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These results compared with the former set show the degree of correspondence between experiment and deduction from the barometric formula. The formula of Laplace agrees most nearly with experiment; the difference admits of a satisfactory explanation if we only consider the different quantities of moisture held in solution by the air, under the very different circumstances of a confined room, and the top of a mountain; and this difference being only about d part would only affect the accuracy of about 10 metres even in the height of Chimborazo; and after all, the ratios which the formula gives, being so many means deduced from a great number of observations, and so many conclusions deduced from operations on a large scale, and applied to those on a small, are more proper to give confirmation to the results of experiment than to receive it from them.

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In a note appended to the beginning of the second memoir, M. Ramond quotes an account of a more recent determination of the ratio of the weights of air and mercury; which results I: 10463, the air being perfectly dry in the latitude of Paris, temperature 0°, pressure 0-76 m. This result was obtained by MM. Arago and Biot. From it they deduce the barometric coefficient, for lat. 45 in metres, 18316.6 for dry air, and 18351.8 for air saturated with moisture; and for the mean state 18334-2, which is very nearly equal to that adopted by M. Ramond from observation confirmed by geometrical measurement, viz. 18336.

III. Under the head of "Isolated Observations," M. Ramond discusses the question of the decrease of temperature as we ascend in the atmosphere. He has given in the original, a table exhibiting this decrease from a variety of observations, the result of which examination only I have preserved in the foregoing abstract. The reader will find the supposition of an uniform decrease (which M. Ramond took as a mean value convenient for practical purposes), confirmed by reasoning à priori in the valuable paper on Barometrical Measurements, by Prof. Playfair, in the Edinb. Transactions, vol. i. 1788, and since republished in his works, vol. iii. 1822. In this memoir, Part III. the author investigates the law of decrease in the heat of the different strata of air as we ascend. He gives a demonstration, proving, that abstracting from certain anomalies annual and diurnal, as well as from accidental irregularities, the decrease is uniform. This proof is deduced upon the principle, that the sun's rays do not heat the air in their passage through it; a fact established by many concurrent experiments.

IV. In adverting to the necessity of reducing the mercury in the cistern of the barometer to a constant level, M. Ramond has mentioned several contrivances of distinguished foreign artists for this purpose. The accuracy, however, of all such expedients appears very questionable; and as a constant point of departure

in the scale is a very important and fundamental condition when any thing like precision is attempted, it may be proper, for the sake of such readers as may not have had much experience in these operations, to state the mode of making this correction by calculation; a method, which, it may safely be presumed, must be more correct than any mechanical contrivance.

First, we must suppose that we have given the internal diameter of the cistern (supposing it to be truly cylindrical), which we will call (D); secondly, the external diameter of the tube (d); and thirdly, its internal diameter (d'). It is obvious that the increment or decrement of the height of the mercury in the tube (h,) will be accompanied by a corresponding decrement or increment in the cistern (h'); and this, in the inverse ratio of their areas. This ratio will be that of (D2 - d2) to (d'); which we will call (4); and consequently h' = h, (†).

The quantity (h) is here obviously supposed to be measured from some fixed point at which the scale of the instrument becomes accurately true, the mercury in the cistern being exactly at the zero of the scale. Supposing this point to be 30 inches, and (h) the observed height of the mercury, the correct height (H) will be = h ±h, (), h, being + when above 30, and — when below. The ratio () is to be determined once for all for the particular instrument we employ, and the whole operation at each observation is reduced to merely taking the difference of the observed height above or below 30 inches, or the standard point of the scale, multiplying that difference by the constant ratio, and adding to or subtracting from the observed height.

Example.-Suppose from measurement we found

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The fraction thus obtained for each individual instrument is marked with a diamond on the tube near the top by the maker.

The measurements from which these fractions result may be depended upon to the 100th of an inch, as I have been informed by Mr. Cary. In his barometers, the point of no correction, if

it be any other than 30 inches, is distinguished by a mark on 'the scale.*

V. In forming the preceding compendium, one considerable source of abridgment was found in omitting altogether many details on the subject of the hygrometer. This I have been induced to do from two reasons; first, the methods described by M. Ramond apply solely to the use of hygrometers on the old constructions, and are both long, and probably inaccurate, when compared with the more improved methods now generally adopted on the principle of evaporation. Secondly, M. Ramond himself only treats of them as connected with the stationary meteorological observations. He conceives them of little use in the measurement of heights as the following quotations will clearly show :

(Second Memoir, § 3, p. 57.)-Allowing all that can reasona→ bly be done to the error of the instrument, it is still certain that I have made observations at extremely different degrees of humidity; and that nevertheless the effect of this circumstance has been covered in extraordinary cases, by that of more preponderating circumstances by which they were accompanied; and in ordinary cases by even the allowance due to the error of observation. The reason is evident: the factor for the temperature having been empirically determined contains the correction for the mean humidity; and the quantities by which this mean humidity has to be augmented or diminished, are ordinarily too small to affect sensibly results, on which the least accidents produce a greater effect than these quantities.

(Third Memoir, Part III. p. 99.)-It is well known that the mixture of vapour diminishes the weight of air; but we know also the limits within which this action is confined; and if we admit it into the number of causes which determine the variations of the barometer, we are not ignorant that it is far from completely accounting for them. Even when atmospheric air is susceptible of passing naturally to the state of dryness to which we can bring it artificially, the return from this state to that of saturation will only diminish the elevation of the column of mercury, by from a sixtieth to a fiftieth part, according to the temperature of the mixture; but experience proves that the air never approaches to absolute dryness, and that it always retains a considerable dose of moisture, so that the usual variations in this respect will scarcely account for a variation of a 120th or a 100th part. Now the oscillations of the barometer in our climate run through a space equal to at least 1-18th of the total height; and the barometer rises and falls frequently in the oppo

Should the preceding remarks, or any others in this appendix, appear of a more elementary nature than are usually the topics of discussion in scientific journals, the author begs to state, that the whole was originally drawn up with a view to separate publication; and he conceived he was consulting the convenience of many readers in giving the detail and reasons of every part of the operations.

site order to the augmentations and diminutions of humidity. We conclude, therefore, that the effects of this cause are counteracted by those of a cause so preponderating, that, after having compensated the action of humidity, the excess of its own influence extends yet further.

(Instructions, p. 197.)-The hygrometer has not yet been of any utility in the mensuration of heights, and there is little probability that it can be introduced, not only because the correction will be very small, but further because it will be very uncertain, whether we consider the ignorance in which we are of the law which the decrease of humidity in the column of air follows; or the extreme difficulty, if not impossibility, of eliminating this law in the result of experiments always made at the surface of the earth; that is to say, at the very source of those influences which modify partially and irregularly the humidity of the atmosphere. Saussure thought thus, and we are of the same opinion. The mean value of the humidity comprised in the constant coefficient and the factor belonging to the temperature, will occasion less error than a theory ill supported by observations will do; and these errors after all are of such small consequence, that they are not worth the trouble of a calculation, which will only cause a variation in the chances, even if it do not multiply them.

Such are the opinions of M. Ramond on the subject of a correction for the different state of moisture in which the air may be at the two stations; and to his great practical experience we must doubtless pay the highest deference; at the same time it becomes necessary to recollect the great improvements which have taken place in the science of hygrometry subsequently to the date of the methods described and used by our distinguished author. Hence several philosophers of the present day have not considered it undeserving attention to examine into the propriety of introducing the correction in question. Some observations seem to indicate an effect by no means inconsiderate due to the presence of vapour. We may cite the instances of Mr. Greatorex's observations on Skiddaw (Phil. Trans. 1818, Part II.), in which a considerable discrepancy appears to have been connected with some changes in the hygrometric state of the air. The measurements of Messrs. Herschel and Babbage at Staubbach (Edinb. Phil. Journ, No. 12), seem also to have been affected by the same cause. I merely refer to these cases, however, in order to observe in general that should more extended observations show the necessity of an application of the hygrometer in barometrical operations, the formula above investigated will easily admit of the introduction of a variable factor for this correction, instead of the mean value at present involved in the constant coefficient, and slightly modified by the variation of temperature.

An excellent method of ascertaining the elastic force of the

vapour actually suspended in the atmosphere at the time of observation is given in the Edinburgh Encyclopedia, Art. Hygrometry, attributed to Mr. Anderson.* The hygrometer employed may consist merely of two common thermometers; one is essentially necessary to the observer for taking the temperature of the air; and the other is to be compared with it, having its bulb covered with moistened linen; and will but little increase the apparatus.

The details are not of difficult investigation; but as experience has not yet decided on the propriety of introducing the correction, I shall not at present proceed to any further particulars.

ARTICLE VIII.

On Titanium. By M. H. Rose.†

THE Oxide of titanium used in these experiments was procured from the rutile of Saint-Yrieix department de la Haute-Vienne. When this oxide is fused with carbonate of potash, it forms a compound which sometimes becomes gelatinous when muriatic acid is added; but it is never as thick as that formed by silica. Oxide of titanium which has been heated to redness, when moistened and put upon litmus paper, becomes red without affecting the colour of the paper. The effect of this oxide upon litmus is more distinctly shown by putting a small quantity reduced to powder upon a drop of the tincture placed upon a white surface. The oxide becomes red as soon as it is touched by the tincture.

Oxide of titanium forms compounds with the alkalies in which it acts as an acid. It is true that it also combines with acids, forming insoluble compounds which do not possess the properties of salts, but rather of double acids. For these reasons, M. Rose considers the oxide of titanium as an acid, and distinguishes it by the term titanic acid; but states that, like columbic acid and silica (which is considered as an acid by M. Rose), its affinities at common temperatures are extremely weak, on which account it is difficult to ascertain its properties, and especially to determine its saturating power, and the quantity of oxygen which it contains.

The author then states that he used three modes to ascertain its saturating power; first, by examining its combinations with the alkalies; secondly, those insoluble compounds it forms with some acids; and lastly, by combining it with sulphur; and the

* See also the Edinb. Phil. Journ. No. 4, p. 369.

+ Extracted from the Annales de Chimie et de Physique, t. xxiii. p. 353. New Series, VOL. VI.

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