The reduction made, there remained for the final calculation the following data:
| Barometric pressure at the summit of Rainier | 17.708 inches |
| Barometric pressure at mean base | 30.130 inches |
| Mean temperature of air column | 49 deg. F. |
| Latitude of Mount Rainer | 46 deg. 48 min. |
In making the calculation I used the amplified form of Laplace's formula given in the recent publications of the Smithsonian Institution, with the constants there adopted. Perhaps for the general reader it may be important to remark that this formula, besides the barometric pressures, contains corrections for the temperature of the air column; for latitude, and for the variation of gravity with altitude in its effect on the weight of the mercury in the barometer; for the average humidity of the air; and for the variation of gravity with altitude in its effect on the weight of the air. I used the latest edition of the Smithsonian tables, but afterward verified the result by a numerical solution of the formula—the altitude being, as stated at the beginning, 14,528 feet above sea level.
It should be noted as an evidence of the great care and foresight with which Professor McClure planned his work and the success with which he carried it out, that the result of his observations agrees within nine feet with that obtained by the United States Geological Survey in 1895, using, as we may suppose, the most refined methods of triangulation—the latter estimate being 14,519 feet. In connection with so great an altitude, nine feet is an insignificant quantity, and the close correspondence in the results of the two methods of measurement is truly remarkable. I am not inclined to regard it as accidental, but as due to the most careful work in both cases.
Having a full knowledge of all the available data, I am perhaps better prepared than anyone else to pass judgment upon the result set forth; and while it would be folly to give a numerical estimate of the probable error, I feel justified in saying that no single barometric determination is ever likely to prove more accurate than this one of Professor McClure's. At any rate, the outstanding error is now too small to justify the hazard of any future attempts.
From the observations made by Professor McClure while en route to the summit, together with simultaneous records from Seattle and Portland, the following altitudes are obtained:
| Feet above Sea Level | |
| Eatonville | 870 |
| Kernahan's ranch | 1,880 |
| Longmire springs | 2,850 |
| Mazama camp | 5,932 |
| Camp-No-Camp | 12,700 |
| South side Crater Rainier | 14,275 |
The data in these cases were not sufficient to admit an elaborate working-out of the altitude, so that the figures given are to be regarded as rather close approximations, except in the case of Mazama camp, the altitude of which rests upon four observations and is correspondingly reliable.
Professor McClure's barometer had a notable history in mountaineering. To quote the professor's own words:
"It has twice looked upon the beauties of the Columbia river from the summit of Mount Hood. It was the first barometer taken to the top of Mount Hood, and gave the true elevation, 11,225 feet, in place of 17,000 or 18,000 feet previously claimed. This barometric measurement of Mount Hood was made in August, 1867, by a government party under the direction of Lieutenant R. S. Williamson. The second barometric measurement of Mount Hood was made with the same instrument in August, 1870, by Professor George H. Collier."