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in fact, treat only of effects, of spaces described, and of the mathematical laws of their description-But, considering the great and general merit of the performance before us, we seem to ourselves degenerating into petty and peevish criticism; we turn, therefore, with satisfaction, to certain passages in which the author's perspicuity, accuracy, and mental vigour, eminently shine forth.
These mechanical affections of matter have been very generally called powers or forces; and the body conceived to possess them is said to act on the related body. This is figurative or metaphorical language. Power, and force, and action, cannot be predicated in their original strict sense, of any thing but the exertions of animated beings; nay, it is perhaps only the exerted influence of the mind on the body which we ought to call action. But language began among simple men; they gave these denominations to their own exertions with the utmost propriety. To move a body, they found themselves obliged to exert their strength, or force, or power, and to act. When speculative men afterwards attended to the changes of motion observed in the meetings or vicinity of bodies, and remarked that the phenomena very much resembled the results of exerting their own strength or force; and when they would express this occurrence of nature, it was easier to make use of an old term, than to make a new one for things which so much resem. bled; because there are always such differences in other circumstances of the case, that there is little danger of confounding them. We are not to imagine that they thought that inanimate bodies exerted strength, as they themselves did. This was reserved for much later times of refinement.-In the progress of this refinement, the word power or force was employed to express any efficiency what eyer; and we now say, the power of aqua fortis to dissolve silver -the force of argument-the action of motives, &c. &c.
To this notion of conveniency we must ascribe, not only the employment of the words power and force, to express efficiency in general, but also of the terms attraction, repulsion, impulsion, pressure, &c. all of which are metaphorical, unless when applied to the actions of animals. But they are used as terms of distinction, on account of the resemblance between the phenomena and those which we observe when we pull a thing toward us, push it from us, kick it away, or forcibly compress it.
Much confusion has arisen from the unguarded use of this figu rative language. Very slight analogies have made some animate all matter with a sort of mind, a wong buy, while other resem blances have made other speculatists materialize intellect itself.
The very names which we give to those powers which we fancy to be inherent in bodies, shew that we know nothing about them. These names either, like magnetism, express a relation to the par ticular substances which we imagine possess the power, or they express something of the effect which suggested their existence. Of this last kind are cohesion, gravity, &c. They are almost all verbal derivatives, and should be considered by us merely as abbre viated descriptions or hints of the phenomena, or as abbreviated
references to certain bodies, but by no means as any explanation of their nature. The terms are the worse by having some meaning. For this has frequently misled us into false notions of the manner of acting. Perhaps the only strict application of the term action is to the effect produced by our exertions in moving our own limbs. But we think that we move other bodies, because our own body, which is the immediate instrument of the mind, is overlooked, like the plane in the hand of the carpenter, attending to the plank which he dresses.' P. 90.
And again, when he tells us not to trouble ourselves about the intimate nature of forces:
The only safe procedure is to consider all the forces which we observe in action as mere phenomena. The constitution of our mind makes us infer the agency of a cause, whenever we observe a change. But, whether the exertion of force shall produce motion or heat, we know not, except by experience, that is, by observation of the phenomena. Nor will speculations about the intimate nature of these forces, and their manner of acting, contribute much to our useful knowledge of mechanical nature. We gain ali that is possible concerning the nature of those faculties which accompany matter, or are supposed to be its inherent properties, by noticing the laws according to which their exertions proceed. Without a knowledge of these laws, the other knowledge is of no value.' P. 94.
The three laws of motion,-to which, in our opinion, much greater importance than is necessary has been attached,-are discussed by the author in a very masterly manner. After stating the various opinions concerning the first law, and the proofs that have been attempted of it, he adds the following excellent observation:
All these differences of opinion may be completely settled, by adhering to the principle, that "every change is an effect." It is a matter of fact, that the human mind always considers it as such. Therefore, the law is strictly deduced from our ideas of motion and its causes; for, even if it were essential to matter gradually to diminish its motion, and, at last, come to rest, this would not inva◄ lidate the law, because our understanding would consider this diminution as the indication of an essential, or, at least, a universal property of matter. We should ascribe it to a natural retarding force, in the same way that we give this name to the weight of an arrow discharged straight upwards. The nature of existing matter would be considered as the cause, and we should estimate the law of its action as we have done in the case of gravity; and, as in that case, we should still suppose that were it not for this particular property, the material atom would continue its motion for ever undiminished.' P.104.
The remarks on the second law, which is properly charac terized as a tautological proposition, are not less excellent. The author does not directly say the law is nugatory, but we fancy we discern a struggle between his good sense and his ve
neration for the great and illustrious propounder of these
We do not, however, approve of all that the learned professor has said relatively to this second law of motion. In proving the law, or rather in rendering probable the truth of the law, considered as a physical law, he introduces arguments to which we are unable to affix any precise notion. We instance one: A force, which we know to act equably, produces equal increments of velocity in equal times, whatever these velocities may be.' What kind of force does the author allude to? Does he know any thing of the equable action of the force, except from the fact of 'equal increments of velocity'? We suspect the proposition, if it means any thing, is tautological; but besides this objection, the professor's manner of writing is here very faulty: he ought not obscurely and allusively to have couched in general terms the results of particular experiments; but plainly and particularly to have stated those experiments, and then to have drawn his inferences.
But our objections and censures are not at an end: we like still less than the part just reprehended, what the author has said relatively to the question of the forces vives; we thought this question had been settled, and that every philosopher understood the dispute to be verbal, and why it was verbal. Dr. Robison might have learnt this from a writer he often quotes, from Dalembert: for, if our memory does not fail us, he has shown that according as you define force, its measure may be the mass into the velocity, or the mass into the square of the velocity; that is, the measure of force A, or of moving force, may be mv, and the measure of force B, or of mechanical force, may be mv': from applying the same term (force) to two different things, great mistakes arise; and, for a moment to descend from our grave chair of criticism, the cause of Master Slender's mistake was somewhat similar, when he carried off, instead of Anne Page, a lubberly boy dressed like her, in white.
We find some difficulty in conceiving how Dr. Robison, who in many parts so clearly explains what the mathematician ought to understand by the terin force, could dictate the follow⚫ing passage.
The same conclusion may be deduced from our notions of a constant or invariable force; it is surely a force which produces equal effects, or changes of motion, in equal times. Now equal augmentations of motion are surely equal augmentations of velocity. We find this notion of an invariable accelerating force confirmed by what we observe in the case of a falling body. This receives equal additions of velocity in equal times; and we have no reason to think that this force is variable. We should therefore infer, that whatever force it imparts in one second, it will impart four times as much in four seconds. So it does, if we allow a quadruple
velocity to indicate a quadruple force; but in no other estimation of force.' P. 112.
On the subject of the composition of forces, Dr. Robison properly animadverts on those faulty proofs by which the composition of forces is immediately inferred from the composition of motions; and he notices the more rigorous demonstrations of Daniel Bernouilli, Foncenex, and Dalembert. We are rather surprised that the professor should have passed over unnoticed the demonstration of Laplace; and that he should have let slip so fit an occasion for the exercise of his acuteness and sagacity, and (we may add) for the indulgence of his spleen, against that great mathematician. It is hardly fair to give our opinion without the grounds of it: but we cannot briefly enter into particulars; the demonstration of the author, in our opinion, is not free from objection..
In the section concerning accelerating and retarding forces, the author, notwithstanding his own arguments, seems not entirely delivered from verbal thraldom. He says: "Indeed all that we know of force is, that it is something which is always proportional to this is indeed all that is necessary to be :'
known; and if the author had been under the influence of the same good sense that dictated the preceding sentence, he would not have swelled his book by attempting to prove that no finite change of velocity is generated in an instant, by any accelerating or retarding force.'
On the subject of deflecting forces, and of central forces, the author gives several of the propositions contained in the second section of Newton; the first proposition in the eighth, the first and second propositions of the ninth, and some of the first propositions of the eleventh, section. He then passes on to plane astronomy, which he treats with great neatness and perspicuity, particularly the subject of the calendar; and we subjoin an extract rather for its perspicuity, its easy and flowing style, than for its novelty of information and depth of thought.
Astronomy, like all other sciences, was first practised as an art. The chief object of this art was to know the seasons, which, as we have seen, depend either immediately, or more remotely, on the sun's motion in the ecliptic. A ready method for knowing the season seems, in all ages, to have been the chief incitement to the study of astronomy. This must direct the labours of the field, the migrations of the shepherd, and the journies of the traveller. It is equally necessary for appointing all public meetings, and for recording events.
Were the stars visible in the day-time, it would be easy to mark all the portions of the year by the sun's place among them. When he is on the foot of Castor, it is midsummer; and midwinter, when he is on the bow of Sagittarius. But this cannot be done, because his splendour eclipses them all.
The best approximation which a rude people can make to this, is to mark the days in which the stars of the zodiac come first in sight in the morning, in the eastern horizon, immediately before the sun-rise. As he gradually travels eastward along the ecliptic, the brighter stars which rise about three quarters of an hour before the sun, may be seen in succession. The husbandman and the shepherd were thus warned of the succeeding tasks by the appearance of certain stars before the sun. Thus, in Egypt, the day was proclaimed in which the Dog-star was first seen by those set to watch. The inhabitants immediately began to gather home their wandering flocks and herds, and prepare themselves for the inundation of the Nile in twelve or fourteen days. Hence that star was called the Watch-dog, Thoth, the Guardian of Egypt.
"This was therefore a natural commencement of the period of seasons in Egypt; and the interval between the successive apparitions of Thoth, has been called the natural year of that country, to distinguish it from the civil or artificial year, by which all records were kept, but which had little or no alliance with the seasons. It has also been called the Canicular year. It evidently depends on the sun's situation and distance from the Dog-star, and must therefore have the same period with the sun's revolution from a star to the same star again. This requires 365d 6h 9' 11", and differs from our period of seasons. Hence we must conclude that the rising of the Dog-star is not an infallible presage of the inundation, but will be found faulty after a long course of ages. At present it happens about the 12th or 11th of July.
This observation of a star's first appearance in the year, by getting out of the dazzling blaze of the sun, is called the heliacal rising of the star. The ancient almanacks for directing the rural labours were obliged to give the detail of these in succession, and of the corresponding labours. Hesiod, the oldest poet of the Greeks, has given a very minute detail of those heliacal risings, ornamented by a pleasing description of the successive occupations of rural life. This evidently required a very considerable knowledge of the starry heavens, and of the chief circumstances of diurnal motion, and par ticularly the number of days intervening between the first appearance of the different constellations.
Such an almanack, however, cannot be expected, except among a somewhat cultivated people, as it requires a long continued observation of the revolution of the heavens in order to form it; and it must, even among, such people, be uncertain. Cloudy, or even hazy weather, may prevent us for a fortnight from seeing the stars
'The moon comes most opportunely to the aid of simple nations, for giving the inhabitants an easy division and measure of time. The changes in her appearance are so remarkable, and so distinct, that they cannot be confounded. Accordingly, we find that all nations have made use of the lunar phases to reckon by, and for appointing all public meetings. The festivals and sacred ceremonies of simple nations were not all dictated by superstition; but they served to fix those divisions of time in the memory, and thus gave a comprehensive notion of the year. All these festivals were