every one; but to distinguish the features of hills, mark their connexion, judge of their comparative height, steepness of slope, and other points relative to ground, which are the most important considerations in a purely military plan, requires a real knowledge of the art. And it may further be observed, that few officers who have not had the advantage of a military education can ever understand ground as exhibited in a plan.

MILITARY PLANS and SKETCHES, like extensive Trigonometrical Surveys, are obtained by means of triangulation from a measured base line: in the former, however, we do not look for the minute accuracy which is imperative in a regular survey; being in general rapidly executed, and with the aid of inferior instruments.

a

"The most suitable instrument for taking the bearings of objects in military sketches, or for making a survey of line of road or railway, is the surveying, or prismatic compass. With this instrument, and either the land-chain or pacing, or both combined, a survey of an extensive tract may be made with great accuracy, provided the lines of measurement be well arranged. The survey may be entered in a field book, and the details sketched after the

* A slight acquaintance with the elements of Geometry is necessary to enable a student to comprehend the subject fully. A knowledge of Trigonometry is not absolutely required in this kind of surveying.

B

general outlines have been plotted; but, in many cases, it is desirable to plot the work in the field, so as to bring home the drawing complete.

"The method of surveying by the prismatic compass appears to be so valuable for military purposes, and the process so well adapted to teach the principles of surveying, that it might perhaps be found worth consideration, whether it should not be perfectly understood, before a theodolite be put into the hands of a pupil."

It is believed that the experience of most officers who have had extensive practice in military surveying, will fully bear out the opinion given in the above extract.

The prismatic surveying compass, known as Schmalcalder's, when mounted on a stand, and having the card divided to 20', that is, each degree into three parts, enables the observer to obtain a bearing to within three or four minutes of a degree, by estimation.† When held in the hand, a bearing may be depended on to about fifteen minutes, provided the wind is not so violent as to shake the hand of the observer. It is usual to watch the vibration of the divisions on each side of the vertical hair, and take the mean. I have found it most convenient when the card is divided into twice 180 degrees, instead of having it numbered from 0 to 360 degrees. The stand is better without levelling screws; they are troublesome, and, I may say, almost useless; for the card is easily made level enough to play freely, by shifting the legs of the stand: these should be between five and six feet long, for the convenience of

* See contribution to Professional Papers, Royal Engineers, by Samuel B. Howlett, Esq., Chief Draftsman, Ordnance. Vol. I., page 62.

† Messrs. Troughton and Simms, 138, Fleet Street, have lately divided the cards of their smallest prismatic compasses to twenty minutes instead of to half a degree, as formerly, which I consider to be a great improvement.