ably greater than was assumed by Dr. Hutton; being, instead of 2.5, somewhat more than 2.7: and, according to the calculation founded on this fact, the mean density of the earth is brought out to be not less than 4.866997.

"The measure thus obtained for the mean density or mean specific gravity of the earth, is above that of any of the precious stones, and is nearly a mean between the results of Dr. Hutton and Mr. Cavendish. According to the former, density =4.481: according to the latter, it is 5.48, the mean of which is 4.98. The difference between this and the last of our results is nearly .1, or less than a forty-fifth part."

On the whole, from the survey and calculations now before us, it may be determined that the mean density of the earth cannot be less than 4.5588, nor greater than 4.867: the mean of which is nearly 4.713. The curious reader will find much to interest him in this paper, that we can neither abridge or transcribe; and in perusing it he will not fail to admire the patient and exact manner in which Mr. Playfair proceeded to collect his facts, and the scrupulous caution which he employed in founding any conclusions upon them.

The only other article which we shall mention at present, is that on the "Causes which affect Barometrical measurements." Every one is aware that in measuring the heats of mountains by means of the barometer, various allowances and corrections are made for difference of temperature, and for the increased condensation which attends the diminution of heat in the more elevated regions of the atmosphere. These allowances, however, involve in them an hypothesis that is not well understood, and of course not clearly expressed; whilst there are several circumstances which affect the density of the atmosphere, that have either been wholly omitted, or improperly introduced.

M. Deluc had discovered that, at a certain temperature, 69 of Fahrenheit, the difference of the logarithms of the height of the mercury in the barometer, at an upper and a lower station, gave the height of the former of those stations above the latter in 1000ths of a French toise; but that at every other temperature above or below 69, a correction of .00223 of the whole was to be added or subtracted for every degree of the thermometer. As the degree of heat lessens, the expansion of the air is likewise lessened, and its effect of that fluid on the barometer, by being denser, is encreased, and counteracts the influence of the rarity which belongs to the more elevated parts of the atmosphere. By observations more accurate than those of Deluc, it has been found that

the temperature at which the difference of the logarithms gives the height in English fathoms is 32°; and that the correction of other temperatures is .00243 of that difference, for every degree of the thermometer. The manner of estimating the temperature of the air, adopted in all these observations, was the same: an arithmetical mean was taken between the heights of the thermometers at the upper and lower stations, and was supposed to be uniformly diffused through the column of air intercepted between them. Nor is there any reason to suspect that this method was attended with any material error, though both M. Deluc and General Roy were dissatisfied with it; for it may be rendered very probable, as Mr. Playfair has shewn in the paper now under consideration, that the diminution of heat, in ascending the atmosphere, is uniform and proportional to the height.

But, admitting the fact now stated, and allowing that there is just reason to conclude that the decrease of heat in the superior strata of the atmosphere is proportional to their elevation, there is yet no sufficient ground to believe that the condensation produced by that decrease is also uniform. On the contrary, it is proved by experiment, that the variations in the bulk of a given quantity of air are by no means proportional to its variations of temperature. This, we need not observe, is another cause of inaccuracy in barometrical measurements, for which it is not very easy to supply a suitable correction. Nor is it the only irregularity to which the expansion of air by heat, and contraction by cold, appear to be subject. We learn from the manometrical experiments of General Roy, that a given variation of temperature is accompanied with more or less variation of bulk, cording as the air is compressed by a greater or less force. Air, for instance, compressed by the weight of an entire atmosphere, was expanded by the 180 degrees from freezing to boiling, no less than 484 of those parts whereof at the temperature 32 it occupied a thousand. But the same air, when compressed only by one fifth of an atmosphere was, by the same difference of heat, expanded no more than 141 parts; and this, though the heat of boiling water was applied to it for an hour together.

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These inequalities, so extremely difficult to measure or to correct on the large scale, belong entirely to the temperature of the air. There is another which depends wholly on the compression of that fluid. In deducing rules for the measurement of heights by the barometer, it has hitherto been supposed, agreeably to the experiments of Mr. Boyle and M. Mariotte, that the density of the air, while its tempera

ture remain the same, is exactly as the force which compresses it. But we learn that, from certain experiments described in the ninth volume of the Memoirs of the Berlin Academy, the elasticity of air of the temperature 55°, or the compressing force increases more slowly than the density; so that if the compressing force be doubled, the density will exceed the double by about a tenth part.

Mr. Playfair adds one other correction as applicable to the barometrical measurement of the height of a mountain, which we do not very clearly comprehend. This, according to him, arises from the diminution of gravity, whether we ascend or descend from the surface of the earth. The effect of this diminution, he adds, is to produce a twofold error; because, on the supposition of uniform gravity, the weight of each particle of air is computed too great, and the weight of the column of mercury that is not on the surface, is also reckoned too great. The effect of both these errors is of the same kind, tending to make the height less than it is in reality; yet it is only the first of them, and that too the least considerable which has hitherto been taken into account.

Passing over the mathematical reasoning, of which the object is to determine the amount of the several corrections, we shall rest satisfied with recapitulating those corrections themselves, so necessary to an accurate result in barometrical measurements.

The first is that suggested by M. Deluc, and respects solely the temperature of the air, and the allowance to be made for every degree in the rise or fall of the thermometer.

The second is a correction for the decrease of heat in the superior strata of the atmosphere, and for the first inequality of expansion.

The third correction applies to the second inequality of expansion, or for its variation by a given change of temperature, according to the pressure.

The fourth is to be made on account of the departure of the law which regulates the elasticity of the air from that of the direct ratio of the density.

The fifth provides a compensation for the diminished weight of quicksilver in the upper barometer, compared with that in an instrument placed on the surface of the earth.

The sixth correction is to be made on account of the diminished gravity of the air in ascending from the surface of the earth.

There are several other particulars mentioned in this ingenious paper, well deserving the attention of the philoso

pher, and also of the engineer and practical surveyor. Depending closely on the mathematical reasonings with which they are accompanied, they admit not either of abridgment or partial extract. We recommend them to the attention of the reader.

ART. XI. The Narrative of a Journey, undertaken in the Years 1819, 1820, and 1821, through France, Italy, Savoy, Switzerland, Parts of Germany bordering on the Rhine, Holland, and the Netherlands; comprising Incidents that occurred to the Author, who has long suffered under a total Deprivation of Sight; with various Points of Information collected on his Tour. By James Holman, R. N. and K. W. 8vo. pp. 368. 13s. Rivingtons. 1822.

FEW things would appear more absurd or improbable, primâ facie, than that a person wholly blind should undertake the grand tour. Mr. Holman, nevertheless, has per-, formed this feat with additional wonders. He was unaccompanied even by a servant, and he was unacquainted with the current modern languages of the countries through which he passed. The re-establishment of health by a visit to southern Europe appears to have been the leading motive. of his journey. He quitted England in October, 1819, and travelled by the Diligence to Paris. The same conveyance transported him to Bourdeaux, after a week's residence in the metropolis; and we here meet with the first personal danger to which his infirmity exposed him.

"About nine o'clock on the following morning, being Sunday, the 31st of October, one of our company exclaimed, Voila Bourdeaux! The sound revived me exceedingly, for I was become irritable and impatient, from the length and fatigue of the journey. At twelve o'clock the coach halted, and my fellow-passengers immediately jumped out, leaving me to shift for myself. Of course I concluded that we had arrived at the coach-office, and began to call out loudly for the conducteur to come and assist me in getting out. He immediately presented himself, uttered the now well-known ' toute a l'heure,' and left me. Although I perfectly recollected the unlimited signification of this word in Paris, what could I do? Had I jumped out, I should not have known what step to have taken next, and the rain was falling in torrents. There appeared no remedy, but to sit patiently until it might please some one to

come to my assistance. In a while I heard at least thirty people around the coach, talking a loud and unintelligible gibberish, quite unlike any language of the country which I had hitherto heard; soon afterwards I perceived the carriage undergoing an extraordinary, and irregular kind of motion; the people occasionally opened the door, and made me move from one side to the other, as if they were using me for shifting ballast; I inferred that they were taking off the wheels, with a view of placing the carriage under cover. After this I became sensible of a noise of water splashing, as if they were throwing it from out of hollows, where it had collected in consequence of the rain. It was in vain that I endeavoured to gain an explanation of my being thus left behind in the coach, the only satisfaction I could derive was tout a l'heure,' and the conviction that nothing remained for me but to be patient.

"But patience is more oft the exercise

Of saints, the trial of their fortitude."-MILTON.

"Atlength the motion began to increase, and to my great surprise, after an hour's suspense, I heard the horses again attached to the carriage; the passengers re-entered the coach, and we once more proceeded on our journey!

"It was afterwards explained to me, that these unaccountable proceedings arose, on our having arrived on the banks of the river Dordogne, which enters the Garonne, near Bourdeaux, from the necessity, at this point, of transporting the carriage on a raft for some distance down the stream; that the passengers had crossed the river in a ferry-boat, to a coach waiting for them on the other side, leaving me to float down with the carriage on the raft, or sink to the bottom as fate might determine; in short, I found that, while I supposed myself sitting in the coach-office yard at Bourdeaux, I had actually travelled four miles by water, without having entertained the least idea of such an adventure." P. 18.

We are not among those who join in the prevailing cry against French cookery. The roast beef of Old England, no doubt, is an excellent preservative against the agonies of downright hunger; but those who look for elegant amusement, rather than mere vulgar sustenance, in their meals, cannot but prefer the lighter viands, and more delicate preparations of the multifarious carte. All this we say with a reference to taste only; we might add much more on the score of economy. Mr. Holman, on his entrance to Bourdeaux, had four dishes, (and these were soup" in a silver basin," an entrè of ragouts, a roast chicken, and a dessert) a bottle of wine, and pain à discretion, all for fifteenpence!

VOL. XVIII. JULY, 1822.

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