a retired part of the room, away from the fire, and not exposed to the open doors or passages; but for nice experiments, the observation should always be made in the open air and in the shade, taking especial care that the instrument be not influenced by the radiation of any heated bodies, nor any currents of air; the dew-point is then found by the Rule given on another page, and corresponds exactly with the Dew-Point Hygrometer, an instrument described in "Jameson's Journal," July, 1835, and modified by Dr. Mason. Should the wind be strong upon the instrument, the "Degrees of Dryness Observed," multiplied by 2, gives the "Absolute Dryness," (the "Excess of Dryness" being omitted in the calculation,) because a strong current of air makes the instrument indicate the Excess of Dryness, which is necessary to be added, in a calm atmosphere. If the absolute dryness of an apartment be required, the instrument must be placed in the shade and the dew-point found, which subtracted from the temperature of the apartment, will give its absolute dryness. The reason is obvious, and arises from this law, namely, that air has its dryness doubled for every increase of temperature corresponding to 21° of Fahrenheit's thermometer, and in proportion, for all intermediate temperatures. TABLE OF CORRECTIONS To be used when the Term of Deposition, or Dew-Point, differs from the Temperature of the Air in the Shade. N. B. The principles of these calculations will be found in Professor Daniel's Meteorological Essays, in Mr. Anderson's Essay on Hygrometry, in the Edinburgh Encyclopedia, Vol. xi., and in the Edinburgh Journal of Science, Vol. vii., p. 47, in an excellent article on the Dew-Point Hygrometer, by Mr. Foggo, from which the Table of Corrections has been partly subtracted. The Table of Quantity, by weight, has been taken from Professor Daniel's work on Meteorology, to which the reader is referred for further particulars. EYE GLASSES-are made in a variety of elegant forms, with frames of either black horn, blue steel, silver, or gold, connected with springs in such a SPECTACLES. Those who have oc casion to use Spec tacles, should by all means, attend to the selection of them in person. By trying them on and at the same time availing themselves of the suggestions of an optician, they will not fail to select those most suitable to their eyesight. Oculists recommend, that so soon as the slightest failing in the eyesight becomes apparent to a person, spectacles should be resorted to, as serious injury is often the result of delay, in consequence of the severe strain upon the optical nerve. The best form for the lenses, is the double-convex or double-concave. Wm. Schmolz has an unlimited assortment of well-ground and highly polished glasses, and an equally large number of frames to put them in. Also, Pebbles, Miniscus, etc. DOUBLE-EYE SPECTACLES-are necessary to persons suffering with weak eyes, and are also a great relief to the eyes, when riding in the wind and dust. The glasses are large, shaded either blue, gray, or green, and mounted in fine steel, by which they are firmly clamped to the head. GOGGLES-with white glas ses and protecting gauze Those with colored glasses, are a complete protection to the eye against dust, sunlight, and cold winds. GOLD ASSAYING SCALES-in the most approved style and so delicately balanced as to be affected by the thousandth part of a grain. GOLD ASSAYING WEIGHTS-divided into tenths, hundredths, and thousandths, corresponding with the assay weights of the U. S. Branch Mint. GOLD DUST COUNTER SCALES-assorted sizes, with weights from 10 to 200 Dyspepsia, Bronchitis, Loss of Voice, Scrofula, Curvature. of the Spine, Toothache, Deafness, etc. GALVANIC BATTERIES-are constructed of various forms but consist, essentially, of two different metals, which are placed in some dilute acid which acts on but one of the metals. The galvanic current is conducted by wires fastened to the metals. in solving, mechanically, many problems in astronomy relative to the hour of day at different places; the times of the rising and setting of the sun; the 28 THE SURVEYOR'S AND ENGINEER'S COMPANION. Rules for Solving all Cases of Plane Trigonometry. CASE 1. Given all the Angles and One Side, to find the other Side. RULE. As sine of the angle opposite the given side, is to sine of the angle opposite the required side, so is the given side to the required side. CASE 2. Given two Sides and an Angle opposite one of them, to find the other Angles and Side. RULE. As the side opposite the given angle, is to the other given side, so is sine of the angle opposite the former, to sine of the angle opposite the latter. CASE 3. Given Two Sides and the included Angle, to find the other Angles and Side. RULE.-Subtract the given angle from 180° and the remainder will be the sum of the two unknown angles; then say, as the sum of the two given sides is to their difference, so is tangent of half sum of unknown angles, to tangent of half their difference. Add this half difference of the unknown angles to, their half sum for the angle opposite the greater side, and subtract it from the half sum for the angle opposite the less side. CASE 4. Given the Three Sides to find the Angles. RULE.-Upon the longest side let fall a perpendicular from the opposite angle. This perpendicular will divide the base into two segments and the triangle into two right-angled triangles; then say, as the given base is to the sum of the two other sides, so is the difference of those sides, to the difference of the segments of the base. To half the base add half the difference of the segments for the greater segment, and subtract it from half the base for the' less side; then proceed as in Case 2. RULE 2.-Add together the arith. comp. of the logarithms of the two sides, containing the required angle the log. of the half sum of the three sides and the log. of the difference of the half sum and the side opposite the required angle. The half the sum of these four logarithms will be the logarithmic cosine of half the required angle. |