« AnteriorContinuar »
the subjects of numerous experiments; in the first instance at common temperatures, and subsequently when the temperature was considerably raised. The former condition distinguishes those experiments which are comprised in the first section of the paper. The general conclusion to this section is given in the following words:
“From the facts which have been stated, it appears, that when the compound combustible gases mixed with each otber, with hydrogen, and with oxygen, are exposed to the platinum balls or sponge, the several gases are not acted upon with equal facility; but that carbonic oxide is most disposed to unite with oxygen; then olefiant gas; and lastly, carburetted hydrogen. By due regulation of the proportion of hydrogen, it is possible to change the whole of the carbonic oxide into carbonic acid, without acting on the olefiant gas or carburetted hydrogen. With respect, indeed, to olefiant gas, this exclusion is attended with some difficulty, and it is generally more or less converted into carbonic acid and water; but it is easy, when olefiant gas is absent, so to regulate the proportion of hydrogen, that the carbonic oxide may be entirely acidified, and the whole of the carburetted hydrogen be left unaltered. This will generally be found to have been accomplished, when the platinum ball has occasioned a diminution of the mixture, at about the same rate as atmospheric air is diminished by nitrous gas, when the former is admitted to the latter in a narrow tube.” P. 276.
In the 2d section, Dr. Henry proceeds to trace the effects of the platinum on gaseous combinations at increased temperatures. He conceives that the effect of varying the proportion of free hydrogen to the compound combustible gases, on the degree of action which is excited by the platinum sponge, will perhaps admit of being explained by examining the facts already stated in connexion with the degrees of combustibility of the compound gases under ordinary circumstances. He then states the order of their combustibility, as given by Sir H. Davy; and, observes, that it is precisely in this order that the three compound gases require hydrogen to be added to them, in order to be rendered susceptible of being acted upon by the platinum sponge. He hence considers it extremely probable, that the temperature produced by the union of the hydrogen and oxygen, forming part of any mixture, is the circumstance which determines the combustible gases to unite or not with oxygen, by means of the sponge.
Dr. H. however thought it further desirable to ascertain the exact temperature at which each of these gases unites with oxygen by means of the
the gases mixed with oxygen enough to saturate them, were severally exposed in small retorts, containing a platinum sponge, and immersed in a mercurial bath, to a temperature which was gradually raised till the gases began to act on each other. In this way a vast number of results, equally new and interesting, were obtained, which it will be impossible here to particularize. The 3d section, we shall only add, contains the application of the facts before ascertained to the analysis of mixtures of the combustible gases in unknown proportions.
The whole paper, it is obvious, must be carefully perused by any one who wishes fully to appreciate its excellence; and as a model of accurate and elegant experimental inquiry, we cannot too highly recommend it to the attention of the student:
Our readers will hardly need to be reminded of the valuable investigations respecting the deviation of the needle on shipboard, from the action of the iron of the vessel, which we have from time to time brought under their notice, and in which so much has been effected by Mr. Barlow, of the Royal Military Academy. To these researches some addition has lately been made by Mr. G. Harvey, of Plymouth. This gentleman's name will be known to our readers as connected with various able investigations on the effect of magnetism on chronometers, &c. He has lately turned his attention to the magnetisin of ships; and an elaborate paper on the subject is inserted in the volume of the Transactions now before us (No. 20). Whether the author has ever heard of Mr. Barlow's discoveries we do not know; but it would seem rather strange, that in taking up an inquiry so closely connected with that in which Mr. B. has made such distinguished advances, he should not so much as allude to any previons researches on the subject. It may certainly be said, that his object was not identicaiiy the same as that of Mr. B. or any earlier inquirer; but, from the introductory paragraph, it appears to us to differ very little.
“It having appeared, from many unquestionable experiments, that the varia, tion of the compass as determined on shipboard, is subject to remarkable anoma lies arising from the unequal influence of the iron distributed through the various parts of the vessel, and from the changeable intensity of the same, occasioned by the different directions of the ship's bead with respect to the magnetic meridian, and from its different situations on the surface of the earth, it seemed desira ble that some attempt should be made, to discover in what way the attractive forces are distributed throughout the vessel, and particularly in the yicinity of the binnacle, by a series of careful experiments.”—p. 310.
Such was Mr. Harvey's object. He accordingly made experiments on board several vessels; some having no iron on board, except what was employed in their construction; others in a state of complete equipment. Several stations were fixed upon in different parts of the vessel, having a reference to certain imaginary planes, and the intensity of magnetic action at each station was to be determined with the ship's head in several different positions, with respect to the magnetic meridian. The instrument employed for determining the intensity was similar to that denominated the apparatus of Coulomb; consisting of a magnetized cylindrical bar, two inches and a half long, and ths of an inch diameter, delicately suspended by a single fibre of a silk-worm to the extremity of an adjusting screw, which worked in the cap of the glass vessel inclosing the bar. A brass wire likewise passed through the cap, having its lower end bent into an angular form, for the purpose of placing the bar in a direction at right angles to the magnetic meridian, previous to its being allowed to oscillate. On the different days devoted to the experiments, before visiting the ship, the time of making fifty vibrations of the needle was determined in the centre of a meadow, and of which the substratum was clay-slate, by a mean of six sets of experiments, performed with the utmost care; the time being registered to quarter seconds. The instrument was VOL. VII. No. 40.-Museum.
then taken on board, and placed in succession at the different stations, previously assumed in the ship, and the mean of six sets of experiments determined at each station with the same care as on land. The times of performing the oscillations on shore, and at each of the assumed points in the ship, necessarily gave the mag. netic intensity at each station in terms of the terrestrial intensity, and which in this case was represented by 100.
The results of extensive observations conducted in this way on board four different vessels, with others of a more limited kind on board six others, are given in tables. From a comparison of these, it appears that the changes of intensity under the different circumstances are remarkably great and irregular. It is impossible here to describe them particularly, as they are only to be explained by a reference to the sections and plans of the ships with which the paper is illustrated. Mr. H. calculated the position of the centre of force in each case, which was found to undergo very curious changes; in one instance he found the locus of it to be a curve of double curvature.
The magnetisın arising from position is, it is well known, of a very variable kind; developing its intensity in some situations of an iron mass, with singular energy and force, and in others exhibiting only an action of the feeblest kind. These changes, manifesting their influence in an instantaneous and rapid manner, are considered by Mr. Harvey to account for many of the anomalies in the course of these experiments, from the numerous alterations which took place in the bearings of the ship's head, and the consequent change of intensity of every mass of iron.
From a subsequent set of experiments he concludes, that the changes of magnetic intensity, at any station in the vessel are regulated by laws analogous to those which influence simple masses of iron. The variations of intensity, however, at the several stations were of a very unequal kind. In some parts of the ship the alteration of a quarter of a point in the direction of the head was productive of a greater change than the variation of an entire point at some other stations. Nor does the change of intensity at the same point appear to be proportional to the alteration in the direction of the ship's head. Such inequalities must however be considered as necessary consequences of the irregular distribution of the iron, and of its inequality of action.
One of the most curious of Mr. H.'s results is, that no definite relation appears to exist between the magnitudes of the ships, and of the resulting intensities; but it seemed in general, that the changes of intensity were considerably greater on board small ships than in those of larger size.
We omitted to allude to one interesting topic, which is adverted to near the beginning of the paper. Mr. H. found that the protimity of houses sensibly affected the instrument, when carrying on his operations on land; and it was to guard against this that he chose this station is an open field. This fact was observed some years ago (though Mr. H. does not allude to it) by Professor Hansteen, who concluded, from observations made on the effects of a tower at Copenhagen on an apparatus like Mr. Harvey's, for measuring the intensity, that every vertical mass on the earth's surface, composed of any inaterials whatever, acquires a species of magnetic power, having a south pole above, and a north pole below. [See Edinburgh Philosophical Journal, No. 8, p. 299.) We may here take occasion to remark, that the fact observed by Professor Hansteen appears to us not unlikely to afford a satisfactory solution of several apparent anomalies which the magnetic needle has presented to the notice of different observers; among these there is one of a very remarkable kind, which we alluded to in our Review of the Philosophical Transactions, 1823, Part II. In a paper in that volume, by Mr. Barlow, a remarkable difference is mentioned between the indications of the needle in and out of doors. Mr. B. seems very doubtful as to the nature of this effect, and is rather inelined to ascribe it to the influence of light. [See bis paper, p. 340, &c.] We have long been rather disposed to attribute it to some modification of the same effect as that mentioned by Hansteen, and which we think by no means unlikely to be of a thermo-electric nature.
In the present improved state of navigation, the accuracy of chronometers is a point of the most essential importance, and one to which accordingly the greatest attention has been given, both in respect to the construction of the machine, and to the investigation and prevention of external causes of inaccuracy. It has been shown by Mr. Fisher, astronomer to the Arctic expedition, and subsequently by other experimenters, that chronometers are sensibly affected by magnetism; but besides this source of irregularity in their rates, there is another, which appears hitherto to have been overlooked; this is the variable density of the medium in which the balance performs its vibrations. To this curious point the attention of Mr. Harvey has been directed, and in the paper which he has laid before the Royal Society, and which appears in the present volume of the Transactions, he conceives it will clearly appear, that changes in the density of the air will produce very sensible alterations in the rates of delicate chronometers.
His first experiments consisted in subjecting chronometers to a less pressure than that afforded by the ordinary state of the atmos. phere. This was done by partially exhausting a receiver under which they were placed; with several chronometers it appeared that the rate increased with the diminution of pressure; but with others, exactly the reverse took place. The same experiment was varied, by transporting the chronometers to a place considerably elevated above the sea, and comparing their rate with that which obtained at the level of the sea. Here again the same difference of rate, with the same irregularity as to particular chronometers, was exhibited. This was tried in several different places, and calculated on the same data for various others at different elevations.
Thus, although it appears from these experiments that the alterations of rate in the same chronometer depend on the density of the medium in which it is placed, yet the magnitudes and characters of the changes in different time-keepers are very dissimilar, which would seem to arise from some peculiarity of construction.
From these effects of a diminution of atmospheric pressure, it was inferred by the author, that from an increase results entirely the reverse would arise; that is, that each particular instrument, if it gained, by being placed in air of a less density than the ordinary state of the atmosphere, ought to lose by being subjected to a greater. Accordingly by introducing dillerent time-keepers into a condensing engine, furnished with an appropriate mercurial gauge, those opposite results actually took place.
Thirdly, to obtain alterations of rate of the most striking and remarkable kind, the effects of suddenly removing the instruments from condensed into rarified air, and vice versa, were estimated by a series of careful experiments. The results were exactly such as might have been expected, and differed in different instruments as before.
The fourth point to which Mr. Harvey's attention was directed was, to consider how far the ordinary changes in the density of the air may be likely to exercise an influence on the rate of a. chronometer. The range of the mercurial column in London, may on an average be estimated at 21 inches, and the author thinks there can be no doubt, but the difference produced in the density of the air by such a range, must, if the transition be at all sudden, and the difference constant for 24 hours, or even less, be sufficiently considerable to affect the majority of chronometers; he found however, a great difference in this respect existing among the chronometers which he tried; the change of density which in one machine would occasion an alteration of rate amounting to several seconds, in another, would scarcely produce any sensible effects; and he found, during the whole of the experiments, a considerable difference in this particular, between pocket and box chronometers; the former being most readily affected by alterations of atmospheric density.
After stating the particulars of these experiments, Mr. H. makes the following interesting remarks:
“From these experiments it may therefore be inferred, that a difference in the density of the atmosphere represented by a quantity less than an inch of quicksilver, if continued for a day, was capable of affecting all the chronometers employed; and this is an atmospheric change by no means uncommon in this variable climate. Nor is it indeed necessary that the alteration of density should even continue for twenty-four hours, since from the change of rate being instantaneous (as will be proved in a subsequent page), six hours will be sufficient in some cases to disclose it. In cases however where the variations of the mercurial column are but small, and its transition from one state to another marked by a gradual character, the effect on the generality of chronometers, is scarsely if at all perceptible. With a difference in the mercurial column of an inch and three quarters, or two inches, I have little doubt but all time-keepers will be influenced; and it is moreover known, that from a species of reaction in the atmospherical columns, it