DECLINATION. 50 DECOMPOSITION. B ■S„ S c3 ei o E5 .9 'I II STATE OE Station 11 K§ '■§ >i TEBRITOKY ? ^ ■£3 Z 53 £_:j s2 <€ z,"^ 'H~ >5 < < "-3 Wisconsin Milwaukee 41 04 87 53 — 3 18 +5.0 Superior City 4B 40 92 04 — 8 03 +2.8 Wyoming Cheyenne 41 08 104 49 —14 24 +3.0 Cuba Havana 23 08 82 22 — 3 05 +2.5 Santiiigo 20 00 75 50 — 1 50 +2.0 Porto Rico San Juan 18 28 66 OS + 1 04 +2.5 Ponce 18 01 66 36 + 52 +1.0 Hawaiiaa Islands Hc)nolulu 21 18 157 52 —10 26 —1.5 Waimea 20 02 155 38 — 9 20 — S.O Philippines Manila 14 35 .20 58 (East) — 53 — See Compass; Dipping Needle; Magnetism, Tekkestrial. Under the latter will be found charts showing the isoclinic and isogonic lines. Consult the Reports, Bulk-tins, and Charts pub- lished from time to time by the Division of Ter- restrial ilagnetism of the United States Coast and tteodetic Survey. DECLINATION NEEDLE. An instrument for determining magnetic declination. In this instrument, whicii is also known as a declino- meter, there are two things essential — the means of ascertaining the astronomical meridian, and a needle for showing the magnetic meridian. The common form of declinometer consists of a tri- pod, provided with leveling screws, and support- ing a pillar, on which is fixed in a horizontal phine a graduated circle. A compass-box with vernier attached is placed on this divided circle and is free to move aboit a pivot at the top of the pillar. Tn'o uprights arc fixed to the side of the compass-box. on which rests the axis of a telescope. A graduated arc is fixed to the bottom of the uprights, and the angle of eleva- tion of the telescope is read with a vernier on the arm attached to the axis of the telescope. A IcA-el is also hung on the axis of the telescope, for adjusting the instrument. Inside the com- pass-box is another graduated circle, the line joining the zero-points of which is parallel to the axis of the telescope. All the fittings are of brass or copper, iron, of course, being unsuitable. The compass-box and telescope move round as one piece on an axis passing through the centre of the divided circle. When an observation is made, the telescope is pointed at a star whose position with regard to the astronomical merid- ian is known at the time of observation. The telescope w-ith the compass-box is then revolved the proper number of degrees until its axis is in the meridian of the place. If. when the tele- scope is in this position, the north end of the needle stands at the zero-point of the inner circle. the declination would be O = zero; but if it lie east or west, the declination is shown by the degree at whic'h the needle stands. It is difficult to construct a needle so that the line joining its poles exactly coincides with the line joining its visible extremities. If this coincidence be not perfect, the geometrical axis of the needle, according to which the reading is made, lies to the right or left of the magnetic axis, and consequently of the true reading. To remedy this, the needle is so made that it can rest either on its lower or upper surface. In finding the true reading, the position of the needle is marked, and then it is turned upside down, and again marked, the mean of the two readings giving the true one. The most accurate method of determining declination is with the magnol- ometer (<l.v. ). and theodolite. This instrument is employed by the Division of Terrestrial ilag- netism of the Lnited States Coast and lieodetic Survey, and the apparatus and results are de- scribed in the annual reports. The declination of the needle may be also a.scertaiiied by the "dipping needle' (q.v.). The ordinary surveyor's compass, which must be used by making allow- ance for declination, is a declination compass. See Compass; Engineering Instrlments; .MA(;NETis>t, Tekkestrial: and !M.i:.net(imi;tkr. DECLINE AND FALL OF THE ROMAN EMPIRE. A celebrated work by Edward Gibbon (177G-S8), the first real attempt at scientitio history iu English. It contains an account of the growth of Christianity, which subjected the author to fierce attacks by the leading Church- men of his day. DECOCTION (Lat. decoctio, a boiling down, decoction, from de, down + coquerc, to cook, boil). The term applied in pharmacy to a solu- tion procured by boiling an organic drug in water. One hundred cubic centimeters of a de- coction usually contain five grams of drug. DECOMPOSITION (Fr. decomposition, from Lat. de, down, away + coiiipoitcrc. to put together, join, unite). A term employed to sig- nify the breaking up of compounds into simpler coiiipounds or altogether into their chemical ele- ments. The number of substances that are being decomposed is as a rule smaller, of course, than the number of substances produced by the de- composition. However, in the class of chemical transformations known as 'double decomposi- tions.' the number remains unchanged. Thus. AB and CD, two compounds, inade up each of two ele- ments, may. on being brought into contact, under- go decomposition, the result of which would be again only two compounds. .C and BD. Exam- ples of this class of reactions are very numerous in chemistry. But the term decomposition, im- qualified, is generally applied, as stated above, to transformations involving an increase in the number of substances. Such decompositions are usually caused by the influence of some physical factor, such as heat, electricity, etc. fleat is one of the most common of decomposing agents : mer- curic oxide (TTgO), when heated, breaks up into the invisible gas oxygen (O.) and the liquid metal mercury (Hg) ; limestone (CaCO.) decom- poses into quicklime (oxide of calcium. CaO), and carbon dioxide (CO™) ; coal and wood decom- pose into a great variety of useful materials, in- ( lulling charcoal, illuminating gas, and coal tar. F:i(rtricit!i is a potent decomposing agent: under its influence, water (H.O) is broken up into oxy- gen (O;) and hydrogen (H,). and metallic salts are broken up into their constituent metals and acids, electrolytic processes being consequently much used both for scientific and industrial pur- poses. Lifiht effects many decompositions, as of the silver salts used in photography, of nitric acid, of hydrogen peroxide, and a number of other substances. Percussion explodes nitroglycerin, and even a touch causes iodide of nitrogen to de- compose violently. Fcrmenfs (minute vegetable and animal organisms) cause many decomposi- tions, such as the breaking up of dextrose (CjHijOj), in the presence of yeast, into alcohol
