polar semi-axis (c) and semi-axes a (maximum) and b (minimum) in the equatorial plane, then so that the inverse flattening varied between 309.4 and 286.8.

[31] In James (1863) there is a discussion of the European cooperation proposed by Otto Struve, namely that the Triangulations of Russia, Prussia, Belgium, France and Britain be connected so that arc along the parallel at 52°N could be measured.

The accuracy of the cross-channel survey was poor by the standards of 1860, and there were also doubts about the Triangulation in Valentia, so it was decided to repeat both sets of measurements.

The results would also have implications for combining arc measurements to determine the Figure of the Earth, so for this reason non-European standards were also included.

Clarke found that nominally identical standards differed by small amounts measurable in millionths of a yard (or thousandths of a millimetre).

Moreover, many US legal documents include boundary definitions in terms of latitude and longitude values defined on that ellipsoid.

In addition to his contributions to the reports of the Ordnance Survey, Clarke also published a small number of papers in the learned journals.

The title pages of many of the reports mention only Colonel Henry James, Superintendent of the Ordnance Survey, but in every case it is made clear that Clarke was de facto author.