« السابقةمتابعة »
others engaged in marine pursuits, who may have peculiar facilities for adding to the scientific interest of the Society's proceedings; such Associates to be in every case recommended by the council, and to have the same privileges as honorary members—their number to be limited to twenty-five." This plan, there is little doubt, may be productive of much good, and it is hoped will bo adopted by some other societies. It offers a stimulus to tho intelligent shipmaster, and tends to increase his self-respect, by showing that he is held in respect by those who appreciate his efforts to advance science and his own mental culture.
Although it is hoped that in the course of timo some tangible results may be obtained in several branches of science, the writer, being chiefly interested in the science of zoology, determined to make a beginning by causing to be prepared plain directions for the study and preservation of animals in all parts of the world. It being evident that, if we are to expect anything from the mercantile marine, its members should be definitely informed as to what we wish them to do, a committee of the Literary and Philosophical Society was appointed, at the writer's suggestion, to draw up such plain directions as should not fail to be sufficient for the end in view. Tho preparation of such a paper was entrusted to Mr. T. J. Moore, curator of the Liverpool Free Public Museum, a gentleman well qualified for tho task; and having received the sanction of the Committee, the paper was published as an Appendix to the 'Proceedings of tho Literary and Philosophical Society' for 1801-62. It is entitled, " Suggestions offered on the part of the Literary and Pliilosophical Society of Liverpool to Members of tho Mercantile Marine who may be desirous of using tho advantages they enjoy for tho promotion of Science, in furtherance of Zoology," pp. 51. This pamphlet, containing full directions for the preparation of all kinds of animals, methods of study, and lists of text-books and useful apparatus, has b«en separately published by the Society, for distribution in quarters where it is likely to provo useful. It is desirable that such manuals for other sciences should be also carefully compiled, in order that every intelligent seaman may have scopo to exercise his talents in whatever direction his own tastes may conduct him; and thus, there can be no doubt that a useful and valuable body of scientific information would be collected to aid the researches of men of scienco at home.
It is much to be regretted that a united body of members of the mercantile marine, such as tho Mercantile Marine Sendee Association of Liverpool, should not enter cordially into a scheme which they have themselves acknowledged to be one fraught with usefulness. Had tho executive council of this Association shown an ordinary interest in its progress, still greater advances would already havo to be recorded; but the writer is sorry to have to report that he has not met with that assistance and cooperation from that body which he felt entitled to look for. Although from the first invited to cooperate in the plans proposed, no steps have been taken by them, beyond tho tardy publication of some valuable suggestions urged upon them by one member of the council (since resigned) and one of the most intelligent members of the service. This lukewarmnoss of a body of men who, by their example, might be of the most material assistance is likely to retard, although not to destroy, tho prospects of the scheme; and could the services of a small and active committee of influential gentlemen bo secured, success must ultimately crown their efforts.
There can bo no doubt whatever that it is to the rising generation of seamen that we must chiefly look for the fruits of any scheme of improved education which may bo adopted in the present day, and such establishments as the ' Conway' training-frigate in the Mersey are powerfully useful to that cud; still, in order to collect together the elements of scientific industry and laudable ambition, which doubtless exist, scattered among the present body of merchant-seamen, it is desirable, as a beginning, to offer a certificate of merit to such commanders and other officers as hold the extra certificate of the Marine Board, or who keep the meteorological log-book supplied by the Observatory, or who show in various other ways a desire to improve their minds and to encourage industry in thoso under their charge. It must strictly bo borne in mind, however, that the sea is tho only placo where the sailor's mind can be properly influenced. Churches, schools, and sailors' homes on shoro are only attended by thoso whom better influences at sea have inclined for good. Masters of vessels, therefore, who encourage their apprentices to continuo their studies at sea, and who open schools for the purpose of teaching those who have had no benefits of education on shore, are in the first placo well deserving of some reward, such as a certificate of merit, which should be so constructed and signed as to carry some weight. The nature, therefore, of this certificate, and by whom it should be signed, are questions of great importance to the success of the movement, and would require mature consideration. If the Committee of Council on Education or tho Board of Trade, or both, could be induced to take an active and official interest in the matter, the difficulty would be at once solved.
It should be mentioned, as a practical encouragement of some value, that the Colonial and Continental Church Society (9 Serjeants' Inn, Fleet Street) has, through Captain Anderson, offered to grant libraries for sailors afloat, on the following conditions:—1. The Council of the Mercantile Marino Service Association are to recommend to them four captains each year, to each of whom the above Society will grant a library, value £5. 2. It will be understood that it is desirable to select such captains as have communication with our colonial possessions.
Enough has now been said and done to prove that thore is a current at work, setting in the right direction; and we can only now leave tho matter to time, feeling fully assured that it will go on, and bear ultimate fruit, both in tho advancement of science and in the elevation of the character of the merchant-seam an.
Provisional Report of the Committee appointed by the British
Members of tho Committee:—Professor A. "Williamson, F.R.S.; Professor C.
The Committee rogrct that they are unable this year to submit a final Report to the Association, but they hope that the inherent difficulty and importance of the subject they have to deal with will sufficiently account for the delay.
The Committee considered that two distinct questions were before them, admitting of entirely independent solutions. They had first to determine what would bo the most convenient unit of resistance; and secondly, what would be the best form and material for the standard representing that unit. The meaning of this distinction will bo apparent when it is observed that, if tho first point were decided by a resolution in favour of a unit based on Professor Weber's or Sir Charles Bright and Mr. Latimer Clark's system, this decision would not affect the question of construction; while, on the other hand, if the second question were decided in favour of any particular arrangement of mercury or gold wire as the best form of standard, this choice would not affect the question of what the absolute magnitude of the unit was to be.
The Committee have arrived at a provisional conclusion as to the first question; and the arguments by which they have been guided in coming to this decision will form the chief subject of the present Report.
They have formed no opinion as to the second question, or the best form and material for the standard.
In determining what would be the most convenient unit for all purposes, both practical and purely scientific, the Committee were of opinion that the unit chosen should combine, as far as was possible, the five following qualities.
1. The magnitude of the unit should be such as would lend itself to the more usual electrical measurements, without requiring the use of extravagantly high numbers of ciphers or of long series of decimals.
2. The unit Bhould bear a definite relation to units which may be adopted for the measurement of electrical quantity, currents, and electromotive force; or, in other words, it should form part of a complete system for electrical measurements.
3. The unit of resistance, in common with the other units of the system, should, so far as is possible, bear a definite relation to the unit of work, the great connecting link between all physical measurements.
4. The unit should be perfectly definite, and should not be liable to require correction or' alteration from time to time.
5. The unit should be reproducible with exactitude, in order that, if the original standard were injured, it might be replaced, and also in order that observers who may be unable to obtain copies of the standard may be able to manufacture one for themselves without serious error.
The Committee were also of opinion that the unit should be based on the French metrical system, rather than on that now used in this country.
Fortunately no very long use can bo pleaded in favour of any of the units of electrical resistance hitherto proposed, and the Committee were therefore at liberty to judge of each proposal by its inherent merits only; and they believe that, by the plan which they propose for adoption, a unit will be obtained combining to a great extent the five qualities enumerated as desirable, although they cannot yet say with certainty how far the fourth quality, of absolute permanency, can be ensured.
The question of tho most convenient magnitude was decided by reference to those units which have already found some acceptance. These, omitting
copper wire weighing one hundred grains (a unit proposed by Professor Wheatstone in 1843) and one mile of copper wire of -j^th in. in diameter, and weighing consequently about 84£ grains per foot. The smaller units had generally been used by purely scientific observers, and the larger by engineers or practical electricians.
Intermediate between the two lay Dr. "Werner Siemens's mercury unit, and the unit adopted by Professor W. Thomson as approximately equal to one
for the moment Weber's
, were found to range between one foot of
hundred millions of absolute
The former is approximately equal to
371 feet, and the latter to 1217 feet, of pure copper wire J^th in. in diameter at 15° C. Both of these -units have been adopted in scientific experiments and in practical tests; and it was thought that the absolute magnitude of the unit to be adopted should not differ widely from these resistances.
The importance of the second quality required in the unit, that of forming part of a coherent system of electrical measurements, is felt not only by purely scientific investigators, but also by practical electricians, and was indeed ably pointed out in a paper read before this Association in Manchester by Sir Charles Bright and Mr. Latimer Clark.
The Committee has thus found itself in the position of determining not only the unit of resistance, but also the units of current, quantity, and electromotive force. The natural relations between these unite are, clearly, that a unit electromotive force maintained between two points of a conductor separated by the unit of resistance shall produce the unit current, and that this current shall in the unit of time convey the unit quantity of electricity.
The first relation is a direct consequence of Ohm's law; and the second was independently chosen by Weber and by the two electricians above named.
Two only of the above unite can bo arbitrarily chosen; when theso are fixed, the others follow from the relations just stated.
Sir Charles Bright and Mr. Latimer Clark propose the electromotive force of a Daniell's cell as one unit, and choose a unit of quantity depending on this electromotive force. Their resistance-unit, although possessing what we have called the second requisite quality, and superior consequently to many that have been proposed, does not in any way possess the third quality of bearing with its co-unite a definite relation to the unit of work, and has therefore been considered inferior to the equally coherent system proposed by Weber many years since, but until lately comparatively little known in this country.
Professor Weber chose arbitrarily the unit of current and the unit of electromotive force, each depending solely on the unite of mass, time, and length, and consequently independent of the physical properties of any arbitrary material.
Professor W. Thomson has subsequently pointed out that this system possesses what we have called the third necessary quality, since, when defined in this measure, the unit current of electricity, in passing through a conductor of unit resistance, does a unit of work or its equivalent in a unit of time*.
Tho entire connexion between the various unite of measurement in this system may be summed up as follows.
A battery or rheomotor of unit electromotive force will generate a current of unit strength in a circuit of unit resistance, and in the unit of time will convey a unit quantity of electricity through this circuit, and do a unit of work or its equivalent.
An infinite number of systoms might fulfil the above conditions, which leave the absolute magnitude of the unite undetermined.
Weber has proposed to fix the series in various ways, of which two only need be mentioned here—first by reference to the force exerted by the ourrent on the pole of a magnet, and secondly by the attraction which equal quantities of electricities exert on one another when placed at the unit distance.
In the first or electro-magnetic system, tho unit current is that of which the unit length at a unit distance exerts a unit of force on the unit magnetic pole, the definition of which is dependent on the unite of mass, time, and
• Vide "Application of Electrical Effect to the Measurement of Electromotive Force," Phil. Mag. 1851.
length alone. In the second or electro-static system, tho series of units is fixed by the unit of quantity, which Weber defines as that quantity which attracts another equal quantity at tho unit distance with tho unit force.
Starting from these two distinct definitions, Weber, by the relations defined above, has framed two distinct systems of electrical measurement, and has determined the ratio between the units of the two systems—a matter of great importance in many researches; but tho electro-magnetic system v\ more convenient than the other for dynamic measurements, in which currents, resistances, &c, are chiefly determined from observations conducted with tho aid of magnets.
As an illustration of this convenience, we may mention that tho common tangent galvanometer affords a ready means of determining the value in electro-magnetic units of any current y in function of the horizontal component of the earth's magnetism H, tho radius of the coil It, its length L, and the deflexion 8.
y=tang. $ -j-.
In this Report, wherever Professor Weber's, or Thomson's, or the absolute system is spoken of, tho electro-magnetic system only is to bo understood as referred to. The immense value of a coherent system, such as is here described, can only bo appreciated by those who seek after quantitative as distinguished from merely qualitative results. The following elementary examples will illustrate the practical application of the system.
It is well known that tho passage of a current through a metal conductor heats that conductor; and if we wish to know how much a given conductor will bo heated by a given current in a given time, we have only to multiply the time into the resistance and the square of the current, and divide the product by tho mechanical equivalent of the thermal unit. The quotient will express the quantity of heat developed, from which the rise of temperature can bo determined with a knowledge of the mass and specific heat of the conductor.
Again, let it be required to find how much zinc must necessarily be consumed in a DanielFs cell or battery to maintain a given current through a given resistance. The heat developed by the consumption of a unit of zinc in a Daniell'8 battery lias been determined by Dr. Joule, as also the mechanical equivalent of that heat; and wo havo only to multiply tho square of tho current into the resistance, and divide by tho mechanical equivalent of that heat, to obtain tho quantity of zinc consumed per unit of time.
Again, do we wish to calculate the power which must necessarily be used to generate by a magneto-electric machine a given current of (say) the strength known to be required for a given electric light.
Let the resistance of the circuit be determined, and the power required will be simply obtained by multiplying the resistance into tho square of the current.
Again, the formula for deducing the quantity of electricity contained in tho charge of a Leyden jar or submarine cable from the throw of a galvanometer needle depends on the relation between the unit expressing the strength of current, the unit of force, and the unit magnet-pole. When these are expressed in the above system, the quantity in electro-magnetic measure is immediately obtained from the ballistic formula. In estimating tho value of the various insulators proposed for submarine cables, this measuro is of at least equal importance with tho measure of tho resistance of the conductor and of tho insulating sheath; and the unit in which it is to be expressed would bo at once settled by the adoption of the general system described.