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Show 50 minations were copied into the proper record- book, grouped by seasons and parties, and the locality of each point described as nearly as practicable: afterward the whole were thoroughly indexed for facility of reference, and the final results furnished the topographers for inscribing upon their maps. t). Since April 1 the force has been engaged principally in preparing condensed tables and summaries of the observations takeu at the hourly stations established by the survey since its organization. These are quite numerous, and the work of preparing the results for publication is slow and tedious. I hope, however, that it will be completed by the expiration of the present year. ANEROID BAROMETERS. Since the organization of the survey, aneroids have been used by the topographers and { geologists for relative altitudes, and, if properly handled, are a very useful and convenient instrument. Prior, however, to the season of 1873, so little was known by the individuals using them of their action and capabilities under the necessarily rough usage they must encounter in the field, and so much faith was placed in their indications and in the reports of too interested individuals as to their accuracy and constancy, that frequent comparisons with the more reliable mercurial barometers, which are absolutely essential if results of value are to be confidently expected, were neglected, and when the mass of aneroid work came to be examined and computed, it was found useful only as indicating the utter worthlessness of this machine for absolute altitudes when not used in connection with the mercurial barometer or the level. Like all other mechanical combinations of levers, screws, and springs, they are subject to continual shifting of [ parts when subjected to the jars and jolts encountered in ordinary use in the field, and it is essential that a continual watch be kept upon their indices of error by comparison with a more constant instrument. Aneroids on this survey have been known to keep a nearly constant index- error for months; but this is an exception to the general rule of change. I give below, in this connection, a series of comparisons of several aneroids with the cistern- barometer at 32° Fahrenheit, copied difeotly from the field- transcripts in the order in which the comparisons were made. TABLE OF ANEROID COMPARISONS, 1874. Bate. July 29- 30 July 30- 31 July 31 to August 1.. August 1- 2 August 9- 5 August 5- 6 August 6- 7 August 10- 17 August 17- 18 August 18- 84 August 84- 86 August 38 August 29 August 89- 30 November 3- 4 | i 1 95.148 84.313 83.962 83.180 83.114 81.413 84.211 22.657 83.021 83.701 22.245 21.839 21. 873 31.907 22.888 Aneroids, errors of, on oietern. + 0.762 + 0.782 + 0.908 + 0.829 + 0.883 + L136 - 0.033 + 0.279 + 0.334 + 0.499 + 0.567 + 0.594 + 0.031 + 0.021 + 0.016 - 0.008 - 0.041 - 0.091 - 0.064 - 0.482 + 1.967 + 3.137 + 3.039 + 3.173 + 0.559 + 0.535 + 0.558 I. * + 0.863 + 0.921 + 0.949 3 ® « J £-" 0a nS + 0.357 + 0.361 + 0.363 + 0.374 + 0.369 + 0.340 + 0.337 A difference not exceeding 0.03 inch may he allowed between any two comparisons for parallax, incapacity of the observer to subdivide the scale closely, and to the fact that the weight of the machine itself affects its indications, and an observer may not, even though cantioned on this point, hold the instrument in the same position when taking the two readings, when one may be made by daylight and the other by candle- light. This error of 0.03 inch between two readings would be very considerably redaced when two sets of readings, of three or four comparisons in a set, are taken. A simple application of the doctrine or probabilities will convince one that the changes visible in the index-errors of the aneroids from those comparisons where from three to thirty observations have been made at each camp are not so much due to errors of observation as to changes in the zero- point of the instruments themselves. In the table, the height of the barometer, reduced to 32° Fahrenheit is also given, to show, if such evidence is conclusive. |