pafs from the one furface to the other, and each time this takes place, the mufcle is thrown into action. Although the nerves are the conductors of the electricity, yet it is only when acted on by ftimuli, that they excite, as it were, the electricity of the mufcles. In regard to voluntary motions, the will of the animal is what difpofes the nerve to do this; and, in regard to the fpontaneous actions of animals, the author thinks the nerves of each organ to be affected by certain specific ftimuli.

"Whenever the nerves feel a ftimulus, the electrical fluid is excited, and then its effects are made manifeft. Under these circumftances, that takes place, which we have noticed when fpeaking of voluntary motions, that is to fay, the electricity paffing from one state to another, and changing its mode of existence, produces the contraction. Its equilibrium is never established. A new caufe again excites this fluid, and another new effect takes place, which is relaxation. The electricity exifts again in two oppofite ftates; thus if it be again excited, the phenomena, fimilar to the former, will again recur. If by any accident the fluid finds the equilibrium, the organ in which it oc curs becomes paralytic." P. 216.

We ought to remark, however, that with respect to the mufcles of involuntary motion, the author fuppofes it is the nerves alone which are diftributed on them that are charged pofitively and negatively. This opinion he fupports by the following reafons:

"Ift, The coating and exciter do not produce any change in these mufcles.

"zd, These muscles do not poffefs fo large a quantity of nerves, as is diftributed upon those which are fubjected to the command of the

will.

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3d, The fabric of the former differs from that of the latter.
4th, Their movements are likewise different.

5th, The electricity difcharged by the nerves, is a cause fufficient to produce the effect fought for.

6th, Some experiments, as for inftance, that of the learned Cigna with filken ribbons, prove that electricity may exist naturally in fubftances, in the two oppofite ftates." P. 217.

The author does not think that the nervous fluid is fecreted by the brain itfelf, but by the blood veffels. After adducing feveral facts from Dr. Monro's Obfervation on the Nervous Syftem, and from other authors, which ftand in contradiction to the opinion of the brain being the fecreting organ of the nervous fluid, and, after endeavouring to how that the great quantity of blood fent to the head is chiefly deftined for the nutrition of the brain, he adds:

It is, however, probable that the blood veffels may have another office, viz. that of feparating the electrical fluid in the brain (by the brain I mean, all the mafs contained within the cranium), as well as in the medulla fpinalis or nerves.

"The brain in particular, at least in animals, in which it forms an effential organ, ought to have an abundance of this fluid, as it is by means of this fluid that it brings into play the fprings of genius, and that it exercifes its power and influence upon the different parts of the body." P. 237.

In regard to the general doctrines of our author's hypothefis, we have to obferve, that feveral difficulties and objections remain to be explained before it can be adopted; for, first, granting the propofition, that the nervous fluid is formed or fecreted by the arteries, it remains to be accounted for, how animals fhould exhibit figns of this Huid 24 hours after the circulation. is deftroyed; and granting, ftill further, the fluid to be electricity, the difficulty is ftill greater; for as the means for its fecretion are now deftroyed, one or two difcharges (contractions excited by the metals) ought to reftore the equilibrium between the furfaces of the mufcular fibres, after which the animal, or rather the part of the animal, ought no longer to exhibit any phænomena of the fame kind. Secondly, if the nerves are the only mediums neceflary for eftablishing a communication between the furfaces of the mufcular fibres, which are in oppofite states of electricity, where is the use of external conductors, as employed in the whole of the experiments? a ftimulus, applied to the extremity of the divided nerve, ought, according to the author's own notions, to excite the electricity as long as it exifts. Thirdly, it is not analogous to the known laws of electricity, that two different metals fhould be abfolutely requifite to caufe a difcharge between two furfaces of a body, the one of which is charged pofitively, the other negatively, as is the cafe with this Influence.

Our readers will obferve, that thefe few arguments do not go to the denial of a fluid analogous to electricity refiding in animals, but only to our author's manner of explaining the phenomena by it.

We acknowledge our belief in the existence of fuch a fluid in animals. The phænomena exhibited by the Torpedo, the Gymnotus Electricus, the Silurus, and the fih defcribed by Mr. Paterfon; the facts relating to the preternatural and difeafed accumulation of electricity in man, related by Nollet, and in the detached writings of phylicians; the fact mentioned by Cavallo, of the difference between the dead and the living body, in regard to its quality, as a conductor of electricity; BRIT. CRIT, VOL. III. MAR. 1794.

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ale

are circumstances which must have great weight with every impartial perfon.

Upon the whole, however, we think the work before us deferves applaufe, not only from the number of interefting experiments related in it, but alfo for the ingenuity which the author has difplayed, in attempting to reduce them under general principles; which, at leaft, may ferve to excite others to the profecution of the fubject, either with a view of confirming or refuting the hypothefis.

ART. IV. The Theory and Practice of finding the Longitude at Sea or Land; to which are added, various Methods of determining the Latitude of a Place, and Variation of the Compass, with new Tables. By Andrew Mackay, A. M. F. R. S. Ed. Two Volumes. 8vo. 12s. Sewell, Elmiley, and Evans. 1793.

THE

HE fubjects mentioned in this title are of the highest importance to feveral defcriptions of men. An accurate knowledge of them enables the aftronomer to obferve the heavenly bodies with precifion, and impowers the geographer to determine the diftances and pofitions of places on land. The mariner has recourse to them, both in war and peace, in order to guide his veffel to the port for which he is bound, and to fecure the lives of thofe with whom he fails, and the wealth committed to his care, from the danger of fhipwreck.

The neceflity of determining the latitude and longitude for the laft-mentioned purpofes, did not fully appear till towards the end of the 15th century, when De Gama and Columbus had boldly stretched out into the ocean, and by their fucceffes had excited a very general defire of diftant difcovery, adventure, and commerce. The mariner, removed from the fight of his land-marks, attended to the compass, and directed his views to the heavenly bodies for affifting him to afcertain his fituation; but experience foon taught him, that a confiderable portion of science and accurate inftruments were requifite to enable him to derive practical utility from his obfervations. Without them he found he could only conjecture, and even with their afliftance he could only obtain approximations to the truth, which, in the infancy of this kind of navigation, were found to deviate too much from it to be likely to enfure perfonal fafety. This was more particularly the cafe in the early methods for afcertaining the longitude, and, in confequence of the very incorrect determinations then made, many fhips and lives were loft.

In order to fecure themselves against a continuation of such difafters, maritime nations began, about the end of the 16th century, to confider the determination of the longitude as a public concern; and from that period to the prefent, rewards have been held out by princes and affemblies, and particularly by our own parliament, and ftill continue to be held out, to call forth the efforts of individuals to effect this important problem, even within a specified difference from the truth. Every thing, therefore, tending to this great end, has a claim to our ferious attention, and we readily coincide in opinion with Mr. Mackay, and approve of his defign ftated in the following paffage of his Preface:

"In every commercial ftate, any work that has for its object the improvement of the art of navigation, will always be favourably received; and fhould it not in all refpects anfwer the public expectation, the author may ftill have fome claim to indulgence, from his good intentions, and having exerted his best abilities for the good of his country. With this view, the author of the following treatife has bestowed much time and labour to render it as complete and as generally useful as poffible; and though he has not the vanity to imagine it to be by any means a perfect work, he has yet been flattered, that it may be of fervice to navigation, and, therefore, not altogether unworthy of the notice of the public. He ventures then to fubmit it, with much diffidence to their judg ment, and if it shall be found any ways deferving of fuch a character, he will efteem their approbation his highest and best reward.''

The first volume of Mr. Mackay's work confifts of fix books, of the extent and arrangement of which our readers may form a general idea from the contents, which we here fubjoin.

"Book I. In which the principles of the aftronomical methods of finding the longitude at fea or land are explained. Chap. 1. Of the figure and magnitude of the earth. 2. Definitions and principles, &c. 3. Of the fixed ftars. 4. Of the planets. 5. Of

the moon.

"Book II. Upon the inftruments for measuring angular distances at fea, and of the corrections to be applied to thefe obfervations. Chap. I. Of Hadley's Quadrant. 2. Of the Sextant. 3. Of the circular inftrument of reflection. 4. Of the manner of taking a complete fet of lunar obfervations. 5. Of the corrections to be applied to the altitude of an object observed at fea, and to the ob ferved diftance between two objects.

"Book III. Of the method of finding the longitude of a fhip by lunar obfervations. Chap. I. Introduction to this method of finding the longitude. 2. Preparatory problems. 3. Of the methods of afcertaining time, and regulating a chronometer or watch at sea or land. 4. Of the methods of clearing the apparent distance be

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tween

tween the moon and fun, or a fixed star, from the effects of refraction and parallax. 5. Of finding the longitude at fea or land by lunar obfervations. 6. Of finding the longitude at fea or land, by an obfervation of the distance between the moon or the fun, or a fixed ftar, together with the apparent time of obfervation. 7. A new method of finding the longitude and latitude of a fhip at fea. 8. Of finding the longitude at fea or land, by an obfervation of the diflance between the moon and a ftar, not ufed in the Nautical Almanac. 9. Of finding the longitude, by an observation of the diftance between the moon and a planet. 10. Of finding the lon gitude, by an obfervation of the moon's altitude-the apparent time at the place of obfervation, together with its latitude and longitude, by account being given.

"Book IV. Containing various other methods of determining the longitude of a place. Chap. I. Of finding the longitude by an obfervation of the moon's tranfit over the meridian. 2. The method of finding the longitude of a place, by an obfervation of an eclipfe of the moon. 3.The method of finding the longitude of a place, by an eclipfe of the fun. 4. The method of finding the longitude of a place, by an occultation of a fixed ftar by the moon. 5. Of finding the longitude of a place, by obfervations of the eclipfes of the fatellites. of Jupiter. 6. The method of finding the longitude of a fhip at fea, by a chronometer or time-keeper. 7. Of finding the longitude at sea by the variation chart.

"Book V. Containing the demonftrations of the preceding rules and formulæ.

"Book VI. Containing methods of finding the latitude of a place, and the variation of the compafs."

In the methods of finding the longitude at fea, preceding that now offered to the public in Chap. vii. Book iii. "The neceffary elements, befides the obfervations, are, the latitude of the hip, and its longitude by account, together with the time at the thip nearly. It may however happen that the latitude is not accurately known; hence the apparent time at the thip cannot be computed, and confequently the ship's Longitude will remain unknown. To obviate this, the following method was invented, wherein neither the latitude, longitude, nor time, are required as neceffary data, but are found directly from the fame fet of obfervations." The peculiaritics of this method depend upon the folution of this problem. The apparent diflance between the moon and the fun, or a fixed flar, together with the altitude of each being given, to find the latitude and longitude of the place of obfervation. Our author informs us, that he had discovered this method feveral years before he tranfmitted it to Dr. Mafkelyne, in 1787, with a defire he would lay it before the board of longitude. To this request Dr. Mafkelyne fent an immediate