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The observations in each line of the table apply to a period of twenty-four hours, beginning at 9 A. M. on the day indicated in the first column. A dash denotes that the result is included in the next following observation.

REMARKS.

cast.

Sixth Month.-1. Fine. 2. Cloudy: rain in the evening. 3. Cloudy: a shower of hail in the afternoon. 4. Showery. 5. Showery: a heavy storm of thunder about three, p. m. with large hail, and very vivid lightning. 6—13. Fine. 14. Cloudy, and fine at intervals. 15-17. Fine. 18. Fine: overcast. 19, 20. Fine. 21, 22. Over23. Finc. 24. Overcast. 25. Cloudy: some showers in the night. 26. Cloudy. 27. Showery, till about five o'clock, p. m. when there was a very heavy storm of thunder, and rain and hail of considerable size: the lightning vivid, and thunder near and frequent, the wind going round to the W. 28. Cloudy and fine. 29. Showery: a violent storm of hail about three, p. m. with thunder, followed by rain: the hail as large as peas. 30. Calm: overcast.

RESULTS.

Winds: N, 5; NE, 7; SW, 6; W, 3; NW, 6; Var. 2.

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ANNALS

OF

PHILOSOPHY.

SEPTEMBER, 1823.

ARTICLE I.

An Abridged Translation of M. Ramond's Instructions for the Application of the Barometer to the Measurement of Heights, with a Selection from his Tables for facilitating those Operations, reduced (where necessary) to English Measures. By Baden Powell, MA. of Oriel College, Oxford.

(Continued from p. 111.)

IT is from the publication of the excellent work of M. De Luc on the Modifications of the Atmosphere, that we are to date the commencement of those observations which have really contributed to the advancement of our knowledge. With these subsequent additions, a work on the mode of using meteorological instruments would assuredly be highly useful, for the experimental part of the science has been far from keeping pace with the mathematical; and the art of making good observations is as yet far from being carried to the same perfection as that of employing them when made.

It is, therefore, not useless to apprize those to whom these instructions are addressed, that if exactness is the first requisite in a good observation considered in itself, there are also other conditions to be fulfilled, in order to appropriate that observation to the particular purpose in view: that both meteorology and barometrical measurements require the choice of opportune conjunctures that the laws of general physics lay down the principles, but do not always point out their applications: that these are not within the province of dogmatical instruction; that in order to effect happy applications, there must be much skill, New Series, VOL. VI.

M

discrimination, and experience employed; that we do not learn all this as we learn to read, to calculate, or to translate the terms of a formula. Let us read then, and reflect upon the works of De Luc, Saussure, the memoirs of Pictet, the books of philosophers who have devoted themselves to the scientific employment of meteorological instruments; and above all, let us continue our own observations for a long time before we trust to them, and a still longer time before we yield to the temptation of drawing inferences from them; for the goodness of observations depends on a multitude of minute precautions which habit alone renders constantly present to the memory; and the validity of the inferences rests on considerations too numerous to be at the disposal of a mind not rendered familiar to them by time and practice.

Sedentary Observations* for determining the Mean Pressure at a particular Place.

I commence with considering the method of conducting observations of the stationary kind for determining the mean pressure of the atmosphere at any place; for these form one of the principal foundations on which the science of meteorology rests, and above all because they are the means of affording marked periods for the operations of levelling.

Sedentary observations have a very limited utility if they are not comparable; that is to say, if equal pressures are not constantly expressed by equal heights of the mercurial column. We may readily conceive that this fundamental condition is but ill fulfilled by the rude instruments with which the generality of observers content themselves, and often not even by the more costly ones with which the cabinets of the curious are adorned.

It must be presumed then that the observer has a barometer carefully constructed: the mercury perfectly pure and well boiled; the tube as free from air and moisture as it can be rendered: the scale leaving nothing to be desired on the score of accuracy; he believes himself to possess a very good barometer. It is doubtless so far good; but this is not enough; and the chief condition is not fulfilled unless we are assured that it is comparable. What is the interior diameter of its tube? What means have been taken to correct the effects of capillarity? Is it of the siphon or cistern construction? What precautions have been taken to secure a constant level, and an invariable point of departure? Such are some few of the questions which must be satisfactorily answered before the instrument can be allowed to inspire confidence.

Let us, however, suppose all these conditions fulfilled, still it must be further asked, if the barometer be comparable, are the observations equally so? At what hours and under what circum

* "Observations Sédentaires."

stances are they made? Is the temperature of the mercury taken into account? How are the thermometers and hygrometers constructed, and how placed? And what is the system adopted for the reduction of observations to a mean? Such questions are indispensable, yet it is to be feared many observers could not answer them in a satisfactory manner. We have a great number of barometric means collected from numerous observations, and yet no one can say precisely what these means are.

It is thus that many long series of observations are in reality lost to science, and only furnish illusory documents to the philosopher who wishes to draw inferences from the experiments of his predecessors. Let it be our endeavour that these losses shall be the last science has to sustain, and let us furnish our successors with points of comparison less equivocal.

Choice of Instruments.

The siphon barometer is preferable for stationary observations, as it possesses the peculiar property of annulling by compensation the effects of capillarity.

With cistern barometers the correction for capillarity must be applied as given in the table. But besides this, these barometers require the application of some means for reducing the level of the mercury in the cistern to the zero of the scale, a point from which it is continually deviating, as well by the ascent and descent of the mercury, as by the less apparent, though not less real, effect of changes of temperature. This is provided for by giving the cistern such a diameter that the variations of level become nearly insensible. This, however, is not sufficient for very exact observations.

It is almost superfluous to remark the necessity of rigid accuracy in the division of the scale. In barometers mounted in wood, the scale is generally marked on a plate of metal attached to the mounting. This will not satisfy those who are desirous of great accuracy. Heat, cold, moisture, dryness, affect wood in every direction, sometimes increasing and sometimes diminishing the distance of the scale from the point where its divisions are supposed to commence. The scale, therefore, should be entire from zero up to the point of the greatest elevation of the mercury, though it may be divided only in that part through which the range of variation extends. Copper is the best material as being the metal with whose pyrometrical dilatation we are best acquainted. With a scale thus constructed, we know exactly to what the variations in dimension which result from variations in temperature are reducible. They are regular and very small, and may generally be neglected; but we thus know their amount, and can supply the correction.

The vernier should indicate the 1-1000th of an inch; and this is nearly the utmost which the barometer is capable of expressing without ambiguity: the visual ray must of course be perpen

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