The observations at the camp on the mountain slope were commenced at 7, P.M., on August 22d, and continued hourly (with few omissions) until 8, A.M., on the twenty-fourth. The barometer at the summit was hung up at 1:30, P.M., and allowed to stand a half hour in free air, but protected from the direct rays of the sun. It was then adjusted and observed at 2, P.M., 2:15, P.M., and 2:30, P.M., by Mr. Heuer and Mr. Best, independently, and the two records as shown to me were essentially the same. The mean reading of the barometer reduced to 32° Fahrenheit, was 19,941 inches, with an observed air temperature of 41°.7, and wet bulb of 31°.3. The height of Fort Vancouver above Astoria was computed from the mean of the simultaneous observations taken during the months of July and August. The height of the Dalles above Fort Vancouver was deduced from the corresponding observations during twenty-one days in July, together with those for the month of August. The height of the camp on the mountain slope above Fort Vancouver, and also the height of that camp above Fort Dalles, were then separately computed from the daily means of the observations taken at the three stations during August 23d. The difference between the two should give the same result as by the direct calculations between Fort Vancouver and Fort Dalles; but on account of the short period observed on the mountain camp, a plus correction of a little over eight feet was found necessary to the estimated height of that camp to make the three results agree.
It then remained only to calculate the height of the summit of Mount Hood above the mountain camp. The mean of the three observations of the barometer was assumed as the nearest approximation we can have to the mean pressure for that day, as the horary oscillation at the summit is unknown. With regard to the mean temperature for that day, we have no positive data to determine it. We cannot take the observed temperature, as the observations were taken during the hottest part of the day.
By consulting the hourly observations of the thermometer at the camp, I find the range there is between 63° and 43°.7, or nearly 20°; and supposing nearly as great a range of temperature on the summit, I have assumed the mean temperature then for that day to be 34°.
The following is the final result of the computations:
| Stations. | Inter. Alts. | Sea Line. |
|---|---|---|
| Sea level at mean low tide | — | 0 |
| Astoria | 53 | 53 |
| Fort Vancouver | 79 | 132 |
| Camp on mountain slope | 5,820 | 5,952 |
| Summit of Mount Hood | 5,273 | 11,225 |
The computations are made with new tables which will shortly be published, and which give results similar to Plantamour’s formula, based on Regnault’s constants. They give results somewhat higher than if Guyot’s tables had been used, the latter giving the height of the summit, 11,185 feet.
On our return I took a single observation at what is called “Government Camp,” about four miles below the camp on the mountain slope, and another at a place called Stumpville, some eight miles further on the road towards Portland. The results give for the former place 3,864 feet, and for the latter 1,830 feet above the sea level.
The instruments used on the mountain have been returned in excellent order, and compared with the one at Fort Vancouver with most satisfactory results.
It may be asked: Why is it that the results here given differ so widely from some previous estimates? Mount Hood is said to be, by Mitchell’s School Atlas, 18,361 feet, and the Rev. Geo. H. Atkinson with a party, ascended to the summit in August of last year, boiled water with a spirit-lamp, found that the thermometer read 180°, and therefore concludes the mountain is 17,600 feet, and Government Camp 4,400 feet above the sea. The reason is, that the instruments used are unreliable, and this method of computing the altitude defective. With a boiling point apparatus (or thermo-barometer as it is called) of the most approved kind, the results by boiling water are far inferior to those by the cistern barometer; but if the observations are made with a common thermometer, with small spaces for degrees, as was the case in this instance, and the instrument not protected from drafts of air, the results are utterly unreliable, and therefore worse than worthless.