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I have collected a number of observations running through a scale of nearly 21,000 feet of elevation, and at temperatures varying from 1° to +28°. Now whatever law of decrease we may have deduced from theory, from the abstract constitution of the atmosphere, &c. it will be impossible to make it agree with these results. Slow and rapid diminutions of temperature accompany indifferently both great and small elevations; we find them taking place indiscriminately in the higher, middle, and lower regions of the atmosphere; and at all degrees of heat and cold. Yet these are good observations, and have furnished in general very exact measurements, and many of them confirmed by geometrical determination. And they are moreover the same sort of observations which we commonly make, and for which our barometric formulæ are constructed. The measurements are found accurate, because the thermometer consulted at the lower station has eliminated an unknown quantity to. which no theoretical considerations could have assigned a determinate value. A corresponding observation at the base of the column is a fixed point of departure; the extremes of the temperature being once known, correct the calculation; and although the decrease of temperature generally undergoes, in the same column of air, irregularities occasioned by a multitude of accidental causes, which by turns retard and accelerate it, and is sometimes inverted; yet an arithmetical mean taken between the extreme temperatures so well covers these irregularities, that the exactness of the measurements is not at all affected.

It may sometimes happen that we have not corresponding observations; and when we carry a barometer, we are desirous of deducing at once from it nearly the absolute elevation of the place. The expedient hitherto most commonly adopted has been to compare the observed height of the barometer with its mean height at the level of the sea; but in this method there is always an inherent fault: we compare an insulated observation with the mean of a great number of observations. The comparison will only be just in the single case when the barometer happens accidentally to be precisely at its mean height; in general, it may be considerably above or below it; there is then only one chance in favour of the accuracy of the measurement; that is, by allowing for the diminution of temperature according to the approximate elevation, and the error in this correction happening to compensate the former: if, however, the two errors instead of compensating each other, accumulate, the apparent amelioration of the calculation will have no other effect than to increase the inaccuracy. We may, however, proceed thus: Suppose the mean height of the barometer at the level of the sea to be 30-03437 inches at a temperature of 12.5°, a supposition adopted by most philosophers from the observations of Sir G. Shuckburgh. I commence by reducing my observation to the

same temperature by means of Table 4; I then take the difference of the logarithms: this gives a basis for finding the probable temperature at the lower station according to the most ordinary law of decrease. For this purpose it will suffice to reduce the difference to these decimals, and multiply it by the constant number 122. The product expresses in degrees and decimals, the quantity by which we must augment the temperature indicated by the higher thermometer to get approximately that at which the lower ought to stand. By this means the correction for the temperature of the air is applied.

The factor 122 expresses the mean rate of decrease as deduced from the collection of observations before-mentioned, and agrees with the law of decrease adopted by some philosophers from theory; but though this may inspire some confidence in the observations upon which the value of that factor depends, yet it inspires none for this mode of measuring heights. I find that this method does not sufficiently correct the deviations of the simple method; and moreover the errors instead of compensating each other, are almost always on the same side, and tend generally to diminish the real height. This tendency is remarkable; it has little to do with the decrease of temperature assigned; for we perceive it equally in the uncorrected method. It rather concurs with other circumstances to make me suspect what has been most generally considered the mean height of the barometer at the level of the sea. If I can sufficiently confide in my own observations, this mean is taken too low. If it be raised to 30-089 inches, for the hour of noon, and temperature 12.5°, there will be rather more equality between the chances of error in the different modes of calculation in which this mean is employed.

But whatever may be the corrections which we may apply to the approximate methods which have caused this digression, they will be always very faulty methods. We shall never have recourse to then but with distrust, and only in order to estimate within about 30 feet, the elevation of a place when we have no means of obtaining a more precise determination. The barometer after all does no more towards the measurement of heights without corresponding observations than the repeating circle without an exact determination of distances.

Mode of conducting the Observations.

To understand the theory of barometrical measurements is by no means difficult: it is even still easier to learn to calculate the observations well, but the most difficult thing is to perform them well. Very skilful persons have often given extremely deficient observations, as well from want of good instruments, as of good methods; and always from having considered as more easy than it really is, what appears to be a very simple physical experi

ment; which is, however, in itself of an extremely delicate nature, and which frequently does not answer in the way in which we put the question; for this reason, that it is the nature of all experience not to give a satisfactory answer, except to questions very well instituted.

However, we are commonly in haste to draw conclusions; and men had not even thought of improving the instruments, and much less the methods of observing, when already the lesser as well as the greater questions of meteorology, had been investigated, if not considered as resolved, on the sole ground of observations, either insufficient, or suspicious, or which did not really say a word of what they were made to say.

(To be continued.)

ARTICLE IV.

Register of the Rain at Bombay in the East Indies, measured with Howard's Pluviometer daily at 7 a. m. By Benjamin Noton, Esq. of Bombay.

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