APPENDIX. REMARKS ON THE BAROMETRICAL TABLES, WITH DIRECTIONS FOR USING THEM. The following tables have been drawn up during the summer of 1837, and contain an account of the height of the mercurial column in the Barometers at stations in various parts of the State, together with records of the temperature of the instru ments and of the air, at the time of each observation. Many interesting meteorological remarks are also appended to several of the tables. All persons conversant with Barometry, will at once understand the importance of arranging a line of stations for the observations, in such a manner as to give the height of the mercurial column in different places on a sectional line running across the State; and will perceive that our stations were so arranged as to guard against errors, which might arise from local atmospherical changes. It is with great pleasure that I am able to acknowledge the valuable services of those gentlemen, who have aided us in obtaining these important records, and I beg leave to present them my thanks for the services which they have freely rendered to me and to the State. The first step in making Barometrical measurements, in an accurate manner, is to obtain such instruments as are perfectly free from atmospheric air, and it is essential that the mercury should be boiled in the tubes, so as to expel every bubble of air from them. When this is done, on inclining the Barometer suddenly, the mercury will strike the top of the tube with an audible metallic click, and no space will be discovered which is not entirely filled by the mercury. The next object is, to provide against any loss of mercury from the cistern, and to have the means of confining it so that it cannot move while the instrument is carried. If Sir H. Engleford's Barometer is used, it is requisite to cover the leather bag with a very thin layer of blown sheet india rubber, which is impermeable and elastic, so that no rupture can take place on pressing with the thumb-screw. The thermometers must also be perfectly true, and ought to be carefully compared with a correct standard instrument, throughout the scale. After attending to the above circumstances, the Barometers and thermometers must be compared with those left at the several stations, and their differences must be noted, and corrections are to be made accordingly-by adding or subtracting the difference, as the case may require. It is obvious that when due precautions are taken, that no error of any magnitude can arise in Barometrical levelling, and that such measurements are sufficiently accurate for the construction of Geological profiles, and for the measurement of the altitudes of mountains, by which we can form a correct idea of the true relief of the country. The height of each of the stations recorded, may be readily calculated, by means of OLTMAN'S TABLES, which may be found in the Geological Manual by Henry De La Beche, and in the French Annuaire du Bu reau des Longitudes, or those who prefer the Logarithmic method, will find the requisite rules in Rees' Cyclopedia, and in various other works which treat upon the subject of Barometry. OLTMAN's tables are now generally preferred in Europe, and they are extremely convenient, and easy of calculation, insomuch that any person may calculate the height of a mountain from the Barometrical height, in a few minutes, after collecting the data. In the Geological Reports, which I have had the honor to present, may be found the results of several of the most important observations, and the remainder will be calculated as occasion requires. They will serve as the basis of the Geological profiles, which we shall have occasion to draw, in the regular progress of the survey, and will be presented to the Government, with a Geological map, when that part of the work is completed. 1st. The first station Barometer belongs to Rev. Solomon Adams, of Portland, and is placed at the height of 121 feet above high water mark, in Portland. This sum then must be constantly added to each of the heights, calculated from that station. My Barometer placed beside his, June 14th, stood at which sum is to be added to his observations, or subtracted from mine, constantly. 2d. At Brunswick, in Professor Cleaveland's study, June 15th, 11 A. M. Inches. My Barometer stood at 30.02 T. = 62° F. 29.77 T. = 62° F. 00.25 difference, additive to his observations, or deductive from mine, in calculating from that station. 3d. At Gardiner, 16th June, at noon, Mr. R. H. Gardiner's Barometer and mine compared at his house. Inches. Mine stood, 29.85 T. = 65° F. 00.03 difference, additive to his or deductive from my observations, in calculating from that station. The slight differences in the instruments above noted, arise from the difference in the height of the mercury in the cisterns, and their general agreement is a proof that all the Barometers were in good order. |