PUBLIC LIBRARY 150806 ABTOR, LENOX AND TILDEN FOUNDATIONS. 1899. DISTRICT OF CONNECTICUT, ss. SEAL. BE IT REMEMBERED; That on the fifteenth day of May, in the thirty ninth year of the Independence of the United States of America, JEREMIAH DAY, of the said District, hath deposited in this Office, the title of a Book, the right whereof he claims as Author, in the words following, to wit: "A Treatise of Plane Trigonometry: to which is prefixed, a summary view of the nature and use of Logarithms. Being the Second Part of a Course of Mathematics, adapted to the method of instruction in the American Colleges. By Jeremiah Day, Professor of Mathematics, and Natural Philosophy, in Yale College;" in conformity to the act of the Congress of the United States, entitled, "An Act for the encouragement of learning, by securing the copies of Maps, Charts and Books, to the authors and proprietors of such copies, during the times therein mentioned." HENRY W. EDWARDS, Clerk of the District of Connecticui. A true copy of Record, examined and sealed by me, H. W. EDWARDS, Clerk of the District of Connecticut. THE HE plan upon which this work was originally commenced, is continued in this second part of the course. As the single object is to provide for a class in college, such matter as is not embraced by this design is excluded; with the exception of a few things in the notes. The mode of treating the subjects, for the reasons mentioned in the preface to Algebra, is, in a considerable degree, diffuse. It was thought better to err on this extreme, than on the other, especially in the early part of the course. A more concise method may be adopted in the succeeding numbers. The section on right angled triangles will probably be considered as needlessly minute. The solutions might, in all cases, be effected by the theorems which are given for oblique angled triangles. But the applications of rectangular trigonometry are so numerous, in navigation, surveying, astronomy, &c. that it was deemed important, to render familiar the various methods of stating the relations of the sides and angles; and especially to bring distinctly into view the principle on which most trigonometrical calculations are founded, the proportion between the parts of the given triangle, and a similar one formed from the sines, tangents, &c. in the tables. The solutions of oblique angled triangles are made by the common methods. The propositions which are used in particular cases, principally by astronomers, are inserted in a note at the end. On the subject of Trigonometrical Analysis, nothing more than a few of the first principles could be admitted, in a work upon so limited a plan. This number begins with a view of the nature and use of Logarithms, as preparatory to the calculations in Trigonometry. CONTENTS. Page. III. Solutions of Right angled Triangles 52 Solutions of Oblique angled Triangles 67 Geometrical Construction of Triangles 78 111 LOGARITHMS. SECTION 1. NATURE OF LOGARITHMS.* ART. 1. THE operations of Multiplication and Division, when they are to be often repeated, become so laborious, that it is an object of importance to substitute, in their stead, more simple methods of calculation, such as Addition and Subtraction. If these can be made to perform, in an expeditious manner, the office of multiplication and division, a great portion of the time and labour which the latter processes require, may be saved. Now it has been shown, (Algebra, 233, 237,) that powers may be multiplied, by adding their exponents, and divided, by subtracting their exponents. In the same manner, roots may be multiplied and divided, by adding and subtracting their fractional exponents. (Alg. 280, 286.) When these exponents are arranged in tables, and applied to the general purposes of calculation, they are called Logarithms. 2. LOGARITHMS, then, are the EXPONENTS of a series of powers and roots.+ In forming a system of logarithms, some particular number is fixed upon, as the radix or first power, whose logarithm is always 1. From this, a series of powers is raised, and the exponents of these are arranged in tables for use. To explain this, let the number which is chosen for the first power, *Maskelyne's Preface to Taylor's Logarithms. Introduction to Hutton's Tables. Keil on Logarithms. Maseres Scriptores Logarithmici. Briggs' Logarithms. Dodson's Anti-logarithmic Canon. Euler's Algebra. † See note A. be represented by a. Then taking a series of powers, both direct and reciprocal, as in Alg. 207; a1, a3, a2, a1, ao, a-1, a-2, a-3, a-4, &c. The logarithm of a3 is 3, And the logarithm of a1 is —1, of a1 is 1, of a° is 0, of a 2 is -2, Universally, the logarithm of a is x. 3. In the system of logarithms in common use, called Briggs' logarithms, the number which is taken for the radix is 10. The above series then, by substituting 10 for a, be comes 104, 103, 102, 10, 10°, 10-1, 102, 10-3, &c. Or 10000, 1000, 100, 10, 1, 1, 100, 1000, &c. Whose logarithms are 4, 3, 2, 1, 0, -1, -2, -3, &c. of 4. The fractional exponents of roots, and of powers roots, are converted into decimals, before they are inserted in the logarithmic tables. See Alg. 255. The logarithm of a3, or ao.3 3 3 3, is 0.3333, of a or a 0.6666, is 0.6666, 3 These decimals are carried to a greater or less number of places, according to the degree of accuracy required. 5. In forming a system of logarithms, it is necessary to obtain the logarithm of each of the numbers in the natural series 1, 2, 3, 4, 5, &c.; so that the logarithm of any number may be found in the tables. For this purpose, the radix of the system must first be determined upon; and then every other number be considered as some power or root of this. If the radix is 10, as in the common system, every other number is to be considered as some power of 10. may That a power or root of 10 may be found, which shall be equal to any other number whatever, or, at least, a very near approximation to it, is evident from this, that the exponent may be endlessly varied; and if this be increased' or diminished, the power will be increased or diminished. |