A Compilation of the Messages and Papers of the Presidents. Volume 4, part 2: John Tyler

by Unknown

23. The results of the operations of this division are embodied in a map and profiles, which are herewith presented. The degree of reliance to be placed on this map will be best understood from a detail of the methods employed in preparing it.

The river Metis and its branch, the Mistigougeche, were surveyed by an azimuth compass of Smallcaldus construction, and the distances measured by a micrometric telescope by Ertil, of Munich. The courses of the rest of the lines were determined by compasses of similar construction, and the distances measured by chains of 100 feet constructed by Dollond, of London, and Brown, of New York. An exception to this general rule exists in the survey of the eastern side of Rimouski. The courses and distances thus measured, and corrected for the variation of the compass, were compared with astronomic observations for latitude and with longitudes deduced from chronometers. For this reason, as the line on the east side of Rimouski is almost in the direction of the meridian, it was not considered necessary to lose time in measuring it when the latitude of the several camps, determined by observations of the pole star, were taken nightly.

The latitudes of the courses under the direction of Mr. H.B. Renwick were determined by a reflecting repeating circle of Dollond; those on Mr. Lally's by a good sextant. The latitudes and times at Grand Metis, the river Du Loup, and the stationary camp on Mistigougeche and Abagusquash were principally determined from observations made with the Dollond circle. Lunar transits were taken at the river Du Loup, and distances of the moon for longitude at several places on the line. The reliance for the longitudes was, however, principally upon timekeepers, and of these the party was furnished with one box and two pocket chronometers by Parkinson & Trodsham, one pocket chronometer by Molyneux, one by French, one by Barraud, and one by Morrice. Thus, while several could be retained at the station, each party in the field was furnished with two, and the measured distance furnished a check, which, in case of discrepancy, that on which greatest reliance could be placed might be ascertained. It is sufficient to say that the deductions have been in general satisfactory, although the rough motion to which these instruments were subjected in passing through pathless woods, embarrassed by fallen trees and morasses in which the bearers often sunk to the middle, caused changes of rate and even sudden variations. Uncertainty arising from these causes was rendered less to be dreaded from its being possible to refer, as a base of operations, to the excellent survey of the St. Lawrence River by Captain Byfield, of the British navy. With the geographical positions given in his charts our own observations agreed so closely as materially to confirm the respective accuracy of both.

24. The point which in this part of the survey has been kept in view as most important is the determination of the heights. For this purpose the party of Professor Renwick was furnished with the following barometers:

Two loaned by the Superintendent of the Coast Survey, of his own construction; two portable and one standard, by Neurnan; three of the siphon form, by Buntin, of Paris; one by Traughton & Simms; one by Forlin, of Paris; three of siphon form, by Roach & Warner, of New York; two by Tagliabue, of New York, originally on the plan of Durand, but which had been advantageously altered by Roach & Warner in such manner as to admit of the adjustment of the level of the mercury in the cistern.

The stations at which the lower barometers were placed were Grand Metis until the return of the expedition up the river of that name, and the river Du Loup from that time until the close of the survey. At these places all the barometers not actually in the field were suspended and registered at the hours most likely to correspond with the observations of a traveling party, say at 6, 7, 8, and 9 in the morning, noon, 1, 5, and 6 in the afternoon, until as the season advanced and the days became short the earliest and latest of these hours were omitted. Although several barometers were thus constantly observed, no other use of these was made but to determine their comparisons with each other, except one of the barometers of Mr. Hassler, Superintendent of the Coast Survey. This, from its superior simplicity, being, in fact, no more than the original Tonicillean experiment, with a well-divided scale and adjustment of its 0° to the surface of the mercury in the cistern, was found to be most certain in its results. All the barometers used by the parties in the field were therefore reduced to this by their mean differences.

The stations at the two above-mentioned places were near the St. Lawrence. At Metis the height of the cistern of the standard barometer was determined by a spirit level. At the river Du Loup the height of the station was determined by two sets of observations of barometers, taken with different instruments by different observers, and at an interval of a week from each other. The results of the two several sets, which were calculated separately, differ no more than 0.5 of a foot from each other.

On reaching the highest accessible points of the streams on which the parties proceeded toward the height of land, stationary camps were established, as has been already stated. At these series of observations were made at the same hours as at the river stations. The height of the former was then calculated from a series of observations taken at noon and at 1 p.m. for the whole of the time the camp was occupied. The heights of the points at which observations were made by the traveling party were then deduced from a comparison with the nearest contemporaneous observations at the stationary camp. An exception to this rule was made in the observations to the westward of Temiscouata Lake, which were referred directly to those made at the river Du Loup, which was sufficiently near for the purpose.

The height of the stationary camp at Mount Biort having been determined by observations continued for several days, the level of Lake Temiscouata was thence determined by using a set of levels taken with a theodolite by Breithaupt, of Cassel, in 1840. The height of the lake thus deduced is greater than it would appear to be from the barometric observations taken in December, 1840. It had been imagined that a difference in level might exist between the St. Lawrence at Metis and at the river Du Loup. Four days of contemporaneous observations were therefore made at each with a view to the solution of this question. The idea of a difference of level was not sustained by the operation.

The heights of the river stations were measured in each case to the highest mark left by spring tides, and half the fall of that tide as given by Captain Byfield has been added in all cases as a reduction to the mean level of the sea. Opportunities were offered in a few instances for testing the accuracy of the method by different barometers used by different observers at different days on the same point. No discrepancy greater than 7 feet has been thus discovered. In other cases the same observer returned and observed at the same places, and here a similar congruity of result has been found to exist.