Professor Lovering read the following communication, viz. : "Remarks on the Aneroid Barometer, by PROFESSOR J. LOVERING, of Harvard University. "Most of the scientific journals of Europe and America have published descriptions of the new French barometer, as it is called. For the construction of the instrument, and the history of its invention, I may refer to them; and particularly to that contained in Silliman's Journal for September, 1849. "The two ordinary statical ways of measuring forces are, first, by means of gravity, and, second, by means of elasticity. Our common balances to measure weight employ, either the gravity of a known counterpoise, or the elasticity of a spring. In like manner, the weight of a column of the atmosphere is determined by ascertaining the height of a similar column of some known fluid, which it is able to support, or the elasticity of some familiar substance with which it is in equilibrium. The barometer with which we are most familiar employs the first method: the aneroid barometer, which, as its name implies, contains no liquid, is based on the last principle, namely, that of measuring weight by elasticity. "This new instrument is already manufactured, in large numbers, in France and Great Britain; and its adoption is recommended on the ground of economy, as well as of its great compactness. The barometer is now extensively used, not only for tracing out the grand laws of meteorology, but also as a practical guide to the mariner to forewarn him of approaching storms, and an indispensable auxiliary to the man of science in studying the geography of the solid parts of our planet. It is highly important that the meteorologist, the navigator, and the student of general science, should know what degree of accuracy may be claimed for the new barometer, and to what extent they are allowed to trust themselves to its indications. With the hope of assisting those who desire to form an opinion on this subject, I present the following experiments and observations, undertaken originally at the suggestion of Professor A. D. Bache, Superintendent of the United States Coast Survey. The instrument employed in this research was furnished by Professor Bache. It bears the number 1265, and came from the establishment of Lerebours and Secretan, Paris. "A series of experiments was first made with this aneroid barom. eter, to determine the whole range of the instrument. It was placed for this purpose, first under the receiver of a common air-pump, and afterwards under the receiver of a condensing pump. In this way, it was found capable of indicating a change of atmospheric pressure, which would move the column of mercury in an ordinary barometer from about twenty inches up to about thirty-one inches. From the nature of its construction, the index is incapable of moving beyond the point which corresponds to twenty inches of the mercurial barometer, or beyond that which corresponds to thirty-one inches of the same. How accurately its march between these limits agrees with that of the mercurial barometer will appear from an examination of Table I. The pressure of the air in the receiver of the pump was derived from the pump-gage, which was supplied with common mercury, and corrected for level and capillarity. This table shows that, while the index of the aneroid barometer continues to move, it moves farther than the column of mercury. As it approaches its lower limit, it will begin, of course, to move more slowly, and afterwards the differences between its indications and those of the mercury change sign. It is obvious that, in this instrument at least, and with large ranges, similar changes of pressure are not marked by equal quantities of motion in the index, in all parts of the scale. This might be expected in an instrument where no consideration is given to the distinction between the potential and the apparent leverage. Besides this error, which we may call the instrumental error, there appears to be an irregularity in the motion of the index, arising from friction, bending, or some other cause, which would interfere seriously with the accuracy of this ba rometer, even if the arc over which the index moves were so graduated as to indicate the true pressure. "At the meeting of the British Association, in 1848, it was stated by Mr. Lloyd, that one of his friends had made a similar experiment to that I have described, and that the indications of the aneroid barometer agreed with those of the pump-gage to within .01 of an inch. Such is the statement in the London Athenæum, although no mention is made of the subject in the Report of the Association for that year. As we are not informed to what amount of diminished pressure the aneroid barometer was subjected in this case, and whether the difference above mentioned was the result of a single observation or the mean of many, I am not able to say how far my own experiments are at variance with those to which Mr. Lloyd refers. Neither am I able to say how much of the error manifested by my comparisons is fairly to be charged to the general character of the new barometer, and how much is peculiar to the single instrument with which I experimented. I intend, as soon as an opportunity offers, to subject other specimens of the aneroid barometer, both of French and English construction, to the same trial. "My next series of experiments consisted in a comparison of the aneroid barometer, day by day, with the common barometer, under the ordinary changes of atmospheric pressure. The mercurial barometer used for this purpose was made by W. and S. Jones, of London, and is the same as that used by Professor Farrar in the barometric observations published by him in Volume III. of the Memoirs of this Academy. This instrument is furnished with an adjustment for level, an attached thermometer, and a scale of corrections for temperature. This correction, as well as that for capillarity, has been applied to my observations. In this series of experiments it was necessary to know how much the aneroid barometer was affected by a change of temperature. Only a partial compensation is aimed at in the construction of the instrument. An increase of temperature will make the air in the reservoir expand, in the same way as a diminution of pressure. The same increase of temperature, by enlarging the metallic surfaces of the reservoir and increasing its capacity, may sometimes even over-compensate for the increased elasticity of the contained gas. In the instrument which I used, the compensation was deficient, and the amount of the deficiency was determined by exposing the barometer, side by side with a thermometer, to a temperature of 32° Fah., and reading the index, and then exposing it to a high temperature (in some instances as high as 140° Fah.), and then again reading the index. The difference of the two readings divided by the difference of the two temperatures was adopted as the correction for one degree, and was applied to the daily observations. The value of this correction, as obtained from the mean of five experiments, was .0021 of an inch, with the same sign as in the mercurial barometer. To accommodate the scale of the mercurial barometer, the standard temperature adopted was 55° Fah. The aneroid barometer which I used was not provided (as is sometimes the case) with an attached thermometer. A thermometer by the side of it, and not under the same inclosure as the air-chest, does not indicate the exact temperature of the working parts of the instrument. The slowness with which the index returned to its old mark after the barometer had been subjected to excessive heat or cold, and was then restored to the temperature of the room, manifests the importance of having the thermometer inclosed as the test of the instrument. The result of this series of comparisons is given in Table II. Although the agreement is much better than with low ranges, it falls far below the requirements of nice scientific investigations. - "Mr. David Purdie Thompson, in his very recent work on Meteorology, has the following paragraph:-Upon comparison of indications made with the aneroid barometer not corrected for the particular temperature and a very perfect mercurial barometer, given by Mr. Dent, we find that, from forty-nine observations made between the 6th of January and the 23d of February, 1848, the mean difference was 0.037 of an inch, the aneroid being in excess; and from sixty similar observations made with a standard barometer, during December, 1848, and between the 3d and 31st of January, 1849, the mean difference amounted to 0.026 of an inch, the mercurial being in this case in excess over the aneroid barometer. Combining these observations (109 in number), a mean difference amounting to 0.0025 inch is found to exist, the indications of the aneroid being in excess. For general use, the instrument is thus shown to be well suited; for the measurement of heights, it is peculiarly adapted, from its portability and comparative strength; and for nautical purposes we know of no better instrument.' - p. 448. "Now it will be observed that the mean difference in the twentyeight comparisons which I have given of the two barometers amounts to only 0.040 of an inch. So far as can be known from such means, the comparison was as satisfactory as in the first set given by Mr. Thompson. Still, the differences in single comparisons are large : whether larger or smaller than in Mr. Dent's observations, I am not able to say, as Mr. Thompson has not given the individual differences. Provision has been made in the construction of the instrument for diminishing the mean difference, as we may alter the rate of the chronometer. This mean difference has been eliminated from my com parisons, and the differences which are given in the last column manifest, by the signs of plus and minus, the irregularities of the instrument, and the error to be expected from these irregularities in single obser vations. I have arranged the same observations in Table III. according to the sign and the value of the differences. From the sign of the differences it appears that, when the barometers fall, the aneroid falls most, and when the barometers rise, the aneroid rises most. In other words, the aneroid index, moving either way from the place where it agrees with the reading of the mercurial barometer, moves too fast. The experiments with the air-pump indicate the same tendency more unequivocally. For, in those experiments, where the two barometers were moving in a direction which corresponds to a depression of the common barometer, the aneroid always moved the most, so that when the motion of the mercury in the pump-gage is subtracted from the motion of the aneroid index, the sign is always plus; at least, until we approach the lower limit of range. Although this is the general character of the differences, a nice examination of the observations shows that here, as well as in the experiments with the air-pump, there are errors and fluctuations which cannot be traced to any law of the instrument, and against which no provision can be made. Table IV. contains a series of observations made with the view of ascertaining the stability of the levers in the aneroid barometer, and the firmness of other parts of the instrument. The instrument was first read off; and then, after being exposed to diminished pressure, it was noticed with what fidelity and despatch the index returned to its original position when the original pressure was restored. "It must not be forgotten that it is single observations, indicating momentary changes of atmospheric pressure, on which the navigator most relies. In some of the hurricanes to which he is exposed, the barometer occasionally sinks so low as to come within the range of those experiments made with the air-pump. And yet here, if anywhere, the aneroid barometer finds its appropriate sphere. In meteorology, the barometer is the most important instrument of research. The ba rometer alone, of all the instruments employed in this research, is independent of merely local changes, and gages the atmosphere to its upper limit. But the range of atmospheric pressure is so limited, that laborious series of observations, with the nicest barometers that can be constructed, are necessary in order to develop the harmonies |