Some content below taken from the "American Drawing Book" which is in the public domain in the United States of America as of 2013.
18.) Thus far, attention has been directed only to the drawing of straight lines; and, if proper care and study have been bestowed upon the principles laid down, and the hand has been taught to keep pace with the understanding of these principles, the few examples to be given in the drawing of curves will be all that is required, before he is introduced to the great school of ArtĂ˘â‚¬â€ťthe imitation of nature. Let him be advised not to hurry forward too rapidlyĂ˘â‚¬â€ť to gain strength as he goes Ă˘â‚¬â€ť to confine his efforts to what he can accomplish, rather than run the risk of failure, in attempts beyond his power.
19.) Again (2) let the importance of a clear, firm, and well-defined line be urged. "Think before you draw," is as important a maxim as "Think before you speak." Determine well the point of beginning and termination, the direction and form of every line, before you touch your paper. Now is the time to school your hand to this habit; which, when once acquired, will render progressive studies comparatively easy, and hereafter serve you well in your attempts, however far you may pursue the Art of Drawing. A manner of dashing off random lines or touches, as if in search of the true line, betrays weakness and indecisionĂ˘â‚¬â€ťbesides, produces a painful display of the labor the work has cost. The ease apparent in the sketch of a masterhand, that is so captivating, is the result of absence of any appearance of hesitation or doubt. If any were felt, in its execution, it is a secret known only to the artist himself, who should always possess the judgment to look rather to results, than the ostentatious display of the labor of their accomplishment. The examples given will enable the student, by comparison, better to understand what is to be avoided.
20.) In the directions hitherto given, with regard to the drawing of straight lines, the ruled paper afforded a more certain guide than it will be found to be in curves and irregular forms. The straight, or right line, must be the basis, however, upon which to form the true observation and delineation of them. A right line is certain and arbitrary; and, according to the variation of curves and irregular forms from a right line, must be measured their irregularity by the eye, and also expressed, the result of that observation. The faculty of ascertaining and expressing the degree and character of these variations, is a most important acquirement in drawing. Hereafter, in its proper place, more will be said in reference to circles, ovals, etc., as presenting the motive of lines and forms; but, it is important that the pupil should go step by step, and, as far as possible, master one difficulty before he encounters another.
21.) Let him attempt to draw the most simple curve or eccentric line, and he will find it, probably, no easy task to perform with accuracy; and even if measurably successful, at first, to repeat it may be more diflicult. But, if he has a right line from which to mark the variations it becomes comparatively easy. To the beginner, a difficulty naturally will arise as to the existence of these right lines in objects in nature. The eye, by practice and proper education, learns to supply this, and soon becomes accustomed to measure irregular forms by this unerring standard. At present, it is out of place to enter, as fully as may be hereafter necessary, into the explanation of this principle in Drawing; which must be gradually developed to the understanding of the pupil, as he acquires progressive strength in the training of his eye and hand.
22.) In the following examples for practice, the ruled paper will be of essential advantage. Begin, as in the exercises in drawing straight lines, by marking certain points along the ruled line (5), and then connect these points by curves sweeping at first to the middle of the faint lines, above and below the points (example A) these exercises from right to left, as well as from left to right, It is important that suflicient command of hand, to draw lines in any direction with equal facility, should be early acquired. When you can do this with some degree of ease to yourself, as well as accuracy, increase the distance between the points, as B; and after that, draw a line of greater sweep C D: and so on proceed with the rest of the examples. E is but a combination of what you have already done A; and F of C D.Ă˘â‚¬â€ť I K : will be comparatively easy after these, as well as L. In examples M N, observe well the movement of the line as it touches the six faint lines, and the points it marks as it approaches its termination. It starts on the first ruled line, and, making a gradual sweep, turns on the sixth, moves upward to nearly half way between the first and second: again descends to half way between the fifth and sixth, moves upward to nearly half way between the second and third, and terminates between the fourth and fifth. In example N, the same observation, with some little variation, will apply. Endeavor, in the imitation of these examples, to draw them with a clear, unbroken line, without taking the pen from the paper until it is done. Be not discouraged at repeated failures, but try again and again, until you succeed. You doubtless begin to find that you require more than the command of your fingers in drawing: your wrist, and the whole arm, must be brought under proper government. And here, as a valuable assistant, the blackboard can not be too strongly recommended.