"As McNair wrote in a letter to Dunbar D. Scott (March 26, 1898):  Necessity is the mother of invention.  Mine surveying on slopes so steep as to be almost vertical suggested the possibility of accomplishing this by the use of standards on the engineer's transit sufficiently inclined as to admit of a perpendicular telescope beyond the outer rim of the transit plates.

His brother, J. Sharon McNair, 'who was with me as an assistant in the service of the Lehigh Valley Railroad Company at this place [Hazelton, Pennsylvania] and myself (as I now fix the date) in the first week of December, 1874, went to the business place (43 North Seventh Street, Philadelphia) of William J. Young & Sons.  I laid the matter before them and said the only question in my mind was as to the line of sight of the instrument being always through the pivotal point (or center) of the horizontal plates of the instrument.  Mr. Watson, then in charge of the manufacturing, demonstrated this by picking up a metal triangle and revolving it vertically from the acute angle as a center, thus demonstrating at once that the telescope being in line with the base of the triangle, the line of sight would always pass through the center of the instrument when in proper adjustment.'

In a letter to Young and Sons dated March 13, 1875, he says that the purchasing agent of the Lehigh Valley referred him to the Chief Engineer (R. H. Sayre) for permission to have the instrument made.  In a letter to Young & Sons dated February 18, 1876, he states that the new mining transit was received a little more than two months ago.  In this last letter he suggests that the clamp for the vertical circle be placed on the right-hand side instead of on the left, and that the tripod have one sliding leg to facilitate in setting up on side hills or rough ground.  The transit is Young & Sons "No. 4143 and is now (March 26, 1898) in possession of the Lehigh Valley Coal Company office in this place.  It corresponds with Young's catalogue number, as Young's Improved Mining Transit No. 4, and has all of their improvements attached."

William J. Young wrote to McNair in regard to borrowing the instrument for exhibit at the Centennial Exhibition in Philadelphia in 1876, but McNair could not spare it.  Young therefore exhibited a plain one, lacking extras (letter to Dunbar D. Scott dated October 22, 1898).  "Of distinctive mining transits, there are probably more of the inclined standard type in use than any other, all objections to its eccentricity and 'overhang' melting away wherever it has once been used.  It has achieved this recognition without any special recommendation on the part of the makers."

"It was capable of perfect adjustment, and fulfilled in a measure, as did Draper's instrument, the functions of both main and top-telescope.  Its construction did not interfere with the direct and perfect reading of horizontal angles, though a counterweight was always required to preserve the equilibrium necessary in the best instrumental construction.  The eye-prism is detachable at will.  It is inserted between the two lenses of the ocular, forming an image that becomes erect with respect to altitude, but reversed in azimuth.  Except by the use of a diagonal eye-piece, a steep angle of elevation can be observed only in a reversed position of the telescope; but its one disadvantage lies in the fact that in the prolongation of an inclined shaft-alignment it cannot be checked by reversed sight.  In such cases one must rely solely upon perfect adjustment of the instrument."

"We must aver that, while no essentially eccentric type is without faults, that of Thomas S. McNair possesses the least.  It may be called the 'American Eccentric,' and finds favor in many localities."

According to Justus E. Altmiller 1950 successor to Thomas S. McNair as engineer for the four companies, the McNair inclined standard mine transit is especially valuable in surveying coal veins of very steep pitches, as occur in the East Middle Coal Field, and the Schuylkill region of the Southern Coal Field.  Sharp Mountain, in the Schuylkill, has very steep pitches.  Altmiller says that a transit, with a top or side telescope, can be used in the same way, but there is no comparison in accuracy with the inclined standard.

Engineering in 1876 caused McNair to inspect the Buck Mountain Coal Co. property with regard to the amount of coal present and the feasibility of extending the Lehigh Valley Railroad from Eckley to their Owl Hole slope.  A survey was made for a branch railroad to extend from Harleigh to Cranberry."