330 BAROMETER BAROMETRICAL MEASUREMENT to its original position, the spring pulling it out again, and thus counterbalancing the at- mospheric pressure, which tends to make the box collapse. A change in this pressure will of course resist the spring more or less, and this slight motion, multiplied by a proper mechani- cal arrangement, turns the hand seen at the top of fig. 10, and also, with the scale, in fig. 9. As, however, a rise in temperature expands the spring and diminishes its resistance, it will have the same result as an increased atmos- pheric pressure, namely, tend to let the box collapse. Becker, a well-known balance maker of New York, corrects this by introducing into the vacuum in the box a measured but very small quantity of perfectly dry air, the expan- sion of which by heat counterbalances the loss of tension of the spring by the same cause. Experience proves, however, that this kind of compensation becomes inert after a lapse of a few years ; hence a correction for temperature is required, the instrument having a thermom- eter attached, as shown in fig. 9. Unfortunate- ly, this correction must be found by experiment for every instrument, and changes even for the same instrument in the course of time. The coast survey and the Smithsonian institution have therefore pronounced against these ba- rometers. Their objections, however, it is thought, do not apply to their use in the hands of practical surveyors, topographers, civil en- gineers, artists, travellers, and sailors, who all pronounce emphatically in their favor. The observer must however learn to know his in- strument well, or he can do nothing with it on an extended survey. Of course the aneroid can be of no service in the high geodesy of a coast or ordnance survey. In civil engineer- ing, on the contrary, up to the final location line, it is reasonable to expect that it will almost replace the spirit level. In geological examinations it is invaluable. The geologist in tracing outcrops through the woods and where the rocks are entirely concealed, across ravines, and over the shoulders of hills, in a broken country, has only to discover and take the direction of the line of strike, to know by the infallible rise or fall of the index hand to the level of the point of his departure pre- cisely when he is passing up or down over the outcrop of his bed. In countries where the rocks are nearly or quite horizontal, in fact over half the United States, the aneroid is 'to the geologist a whole corps of assistants, and the work of a week can with its help often be done in a day. There is an external index to assist the memory of the house observer from one observation to another. BAROMETRICAL MEASUREMENT. By the per- fection now attained in the construction of barometers, and the skill applied to their use by the best observers, differences of elevation may be ascertained by them with greater accu- racy than by the most carefully conducted tri- angulation at least, in places where the eleva- tions are great and difficult of access. High summits, covered with shifting clouds, involve uncertain errors, arising from constantly vary- ing refraction ; and inaccessible mountains can only be observed under very small angles from the termini of a carefully constructed base line, in some smooth district, at a considerable dis- tance from them. A comparison of results ob- tained by both methods is generally in favor of the barometer. Humboldt noticed this, par- ticularly in the numerous measurements that had been made of the peak of Teneriffe, and, in determining this elevation by the mean results of the various observations, he rejected eight out of nine geometrical measurements, and only one out of four barometrical measurements. Both modes, however, are capable in many lo- calities of a great degree of accuracy, as is shown in the two measurements of Mt. Wash- ington, the first by Prof. Guyot with the ba- rometer, and the second by the officers of the coast survey, in which the difference was only 3 ft. in the height of 6,285 ft. determined by Prof. Guyot. To insure the greatest degree of accuracy, it is essential to use two good ba- rometers, one at the lower and the other at the upper point. If only one be employed, there is a liability of error from a change of atmo- spheric pressure taking place during the time spent in passing from one station to the other. These barometers should have been carefully compared by many observations, and the mean of their variation noted, to be always allowed in the calculation. They should also have been compared with other barometers of known character, and their differences with these noted, and this comparison should be repeated after their use, in the same way as chronome- ters are compared, and their rates noted, be- fore and after a voyage. Repeated observa- tions should also be made at both stations at the same times, and the mean of all be taken, unless some show good reasons for their rejec- tion. It is also important that the two stations be not very far apart. In a distance of 40 or 50 m. there may well be varying conditions of the atmosphere that cause a difference of pres- sure not due altogether to the difference of ele- vation. This cause of error may be avoided by using intermediate stations, and advancing step by step. One point determined serves as the established base for determining the next be- yond. In measuring the heights of the princi- pal summits of the Black mountains of North Carolina, Prof. Guyot used as his starting point the level determined by a railroad survey, 7 m. distant from the nearest hill. The next sta- tion was taken half way to the summit, and by repeated observations at both, continued dur-
