earth. It were to be wished that the tutor would at this part exhibit to his pupil the various phanomena in the heavens themselves; by teaching him thus to observe for himself, he would not only raise his curiofity, but fo fix the impreffions which the objects have made on his mind, that by proper cultivation they would prove a fruitful fource of useful employment; and he would thereby alfo gratify that eager defire after novelty, which continually animates young minds, and furnish them with objects on which to exercise their natural activity. In the third part of this introduction, the received, or Copernican fyftem is explained; by this fyftem, the various phænomena of the heavens are rationally accounted for; it fhews us how to reconcile the real state of things, with the fallacies arifing from the senses; and teaches us that the irregularities obfervable in the motion of the heavenly bodies, are for the most part to be attributed to the fituation from which they are observed. Aftronomy, in common with other branches of the mathematics, while it ftrengthins the powers of the mind, reftrains it from rafh presumption, and difpofes it to a rational assent.

The principles of the Copernican fyftem are further elucidated in the third effay; in which, various planetariums, lunariums, and tellurians,

1

are defcribed. These inftruments, though lef complicated in their construction, and lefs expenfive to the purchaser, than those large ones heretofore made for the fame purpose, are equally, perhaps better, adapted to explain the general principles of aftronomy. In defcribing them, it was neceffary to re-confider many fubjects which had been previously treated; but as they are here placed in another point of view, prefented to the mind under a different form, are generally defcribed in other words, and often with, the addition of new matter, it is hoped that these repetitions, fo far from being an object of complaint, will be found to contribute to the main intention of this work, by conveying further inftruction, fixing it more deeply in the mind, and rendering that obvious, which before might be found difficult.

One part ftill feemed wanting to an introductory treatise on aftronomy, fomething that would gently lead the pupil to a knowledge of the practical part of this science, a branch of aftr nomy to which we are indebted for our present knowledge of the heavens, by which geography has been improved, and by which the paffage of ships over the tracklefs ocean is facilitated.

There

There is no part of mathematical science more fimple and easy, than the measurement of the relative pofitions and distances of inacceffible objects. Yet to the uninftructed, to determine the distance of a fhip on the ocean, to afcertainthe height of the clouds and meteors that float in the atmosphere, to fix the latitude and longitude of places, &c. are problems that have ever appeared to be above the reach of human art; they are, therefore, particularly calculated to engage the attention of young minds, and may be used to encourage diligence, and reward application.

To introduce the pupil to this branch of aftronomy, I have described three inftruments, each of which is fimple in it's conftruction, and two of them of small expence. By these he may find the distance of any inacceffible object, the height of a spire, a mountain, or any other elevation, learn to plot a field, ascertain the altitude of a cloud, a fire-ball, or any other meteor, determine with accuracy the hour of the day, the latitude or longitude of a place, with many other curious problems. In the felection of these, I have to acknowledge the affiftance I received from a very ingenious friend.

I had intended to fubjoin to this preface a lift of astronomical authors, that the reader might know as well where to apply for further information, as the fources from whence I obtained my knowledge of this fubject. But the work has fwelled fo much beyond my expectations, that I am constrained to lay aside this design.

Lately publifhed, in one Volume; 8vo. Price Six Shillings in
Boards,

AN ESSAY ON ELECTRICITY;
Explaining the Theory and Practice of that Useful
Science; and the Mode of applying it
to Medical Purposes.

ΤΟ WHICH IS ADDED,

AN ESSAY ON MAGNETISM. The Third Edition, Corrected and Enlarged. By GEORGE ADAMS, Mathematical Inftrument Maker to His Majesty,

AND

Optician to His Royal Highness the Prince of Wales.

PRINTED FOR, AND PUBLISHED BY THE AUTHOR, No. 60, Fleet-Street.

Juft publifhed by the fame Author,

DEDICATED, BY PERMISSION, TO HIS MAJESTY, In one volume, 4to, price 11. 6s. in boards, (Illustrated with 32 PLATES, and an elegant Frontispiece) ESSAYS ON THE MICROSCOPE;

CONTAINING,

A General History of Infects, their Transformations, peculiar Habits and Economy; an Account of the various Species of the Hydra and Vorticella; a Description of 379 Animalcula; a concife Catalogue of interefting Objects; a View of the Organization of Timber, and the Configuration of Salts when under the Microscope. With a particular Defcription of the most improved Microscopes, and the Method of ufing

them.