The axis is the key of a design or of any composition. An axis in geometry is a line which separates into two equal parts any symmetrical plane figure, or the pole of a surface of revolution or of a regular solid, such as a rectangular prism with a regular base. In architecture the idea of the axis is greater than this. It is in reality a vertical plane through the whole building separating the building into two parts symmetrically, or in such a way that they balance one another.

Although the graphical representation is confined to a straight line, do not forget that it is not simply a line. Take for example a church; in drawing the plan, the axis of this plan will be a straight line separating it into two parts, but this line itself will be only the projection of the central vertical plane which is the axis of the whole building; and the keystones of the vault, the lights which drop from them, the center of the rose window, etc., are in the axis of the church. Notice besides this that the straight line which is the axis of the plan, and the line which is the axis of the front and rear facade, the line which is the axis of the transverse section - these lines are only the traces, all belonging to some axis plane, as it may be called, and this plane is the principal axis.

But there are other minor axes. Parallel to the main axis are the axes of the side arms and between these are the axes of the columns. Running transversely are the axes of the transept, those of each bay, the radiating axes of the chapels, etc.

In laying out the drawings of a church, for example, first place all of these different axes with the utmost accuracy. This method of laying out the drawings of a building by starting with the axes may be best explained by examples. Let us commence by the study of a plan, that of a vestibule, in a public building; e.g., the Hotel des Monnaies at Paris, Fig. 6.

After having drawn the axis 1, which is the principal axis of the building, it will be noticed that there are five bays of the central pavilion which are spaced equally. Of these draw first the extreme axes, 2 2; by dividing the space between axes 1 and 2 into equal parts, the intermediate axes 3 3, will be found. In this way the chances of error would be decreased, for if the axes were placed in the order 1,3, 2, the possible error would be doubled. Now taking the portion to the right, draw first the extreme axis 4, then 5, and divide the space 4 5 into equal parts, which will give the axis 6.

Fig. 6. Plan of Vestibule of Hotel des Monnaies.

Now consider the axes of the rows of columns 7 7. These are to be arranged in relation to the axes 3 3; finally the axes 8 8 are located in relation to the extreme axes 7 7, being checked in relation to the axes 2 2.

In the longitudinal direction the same process will be gone through, placing the first axis 1, then the extremes 2 2; by division 3 3 will be obtained, and dividing the spaces between the axes 1, 2, and 3, into half, the axes 5 and 6 of the columns are obtained. The secondary axes will be placed in the same way. Finally it will be found advisable to check up the different steps by verifying the distances of the secondary symmetrical axes from the central main axis.

In carefully studying the plan, and the different methods of drawing it, the student will become convinced that the methods of spacing the axes are of great importance, and that in this way he will arrive at exactness and will avoid many mistakes.

The student must understand that it is much more difficult to draw a good plan than is popularly supposed; more difficult, perhaps, than anything else, from the mere fact that everything builds up from the plan. In the plan especially, extreme exactness is necessary and the student will do well, in order to become familiar with architectural drawing, to practice the drawing of plans constantly.

Fig. 7. Hotel des Monnaies, Transverse Section of Vestibule.

Section on YY.

Fig. 8. Hotel des Monnaies, Longitudinal Section of Vestibule.

Section on ZZ.

Now let us consider the sections, taking the same example that we have just considered. The student will easily see that the architect cannot study his composition thoroughly without the aid of numerous sections. Two sections, however, are especially necessary, those following the principal transverse and longitudinal axes of symmetry. If the student wishes to draw both of them, he should decide first which one of the two controls the other. See Figs. 7 and 8. He will see that in this case it is the transverse section, parallel to the front elevation. The other, the longitudinal section, is chiefly the projection of elements of the other section. Therefore, in this case the drawing should be commenced by laying out the transverse section.

First, place the axes just as has been done in the plan, 1, 2 2, 3 3, 7 7, 8 8. In regard to the profiles or the parts in section, the first thing necessary is to locate the heights of the essential parts, taking for the first level the main floor A A, next drawing the upper line of the capitals of the columns B B, then the centers of the vaults C D.

Starting with these principal lines, draw in the details, as for example, the heights of the bases in relation to the floor A A. The capitals and heights of the architraves will be located in relation to the line B B. It is evident that if all the measurements were taken from the level of the main floor A A, the least inexactness would affect the capitals, while if the total height of the column A B is once determined, no mistake can be made in the height of the base and that of the capital, and even admitting a slight inexactness, it will be inappreciable on the total height of the shaft of the column.