branches: and carefully-drawn figures of them are annexed. The consequence at which he arrives is-that if we admit the hypothesis of two centres of magnetic force situated within the earth, there will be two and only two points on the earth's surface, at which the needle can take a position vertical to the horizon.

"Whether," he continues, "this be the number actually existing on the surface of our earth, we are not at present in a condition to determine. One such undoubtedly there is, and a second is probable, but its position has not been assigned; neither, from any observations yet published, can it be even approximately determined, nor, therefore, its existence positively affirmed. I am not aware that any observations give reason to suspect the existence of more than these two; and hence, so far as we can judge from the data before us, the conclusion now obtained as a consequence of two magnetic centres of force, is consistent with the phenomena for which the hypothesis is required to account. It is, therefore, a strong argument, in the present state of our actual knowledge of the phenomena of terrestrial magnetism, for the truth of that hypothesis*."

This is the utmost extent to which the solution of the problem has been advanced in this direction: and here we are compelled, for the present, to leave the subject.

ADDENDUM.- -Since this paper has been in type, a memoir by Baron Humboldt has been submitted to the Royal Society, and a Report upon it, drawn up by Mr. Christie and the Astronomer Royal, has been also given in. The object is to urge civilized governments, especially that of England, to establish a few well-appointed permanent magnetical observatories in appropriate positions on the globe. Many of the difficulties to which we have alluded at p. 383, would then be removed: and we have little doubt of this system completely supplying all the data that will be required. We trust the British Government will not be backward in seconding the efforts made to complete a system of inquiries, which in origin and cultivation have hitherto been almost entirely British, and in which, too, Britain has a deeper personal interest than any other nation in the world.

* Phil. Trans., 1836, page 124. He remarks, however, that could we conceive such a constitution of the terrestrial magnet as should have two poles of like kinds, there would always be two points of verticity, and within certain limits of relation between the positions of the poles, the centre of the earth and the magnitude of the earth's radius, there may be four such points.

MISCELLANEOUS INTELLIGENCE.

Precision in Scientific Terms. No. III.

FORCE; POWER; FLUENT-POWER.-"A great source of confusion and error in Mechanical Philosophy, as treated of in books, is an indiscriminate and ambiguous use of the terms, force, power, resistance, and others of correlative import, to signify several different sorts of quantities; accompanied by a corresponding vagueness of apprehension in regard to the existence and nature of these differences. Whether the ambiguity of language is the cause or the effect of the indistinct notions on the subject, I need not inquire. It is certain that they tend mutually to perpetuate each other, and I have no doubt that, collectively, they have done more to retard the progress of mechanical philosophy, and to bring it into disrepute among practical men, than all other causes united. That there may be no misapprehension in regard to the precise meaning I attach to the language I use in speaking of these different quantities, I shall, before I proceed further, point out the differences to which I allude, and the mode in which I shall distinguish them.

Mechanical agency may be contemplated under several different aspects, in each of which respectively, its magnitude is a different species of quantity. My present purpose requires me to notice three of these:—

1. It may be considered with reference solely to the simple pressure or effort, exerted at any point, or indivisible instant of time. In this view its magnitude is expressed simply in pounds; this quantity I call force, or force of resistance.

2. It may be considered with reference not only to the force, or pressure at any point of time, but also with reference to the distance through which it is exerted, but without reference to the time occupied in moving through that distance. In this view its magnitude is expressed by the product of force and distance. The cost and value of all mechanical effects, and, of course, the cost and value of all mechanical power, are proportional to this product; for this reason I designate the quantity resulting from the product of force and distance by the name power. This product is the true measure of mechanical power in all cases, when contemplated as an agent, producing a determinate amount of any of the various ultimate effects aimed at in practical mechanics; for the amount of effect produced will always be proportional to this product in the moving power.

3. Mechanical agency may be contemplated as a quantity depending for its magnitude, not on the total amount of its effects, but on the rapidity with which it produces given amounts of effect; that is, it may be considered not only with reference to the force and distance, but also with reference to the shortness of time occupied in passing through that distance.

In this view its magnitude is expressed by the product and distance divided by time, or which is the same thing, the product of force and velocity. This product of force and velocity is not a measure of power, but of a ratio which power bears to time, or in other words, it is a measure of the rapidity with which power flows out and is brought into action. This quantity, therefore, I call fluent-power.

I will recapitulate these distinctions. VOL. I.

2 E

6

Force is simple pressure, irrespective of duration or motion.

Power becomes developed as this force moves, in proportion to the distance moved through.

Fluent-power is the ratio of this developement to the time in which it takes place.

These are three quantities, which are totally different in their nature, and between which it is of the utmost importance clearly to distinguish, in every branch of mechanical philosophy in which they are introduced, and in none more so than in that which treats of the resistance of fluids.

It may seem incredible to some, that distinctions so obvious and simple, and the propriety, and even necessity, of which are so manifest, can need to be laid down, and insisted upon in the nineteenth century, especially in a branch of science which has been cultivated ever since the days of Archimedes. But if any one to whom this sentiment may occur, will first clear up his own views with regard to these distinctions, and will then in the light of them examine the books, he will be astonished to find what a medley of confusion and error they contain, in every branch of mechanical philosophy to which these distinctions are applicable. Even the Treatise by Olinthus Gregory, who, perhaps, brought as high a degree of mental and mathematical acumen to bear upon the subject, as any who have preceded or followed him, should not be excepted from this remark.

It was an oversight, or misapprehension, of these distinctions, that embarrassed the views of your two correspondents, to whose communications I have already referred, and which has led them to suppose that the different results at which they have arrived, in estimating the perpendicular action of a fluid on a plane oblique to the line of its motion, are at variance with each other. Correctly understood, these results are in perfect accordance, not only with each other but with the common theory. They differ, it is true, not, however, in principle, but only in the nature of the quantity deduced."

The two correspondents referred to, are two highly-respectable Professors in the United States. Mr. Blake, whose remarks we have been quoting, after stating the question between them, and showing that, by the strict use of the terms in the senses he has proposed, the debateable ground between the disputants is narrowed, if not altogether annihilated, proceeds :

"In the foregoing remarks, I have endeavoured to give a prominence to the distinctions I have made between the different aspects in which mechanical agency may be contemplated, corresponding to my views of their importance. That the quantities between which I have endeavoured to distinguish are different, is no new discovery. Their difference has always been recognised, whenever it has been adverted to. It has, however, been so little adverted to, at least in treatises on mechanics, that it is scarcely too much to say, that it has been wholly overlooked. Nothing is more common, not only in loose conversation and writing, but even in the books which profess to teach on this subject, than to find these several quantities spoken of under one and the same name, without any discrimination at all, and evidently without any apprehension of their difference. It is to this circumstance, as I have already suggested, that I chiefly attribute the well-known fact that, in reference to the application

and use of mechanical power, theory and practice have hitherto wooed each other almost in vain. Whenever a good treatise of mechanics shall be given to the public, in which these distinctions are laid down, in limine, as fundamental, and carried out through all branches of the subject, as I have endeavoured to carry them out here, that moment, in my view, theory and practice on this subject will be wedded, and a new era in their history will commence."-Blake, Remarks on the Theory of the Resistance of Fluids. Silliman's Journal, No. LX. 1836.

Without expressing any opinion, at present, upon the accuracy of the definitions given by Mr. Blake, it is impossible not to acknowledge the justice of his observations. The careless, and therefore, the frequently contradictory use of terms, is an old and inveterate disease. The experiment advised by Mr. B. to those who have any doubt upon the subject, must, unfortunately, under existing circumstances, be undergone by at least every selftaught student. It is a painful necessity which ought to be removed, and which would be in a short time, if authors were as anxious and as fearless to be understood as this gentleman. His example, if followed by our own countrymen, who are the greatest sinners in this respect in Europe, would save many a tiresome and unprofitable investigation and many a mischievous dispute.

Safety-Stopper for Steam-Boilers.

EXPLOSIONS of steam-boilers are frequently the effect of an accumulation of sediment within them. This forms a crust on the interior surface, particularly on the bottom, and being a bad conductor of heat, insulates the metal from the water. It is, therefore, extremely favourable to the former acquiring a high and injurious temperature. When this has happened, the difference of dilatation, which sometimes occurs, destroys the adhesion of the sediment, and exposes the incandescent metal to the water. The instantaneous production of steam, which is the consequence of the contact, is often so enormous, that the boiler, unable to withstand the sudden increase of pressure, explodes with more or less violence. To this peculiar case of danger, no one had, up to a very recent period, been very successful in applying a means either of remedy or of prevention. To use distilled or filtered water, to invite the deposit to subside in parts of the boiler not exposed to the fire, and to prevent accumulation, by frequently cleaning out the boiler, were the principal means used. That these have been very inadequate, and that a remedy for the evil, ór a prevention of its occurrence, had been long desired, every one at all acquainted with the causes of destruction of steam-boilers is well aware. Water which, on evaporating, leaves no sediment, cannot be obtained in a vast majority of instances in which steam-boilers are used, and the labour and inconvenience which the frequent removal of the deposit requires, often causes this necessary operation to be deferred far beyond the time that it ought to be done, either for the preservation of the boiler, or the safety of those near it. With this view of the case, and with the recollection of the length of time this particular cause of mischief had baffled the ingenuity of the most successful improvers of the steam-boiler, we can easily feel how eagerly would be received an invention which should faithfully give an indication of this peculiar danger whenever it may be approaching, and which should, if this indication be neglected, avert the danger, by suspending, in proper time, the action of the immediate agent.

Such an invention has been recently accomplished by M. Galy-Cazalat*. It has been patented both in France and in England; and after the usual severe examination of La Société d'Encouragement of Paris has been declared worthy of their large gold medal.

The simple and ingenious apparatus, by which M. G.-C. succeeds in rendering this essential service to the safe application of steam-power, is the following:-it should, however, be premised, that the invention is not intended to be substituted for the present safety and escape-valves, but to be used in addition to them. A small tube is passed through the boiler and its top and bottom, directly over the part where the fire strikes the latter most forcibly. The upper extremity of the tube projects from the boiler, and is terminated by a transferring-cock. The lower extremity is riveted to the boiler-bottom, and its aperture is slightly contracted in its diameter. In the passage of the tube through the boiler, a communication is made between them by a few holes. These are pierced in the tube, and so as to be as high as possible above the regular water-surface level.

The aperture of the lower extremity of the tube is closed by a means which deserves attention. A stopper, which may be either a plug or a ball, of fusible metal, is deposited in the transferring-cock, and by the half-turn of the handle conveyed into the tube: as soon as it has dropped from the cock, it is exposed to the current of steam rushing through the perforations in the tube, and is driven along like the ball in an air-gun. At the end it is arrested by the contracted diameter of the tube, and is so powerfully held there, that it hermetically seals the aperture. This stopper, so applied,

becomes an integrant part of the boiler, and from its position is, like it, exposed to the direct action of the fire; but there is this important peculiarity with regard to the stopper-it never can be, on its upper side, in contact with anything but steam. Now that of the boiler-bottom is always intended to be in contact with water, and may be separated from it by sediment.

If we suppose a boiler-bottom, guarded by a safety-stopper, to be liable to become dangerously heated by either of the sets of circumstances which are the sole causes of this species of mischief, the stopper will melt before the danger becomes serious, the tube will be uncorked, and steam will issue precisely at the point it can be most effective, i. e. directly upon the fire; the latter will therefore be instantly damped, and the usual danger as instantly averted. This alone is a most valuable property in the invention, and sufficient to recommend it to general adoption; but it has this additional advantage in practice; viz., that it operates without stopping the engine; that as it damps, not extinguishes, the fire, the supply of steam is moderated, and never entirely suspended, and that another stopper can, by means of the transferring-cock, be conveyed into the tube, and the aperture again hermetically sealed in a very few seconds after the fusion of its predecessor.

M. G.-C. extends the application of this principle to the sides, as well as the bottoms, of boilers, in order to prevent the mischief which results from a

*Professeur des Mathématiques-Physiques at the Royal College of Versailles.

This cock is similar to the greasing-cock of a steam-engine-its plug is not perforated, but has a chamber only, this receives the article to be transferred; on turning the plug half round, the chamber is exposed to the other part of the tube, and the article drops into it. The use of the cock is to keep the tube closed during the

transfer.

These are total absence of water in the boiler, from deficiencies of supply, &c., and-insulation of the bottom by sediment.