60° Fahr. = 5/9 (60 - 32) = 15·55° centigrade.
The calculation proposed by Lefroy is, however, simpler for Fahrenheit scale. To allow for diminution of boiling temperature, with height from 212°, with barometer at 30 inches, take 511 feet of altitude for the first degree and add 2 feet for each succeeding degree. Thus, taking height of first station = h corrected for 212° Fahr., 30 inches barometer, remembering decrease of barometrical pressure acts the same as increase of height. Then—
| 211° | boil point | h + 511 feet. |
| 210° | " | h + 511 + 513 = h + 1024 feet. |
| 209° | " | h + 511 + 513 + 515 = h + 1539 feet. |
CHAPTER XIX.
MISCELLANEOUS SURVEYORS' AND ENGINEERS' INSTRUMENTS, APPLIANCES, AND ACCESSORIES—CROSS STAFF—MECHANICS' LEVELS AND CLINOMETERS—BONING RODS—FOOTNER'S RAILWAY GAUGE—GIRTH STRAP FOR TIMBER MEASUREMENT—GIRTH TAPES—TIMBER MARKER—SLASHING KNIFE—BILL-HOOK—RECONNOITRING GLASS—TELESCOPE—SUN SPECTACLES—WHISTLES—PIONEER TOOLS—SKETCH BLOCK BOOK—CAMERA—GEOLOGICAL TOOLS—WEALEMEFNA—OPISOMETER—BOUCHER'S CALCULATOR—SLIDE RULES—FULLER'S CALCULATOR—ENGINEERS' POCKET BOOKS—CHRONOMETER—OUTFITS.
846.—Cross Staff.—Those of Tycho Brahé and of Gunter were very elaborate affairs, consisting of a pair of notched cross-bars sliding on a divided rod which gave directions to form any angle in a quadrant from the eye by sliding the bars further from or nearer to it. The surveying cross staff, after better instruments were invented to take angles, became a cross at right angles, sawn upon a disc of wood and supported upon a staff which was pressed into the ground. This was used by looking along the saw cuts to take offsets to the chain, and for setting out buildings. The fixed cross-head was much improved by making it a cross of metal with turned-up ends, down the centre of which vertical saw cuts were made at right angles, Fig. 400. This, in the author's opinion, is still the best form.
847.—Cylindrical heads superseded the open cross-head. The modern instrument in use is the French form, Fig. 401, which is made of octagon brass tube. This is cut with alternate sight slit and opposite window, with vertical hair on each of four rectangular sides of the octagon. On the other four sides there are plain slits subtending 45° to those first mentioned. The octagon tube is mounted upon a socket-piece which fits upon a conical pointed staff. The defect of this cross-head is the closeness of the slits, due to the small diameter of the tube, which renders the direction given for sighting uncertain.
