the fugar-cane, and is, in fome refpects, regarded as its fuperior. He defcribes, through feveral pages, the tree, the fruit, the method of cultivation, and the ufe to which it ferves: indeed he supposes it the fineft vegetable in the world; and from the partiality, he adds, with which it has been always mentioned by circumnavigators, and even in thofe regions in which the bread-fruit abounds, it is natural to fuppofe that it has the preference of this highly boafted and fingular production.

In the account which is here given of the land and water animals, we obferve a paragraph concerning the alligator, which, as it is fhort, we shall infert:

The make of this creature, that feems coated for ftrength, and whofe fcales and colour may deceive, conveys with the idea of danger the lures of deceit; and only floats an apparent log on the furface of the water, to furprize its prey, and hurry it, unfufpecting danger, to the depths below. It is amazing how bold and adroit fome Negroes are in the capture of this fifh. We are told that the Africans will attack the crocodile with knives, and prove victorious in the combat. The Negroes in Jamaica will take the alligator without a weapon, will inclofe it in their arms, and force it on fhore, without fear and without affiftance.' Vol. i. p. 370.

- Mr. Beckford contradicts the opinion that the body of this animal, on account of the contraction of the fcales, is not pliable, and confequently not capable of motion,' (or, as we should rather fuppofe he means, cannot turn without great difficulty.) Of one which he had in his poffeffion, he tells us, he could fcarcely touch its tail with a ftick, before it fnapped it with its mouth.'

According to this writer, the pen-keepers, as they are called, or farmers, who cultivate the foil of this ifland, are generally found to be, if not the most opulent, at least the most independent and the most happy of the inhabitants of this couni try. Yet after an account of the ease and plenty in which they live, it is added,

There is not a country in the world in which there is more room for agricultural improvement, than in the one which I am endeavouring to defcribe; but then the natural indolence of the inhabitants must be removed, their industry awakened, and a flow and progreffive trial of experiments must be made, under the eye of patience and obfervation, before they can fucceed. The land in Jamaica rather wants culture than richness; nor is the idea, and confequently the practice, of keeping it in heart, at all understood, Cultivation is not known as a fcience, but as a routine of duty; and hence the doctrine of manure, and the ufe of the plough, are only confidered as operations of annual recurrence, and not as objects that may either injure or improve: for if the land on which the canes are planted be too much invigorated, they will be too luxuriant to yield returns; whereas if poor land, on the contrary, be well cultivated, the produce will not only be good, but may be great.-The lefs

the

the land is turned up in Jamaica, and expofed to the burning powers of the fun, the longer will it preferve its humidity, and coníequently retain its ftrength. In the most humble productions of the country, it is obfervable that they thrive beft in thofe foils which are the most abundant in flint-ftones; and if they be heaped around a coffee bush, or what may be called a domeftic fhrub, they will certainly maintain, if not increafe, the vegetation.'

In another place it is faid,

The ufe of the plough is certainly not arrived at its period of perfection in Jamaica; it is, without doubt, capable of much improvement to the land, with a proportionate diminution of Negroe labour. The earth is niggard of her gifts, that the hand of induftry may bring them forth; and, however difcouraged we may be by the fterile appearance of the furface, yet let man reflect, that the mine is buried beyond the reach of cultivation; and that in his progress to the refervoirs of gold and filver, he has paffed through many ftrata of earth that would have been productive of the wants and comforts of man.'

Concerning the turtles which annually frequent the coaft of Jamaica, or the islands near it, the author obferves:

"It is well known, that they will not only live for a confiderable length of time without food and water, but out of the last element : and those that I purchafed at the Grand Caymanas *, in my voyage from Jamaica to England, increased very confiderably in weight, notwithstanding they were not given any fuftenance during the paffage. The best of these creatures are fuppofed to be thofe which are caught in the neighbourhood of this ifland: they are not fo large as thofe that the fishermen bring off for fale from Port Antonio and Cuba; but their fat and flesh are reckoned more rich and delicate. The land turtle of Jamaica are among the principal delicacies of the country; and there are but few people who have refided there long, who do not give them a decided preference.'

We have thus prefented our readers with a curfory view of this publication, of which it was the lefs eafy to give an account, as it does not proceed on any exprefs plan, and is not divided into chapters. That the mention which we have made of imperfections and mistakes, is not wholly groundlefs, will probably be perceived by the few fhort extracts that we have inferted: yet, whatever are their faults, we think that these volumes may be read by the public, as they have been by us, with entertainment and information. Befide the ufeful obfervations of other kinds which the author prefents, he not unfrequently introduces religious and moral reflections, and thus adapts his work, in different refpects, to the improvement of the generality of his readers.

* See M. Rev. for Dec. 1774, vol. li. p. 434.

ART.

ART. XII. A Treatise on Practical Aftronomy. By the Rev. S. Vince, A.M. F.R.S. 4to. pp. 204. 15s. Boards. Deighton. .1790.

IN the prefent advanced state of aftronomy, and of the fciences

dependent on it, the knowlege of inftruments, and of the method of employing them in actual obfervation, and of deducing the conclufions which obfervation furnishes, is of fingular moment. To the improvement of modern inftruments; to the accuracy with which they are conftructed; to the combination of their powers; to fkill in ufing them on the feveral occafions to which they are adapted; and to facility and exactness in applying the obfervations that are made to the practical purposes for which they are defigned; we are principally indebted for thofe difcoveries that have extended our acquaintance with the fyftem of nature, and that have accelerated the progrefs of thofe arts and fciences, toward perfecting which, aftronomy is fubfervient: but where are we to obtain the knowlege of thefe inftruments, of their fucceffive improvements, and of the method of ufing them to advantage? We have hitherto been under the neceffity of recurring to various publications, which, on account of their number or price, are not cafily procured, and in which the incidental information that occurs, is lefs fatisfactory than we could with. Writers, whose chief view has been directed to other objects, have contented themselves with recording the refult of their obfervations, without defcribing the apparatus with which they were made; and in works of a more general kind, fome inftruments are wholly omitted, and others are defcribed in a manner not fufficiently minute and accurate.

The work before us fupplies a defect, which has been often regretted. We are glad to find, that it has been undertaken by a writer of Mr. Vince's known character, as a mathematician and aftronomer; and we are happy in an opportunity of recommending it to thofe who need the information which it contains.

This treatife contains the fubftance of a courfe of lectures on practical aftronomy, read in the univerfity of Cambridge, and is defigned to explain the conftruction and ufe of thofe inftruments that are employed in aftronomical obfervations. The ingenious author has availed himfelf of all the information which he could derive from various books, and from the communications of friends, in order to exhibit the latest improvements of the feveral inftruments which he defcribes, and to render this work complete and accurate. In the advertisement prefixed to it, he acknowleges his particular obligations to the Aftronomer Royal, and to Mr. Ramfden.-To the de

4

scription

feription and explanation of the use of each inftrument, he has added rules for computing and deducing the conclufions for which the obfervations are made; and thefe rules are illuftrated by fuitable examples. As a kind of companion to those who have access to an obfervatory, and who have an opportunity of examining and ufing the inftruments which are here defcribed, this treatife is very important and ufeful: but the benefit of it is rendered more general, by accurate figures; a view of which will enable perfons who have not accefs to the inftruments themselves, to understand the author's defcription, and to obtain a fatisfactory knowlege of their conftruction and use.

The author begins with defcribing the principles and use of the VERNIER,, which is a graduated index, now generally applied to all inftruments in which great exactness is required. It is an admirable contrivance for fuperceding the neceffity of thofe minute fubdivifions, which could not be conveniently introduced into the graduated line or scale, to which this is adapted. The principle on which the Vernier is formed, is thus concisely defcribed:

If two equal arcs to the fame radius, or two equal straight lines, be each divided into equal parts, and the number of equal parts in one exceed the number in the other by unity, then will the difference of the equal parts of the two arcs or ftraight lines be a fraction, whofe numerator is the length of the equal arcs or straight lines divided by the product of the numbers of the parts into which each is divided. For if A reprefents the equal arcs or ftraight lines, #and #+1 the number of equal parts into which each is divided, A A then the length of the divifions of each will be and

refpe&ively, and their difference is

A_A A
n n+I

n

E. g. Let

47°, and fuppofe each degree to be divided into three equal parts, then will A be divided into 21 equal parts; and let an arc of the index equal to A be divided into 20 equal parts, then n=20, n+1=21, and the difference of the divifions 1 minute.'

7°

·420

20X21 420

In the fame manner, it will be eafy to adapt the graduated index to an arc, containing any number of degrees and minutes, and to obtain any minute divifions that may be required. Thus, alfo, if a line 1 inch long be divided into 24 equal parts, and another of the fame length into 25; then n=24, n+1=25,

A1 inch, and the difference of the divifions=

I

=

I

24X25 600

After defcribing the principles of the Vernier fcale, and the mode of constructing it, the author proceeds to fhew, how the divifions of it on the limb of the quadrant may be diftinguished and read off. He obferves in a note, that this invention was for fome time afcribed to Petrus Nonius, and from thence was called a Nonius; but it was afterwards difcovered to be the invention of Pierre Vernier, and therefore it is now generally called after his name.' He might have added, that Nonius's method of divifion, explained in his treatife De Crepufculis, printed at Lisbon in 1542, is very different from that of Vernier. It was formed by defcribing, within the fame quadrant, 45 concentric arcs, and dividing the outermoft into go equal parts, the next within into 89, the next into 88, and so on, till the innermoft was divided into 46. By their means, in moft obfervations, the plumb-line or index muft crofs one or other of these circles, very near to a point of divifion: whence, by computation, the degrees and minutes of the arc might eafily be counted. To this invention, fucceeded the method by diagonals, first published by Thomas Digges, Efq. in a treatife intitled, Ala feu Scale Mathematica, printed at London in 1573, who afcribes the invention of it to an ingenious artift, whofe name was Richard Chanfeler. Nonius's divifion by concentric arcs, was improved at different times by Gurtius and Clavius. The laft and beft of Curtius's improvements, confifted in fetting off, on the first concentric arc within the outermoft, the 60th part of fuch a portion of that arc as anfwered to 61 degrees, and from that divifion continuing on through the whole arc the intervals of fingle degrees: by doing which, every divifion in this arc is advanced one minute forwarder than in the firft. At the beginning of the next arc, he takes off the 60th part of 62 degrees, and from that point continues through the whole arc, the intervals anfwering to fingle degrees; and thus each divifion in this arc is advanced two minutes beyond the degrees of the firft; and in this manner he proceeds, till the degrees are divided into the whole. number of minutes that they contain. Vernier's method of divifion is a compendium of this method of Curtius. It was first published in a tract intitled, La Conftruction, l'Usage, les Propriétés du Quadrant Nouveau de Mathematique, &c. printed at Bruffels in 1631. For a farther account of the progreffive improvements which the graduation of fcales has received at different periods, we refer the reader to Robins's Mathematical Tracts, vol. ii. p. 265, &c.

The firft aftronomical inftrument which Mr. Vince defcribes, is Hadley's quadrant. He premises a fhort history of its invention. He then proceeds to the theory and conftruc