« VorigeDoorgaan »
After giving so full an account of the principal doctrines contained in this performance, it is unnecessary for us to recommend it to the attention of our readers. The Author, whatever may be the fate of his theory, is evidently posseiled of uncommon ingenuity; and if he be, as we are assured he is, a very young man, the public may form high expectations from his future labours.
[A] How right soever the result of the example, here given by the Author, may be, in making 122 the temperarure of the two equal portions of water mixed together, at the respective temperatures of 32 and 212; it does not appear to us to agree with the rule which immediately precedes it, and which the Author quotes from Boer haave; nor with the relult of the same example given likewise by Boerhaave. The number 122 does not express (as the rule requires) « balf the excess of the botter above the colder fluid,” which is only 90 degrees, and is the number assigned by Boerhaave *: but it is half the excess, or so, added to the number expressing the temperature of the colder liquor, (32) or Jubftracted from the number expres. fug that of the botter (212) = 122: or, in other words, it is the arithmerical mean, or half the sum, of the two numbers.--It appears remarkable to us, that the very fame example is repcated at the end of this eslay, from M. De Luc; but there the temperature of the same mixture is made to be 180, which is said to be the arithmeti. cal mean between 32 and 212.
[B] It is not merely “ to prevent ambiguity," that Mr. Crawford makes a distinction between the allolute and sensible heat of a body : for that distinction forms the very basis of his whole theory. In order to understand his experiments, and his deductions from them, it is absolutely necessary to attend to the essential distinction, which the experimenoy, already recited by our Correspondent, prove to exist between the sensible and the abjolate heat of bodies. The firit of these is directly indicated by the therinometers: whereas the abfolute heat of a body becomes known to us only by inference and calculation; or by mixing together heterogeneous bodies, which have no known chemical aciion on each other, and observing the changes produced in the senoble heat of the different mixtures, as Thewn by the thermometer. By observing the difference between these results, and those that occur when portions of the same fuid, of different temperatures, are mixed together, it is proposed to de. tect the absolute heat of bodies, or their different capacities for containing heat.
[C] It is rather (ingular that our ingenious Author, as well as - some foreign philosophers, thould have concurred in the opinion, which several of the latier, in particular, ałcribe to Dr. Priestley ; that atmospherical air is changed, in respiration and other processes. into fixed air : especially as Dr. Priestley has more than once, we believe raken notice of this misapprehension of his meaning. This . miitake, in a certain degree, affects some of the Author's conclusions : drawn from his e: periments made on fixed air; on a fupponition
* Boerhaave's Chemistry, tranflated by Shaw, vol. i. pag. 290.
that that the atmospherical air expired from the lungs had been convertedinto fixed air. Air injured by respiration is principally phlogisticaied air, and not fixed air; though it certainly exhibits signs of its containing a portion of the latter principle. The Author therefore tould have made his experiments with phlogisticated air ; and parti. cularly with the air that is phlogisticated by animal respiration.
[D] The Author's theory relating to animal heat may perhaps be molt compendiously and best explained as follows, in addition to the extracts given by our Correspondent. From the experiments made to ascertain the absolute heat residing in bodies, Mr. Crawford infers that a large quantity of fire is contained in atmospherical air, as a conftituent principle; that living animals acquire their heat from this stock contained in the air, by means of the phlogilon contained in the blood; which combining with the air, causes the latter to part with a portion of its absolute heat, or latent fire. In fort, a process is supposed to go on, similar to a chemical eledive attraction. The air,. in respiration, is received into the lungs, copraining a great quantity of absolute heat. The blood is returned from the exo tremities, bigbly impregnated with phlogison ; and by this impreg. nation its capacity for containing heat is diminished. The phlogilloni will leave the blood to combine with the air ; because the attraction of the air to the phlogiston is greater than the attraction of the blood to that principle. On the combination of the phlogiston with the air, the latter is obliged to deposit a part of its absolute heat ; which immediately unites with the blood ; the capacity of which to receive it is at the same time increased by the separation of its phlogiston.
The blood thus dephlogisticated, or deprived of a part of its phlo. gifton, by the process of respiration, afterwards acquires fresh phlo. gifton in the course of the circulation ; and as its capacity for conLaining heat is diminished by this combination, it will gradually part with the heat which it had received in the lungs, and diffuse it over the whole system.--Thus, in the lungs, the blood is continually discharging phlogiston into the air, and absorbing heat from it in return: and in the course of its circulation through the body, it is continually imbibing phlogiston from the system, and emitting heas into it. This heat being more than can be absorbed by those parts of the system which communicate the phlogifton to the blood ; the remainder becomes redundant, or is converted into moving and Jer. fible heat, or that heat which is the object of our senses.
With respect to some of the Author's experiments, we think it expedient to offer an observation which has not been made by our Correspondent. We allude to those, particularly, from which the Author infers that the quantity of absolute heat contained in air is very nearly in proportion to its purity, or to its power in supporting animal life.
Though the Author Thews a minute attention to every circum. fance that can be supposed to influence the results in his experi. ments ;- particularly to the temperature of the vessel in which the mixtures are made ; the time spent in mixing the fubftançes together; the degree of agitation; the temperature of the atmosphere at the time, &c.-yet, in some of them, it must be confessed; the feale is Rev. Nov. 1779.
so very small, that doubts will probably be entertained whether they will justify his dedu&tions from them.
It will be doubted, for instance, whether any decisive conclufion may be drawn from observing the temperatures of a mixture of two Huids, differing in specific gravity fo very confiderably as water, and common, or dephlogisticated, air; though he gives to the latter 100 degrees of heat more than to the former. Thus a pint of at. mospherical air, contained in a bladder, and raised to the tempera. ture of 163 degrees, is immersed in a pint of water at 63 degrees; that is, in a quantity of fluid containing above 800 times more mate ter than itself. Had the atmospherical air contained the same ab. Solute heat with water, the Author calculates that it would have communicated to the water nearly the one-fixteenth part of a degree of heat: . but it communicated to it one intire degree of heat,'-He concludes therefore, that almospherical, air must contain at least 16 times as much absolute heat as water. From similar experiments made with depblogisticated air, he calculates that it communicated three degrees of heat, &c.
Even the greatest of these differences is so small, that there is reason to suspect, notwithstanding all the Author's accuracy and care, that they may poslably have proceeded from causes different from those here assigned. With respect to such differences, there is fome reason to apprehend that they may proceed either from causes wholly unknown and unsuspected ; or from others, the effects of which are too difficult, or minute, to be accurately ascertained.
Notwithstanding this remark, which cannot well have escaped the fagacity of the Author himself; we hope he will profecute his ingenious inquiries, for which he appears to be so well qualified. There are undoubtedly many phenomena in nature, well explained by this hypothesis ; which, as well as the Author's various and well imagined experiments, deferve the attentive confideration of philosophers.
Art. XIV. A Plan of the navigable Canals made, and now making, is
England. Lowndes. T HIS Plan (as we learn, by a new edition of The History -1 of Inland Navigations, published with it) is done from actual surveys, made and drawn by Mr. Hugh Henshall, engineer, and successor to Mr. Brindley; it cannot, therefore, fail of proving very acceptable to the public. It gives a clear and distinct view of one of the most extensive and important general improvements that this nation ever experienced; the consequences of which must be as durable as the existence of these valuable works. When a manufacturing and commercial nation, through' excess of riches, luxury, and taxation, can no longer bring its productions to market at a moderate price ; and when other countries, by reason of cheapness, begin to underfell it, the decay of such rich nation would then be infallible, and extremely rapid, if this natural cause of declenfion were not retarded by the exertions of genius, in the ap
longer brinem excess of riches, ufacturing and
plication of mechanical powers to the business of manufactures ;
by which means alone the destructive effects of abundant riches, and high taxes, can be suspended, and an industrious nation, in such circumstances, be preserved from ruin. The manufactures of Birmingham, Manchester, Sheffield, &c. have long existed upon these principles; and the manufacturers have been enabled, by the ingenuity and variety of their mechanical inventions, to support a rival trade against all Europe, and supply countries (where human labour is much cheaper than in this), with their commodities.
But among all these improvements, there are none so fundamental, so extensive, and so powerful, as that amazing system of water roads, exhibited in the plan now before us; and which realise all the advantages to this nation, long since foretold in various publications on this subject. All the central parts of the kingdom, and almost all the manufactures, now enjoy, or foon will enjoy, the unspeakable advantage of a reduction of more than two-thirds of the price of carrying all heavy articles; many thausands of devouring horses will be rendered unnecessary ;-and if our public measures were conducted with as much wisdom and spirit as the affairs of individuals, and those who take upon them to protect and lead us, would please either to act their part well, or do nothing, there is no doubt but this nation might still go on improving; and that its natural and artificial advantages might support it in wealth, honour, and power, for many ages.
But before we take leave of the Plan under consideration, we cannot help observing one capital defect in the execution of this great system of canals, which, we are well informed, . might easily have been prevented when the act for the Staffordfhire navigation, or the Great Trunk, as Mr. Brindley called it, was obtained. The Duke of Bridgewater's canal, the Staffordshire canal, and consequently, all those that now do, or ever may fall into them, by a narrow and selfish policy, are made to terminate in the Tide-way of the river Mersey; when they ought to have been carried over that river by a grand aqueduct into Lancashire, and to the port of Liverpool; where they might have been joined by other canals and branches from different parts of Lancashire and Yorkshire; and the whole system of canals might reciprocally have communicated with each other, without going into a tide's way, or making it necessary to trans-fhip the goods. This noble plan was proposed at the time, by a zealous and active friend to these undertakings; a meeting was had upon it at Runcorn, the place where the navigation should have crossed the river ; all the parties de*clared, it would be the beft plan for the public intereft ; Mr. Brindley approved of it, and declared it to be practicable, both Cc 2
at this meeting and before the House of Commons ; — where he was very partieularly examined upon this point ;—and yet other views and interests prevailed, and this great work was left to be completed by pofterity, for whose fake we leave this memorandum of a transaction in which they may be interested ; at the same time, referring them for farther particulars to the Journals of the House of Commons.
There is another obvious defect in this fyftem of canals, for want of an extension of the Stafford thire canal from some part near Derby to Chesterfield, which certainly will be removed when the proprietors of the Staffordfhire and Chesterfield canals, and the land-owners between Derby and Chefterfield come to underland their true interests; and to consider the benefit that must arise to them and the public from the proposed communication; by which means, the goods from Staffordshire and the neighbouring counties, would be delivered above fifty miles lower upon the Trent than at present, and the county of Derby receive great advantages in the conveyance of its natural productions, and in many new manufactures and eftablishments, which these improvements never fail to produce.
This new edition of the History of Inland Navigations contains several useful tables of distances, rates of freight, &c.and is, on the whole, a valuable colle&ion of papers relative to the canals that have been projected, and executed, in this country. The last letter in this compilation takes notice of another great mechanical improvement, for which this nation is indebted to the philofophic spirit of the age, and to the abilities of those ingenious philosophers and artists Messrs. Watt and Boulton, whose skill and activity we hope will be amply rewarded by their country *. The writer, after fpeaking of the Jime-kilus, near the Bridgewater Navigation, adds—Nor can I pass filently over the capital and new erected Salt-works, built upon the banks of the navigable canal at Thurlwood, in Chethire, the property of Melirs. Saimon and Purlington. In an adjoining valley, they have fixed a fire-engine, constructed by Mell. Watt and Boulton, which in the waste of three hundred weight of coals (value nine-pence) does in twelve hours throw up, to the height of a hundred yards, not lefs than twenty-four thousand gallons of brine; which is received in a very large refervoir, and from thence conveyed to the salt-pans, where the falt is extracted and loaded into barges, in which it is carried into Staffordshire, Derbyłhire, and the neighbouring counties.'
We have cast up the lengths of the several canals included in the plan, and we find, that they amount to 556 miles, and
* See an account of Mr. Watt's great improvements on the in: vention of the Steam Engine, Rev. vol. lvi. p. 40.