760

THEORY AND PRACTICE OF ENGINEERING.

Book II.

u

Fig. 781.

The !>oundaries of a vessel of any kind, whether those of a vase,cup, or box, are those surfaces and lines which are visible to theeye; they comprise all within and with- y

out, and whatever presents a face.

Zones or Girdles are those bands whichencompass or surround a sphere, as thatbetween B and D.

G L 1, H M K, indicate the zone be-tween F and E: a zone may embrace onlya portion of a sphere, as that indicated atC in the two halves, A, A.

The globe is divided into five zones by the two tropicsand the two polar circles; that comprised between thetropics is called the torrid, its breadth being 47°, or twicethe sun’s greatest declination ; this is divided into twoequal parts by the equator. That zone included betweenthe tropic of Cancer and the arctic circle is called thenorth temperate zone, and that between the tropic ofCapricorn and the antarctic circle the south temperatezone : each of these are in breadth 43° ; that portion com-prised between the arctic circle and the north pole is thenorth frigid zone, and that between the antarctic circleand south pole the south frigid zone.

This division of the earth into zones probably arosefrom the difference observed in the temperature, which ishigher in the equatorial regions than in any other part ofthe earth, in consequence of the sun’s rays being moredirect; to every point of the earth’s surface whose zenithlies between the tropics, the sun is vertical twice in theyear : in the polar regions the temperature is lowest, inconsequence of the obliquity with which the sun’s raysfall, and the length of the winter night. In the countriessituated between these two regions there is a mediumtemperature, increasing as the zenith approaches thenearer of the two tropics, and diminishing as it approachesthe nearer of the polar circles.

Zones are sometimes irregular, as N on the globe Y,the widths of O Q and P R being different.

S is a zone on the globe T, as is V on that of X.

Plans are of various form, as those of A, B, and C: anyfigure having superficial contents is so designated.

The surveyor of land determines by measurement boththe boundaries and superficial content of any portion ofthe earth’s surface; the object being to ascertain thecontent of a single field, or the relative distances andbearings of the various buildings or other objects aroundor within it. In measuring land, all the lines and surfaceswhose contents are to be found are reduced to the samehorizontal plane, on the principle that as corn grows ver-tically, no greater quantity can be produced on the slantside of a hill than would grow on the area which its basecovers : when the lines measured are not horizontal, theymust be multiplied by the cosines of their respectiveinclinations to the horizon. When a country is surveyed,or a large geometrical map is to be formed, it is necessaryto have regard to the earth’s curvature and many othercircumstances.

Ichnographic plans , as shown at D, are made use of toexhibit the foundations of the walls of a building, or thearrangements of the several divisions or compartments.

This term, derived from the Greek, signifies a model,and the drawing of it: in architecture it usually indicatesthe ground plan, as of a fortress, garden, or building:when either of these are laid down properly to a scale,any portion, as well as the whole quantity may be esti-mated, and if the walls or points of supports are distin-guished by colour, their proportions with regard to void

Fig. 783

Fig. 786.