The elements of astronomy; a textbook . recise than those deter-mined by large fixed instruments. J2_But the portability of the instru-ment and its applicability at searender it absolutely was invented by Godfrey ofPhiladelphia in 1730. Fig. 157. — Principle of the Sextant. 555. The principle that theangle between the objects whose images coincide in the sex-tant is twice the angle between the mirrors (or between theirnormals) is easily demonstrated as follows (Fig. 157) : — The ray SM coming from an object, after reflection first atM (the index-mirror) and then at H (the horizon
The elements of astronomy; a textbook . recise than those deter-mined by large fixed instruments. J2_But the portability of the instru-ment and its applicability at searender it absolutely was invented by Godfrey ofPhiladelphia in 1730. Fig. 157. — Principle of the Sextant. 555. The principle that theangle between the objects whose images coincide in the sex-tant is twice the angle between the mirrors (or between theirnormals) is easily demonstrated as follows (Fig. 157) : — The ray SM coming from an object, after reflection first atM (the index-mirror) and then at H (the horizon-glass) ismade to coincide with the ray OH, coming from the From the law of reflection, we have the two angles SMPand PMH equal to each other, each being x. In the same way,the two angles marked y are equal. From the geometric prin-ciple that the angle SMH, exterior to the triangle HMEyis equal to the sum of the opposite interior angles at H andEy we get E = 2 x — 2 y. In the same way, Q — x — y; whenceE = 2Q = 426 APPENDIX. [§556 THE PYRHELIOMETER. 556. The pyrheliometer is an instrument devised by Pouilletfor measuring the amount of heat received from the sun, and hemade with it, in 1838, some of the earliest determinations ofthe solar constant. In Fig. 158 aa is a little snuff-box-likecapsule, made of thin silver, and con-taining 100 grams of water. The bulbof a delicate thermometer is insertedin the water, and the temperature isread at a point m, near the middleof the stem. The disc, ss, enables usto point the instrument exactly to-wards the sun by making the shadowof aa fall concentrically upon it. Theupper surface of the box is just onedecimeter in diameter, and is carefullycoated with lampblack. The instru-ment is used by pointing it towardsthe sun, and first holding an umbrellaover it until the temperature becomesstationary or nearly so, after whichthe umbrella is taken away, and thesun allowed to shine squarely uponthe blackened surface for f
Size: 1513px × 1651px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjec, booksubjectastronomy
