310 



THE CENTURY BOOK OF FACTS. 



in their wanderings, comets are most irregular 

 and eccentric. When first seen, the comet 

 resembles a faint spot of light upon the back- 

 ground of the sky. As it comes nearer, the 

 brightness increases and the tail begins to show. 



The term comet signifies a hairy body. A 

 comet consists usually of three parts : the nu- 

 cleus, a brigiit point in the center of the head ; 

 the coma (hair), the cloud-like 'mass surround- 

 ing the nucleus ; and the tail, aluminous train 

 extending generally in a direction from the sun. 



It is not understood whether comets shine by 

 their own or by reflected light. If their nuclei 

 consist of white-hot matter, a passage through 

 such a furnace would be anything but desirable. 



The discovery of the elliptical orbit of comets 

 is due to Halley. He discovered a comet in 

 1682 which he demonstrated to be a return of 

 the comet described by Kepler in 1607 ; that 

 it had appeared in 1531, and that it was the 

 comet that had appeared still earlier by the 

 same period of seventy-five years, in 1457, and 

 that had caused such consternation among the 

 Christians, who regarded it as a sign, Con- 

 stantinople having just fallen and all Europe 

 being threatened by the Turks. Halley also 

 predicted the return of the comet in 1757. It 

 reached its perihelion in 1759. Its last appear- 

 ance wasin!835. It will be lookedfor in 1911. 



Encke's, Biela's, and the comets of 1843 

 and 1858 are comparatively recent. Others 

 came in 1861, 1874, 1883. In 1881, two com- 

 ets appeared. Some comets of antiquity were 

 very remarkable, and are reputed to have 

 equaled the sun in magnitude. One tail is 

 usually supposed to be the distinguishing mark 

 of a comet, but in 1774 one appeared with six 

 tails, arranged something like a fan. Some- 

 times the tail is separated from the head. 

 Some comets appear at regular intervals, and 

 their approach can be determined with accuracy. 

 Of course we only see those which are attracted 

 by the sun, or those which revolve in the solar 

 system. There must be thousands of other 

 comets which we never see at all. 



Compass, The. The directive power of 

 the magnet seems to have been unknown in 

 Europe until late in the twelfth century. It 

 appears, however, on very good authority, that 

 it was known in China and throughout the 

 east generally at a very remote date. The 

 Chinese annals assign its discovery to the year 

 2634 B. C., when, they say, an instrument for 

 indicating the south was constructed by the Em- 

 peror Hon-ang-ti. At first, they would appear 

 to have used it exclusively for guidance in 

 traveling by land. The earliest date at which 

 we hear of their using it at sea is somewhere 

 about A. D. 300. According to one account, 

 a knowledge of the compass was brought to 



Europe by Marco Polo on his return from 

 Cathay. It was long contended that the com- 

 pass as a nautical instrument was first invented 

 by Flavio Gioja, a native of Amalfi, about the 

 year 1362, and that the-section of the Kingdom 

 of Naples where he was born has a compass for 

 its arms. For this there is no authority what- 

 ever, as the compass was well known as a nau- 

 i tical instrument before his time. The phe- 

 nomena of the magnetic needle which perplex 

 scientists most are that in every place it is sub- 

 ject to variations. By observation at Paris it 

 was found that in 1681 tfye needle varied 2 

 degrees 30 minutes to the west ; in 1865, 18 

 degrees 44 minutes to the west. At London, 

 between 1580 and 1692, the needle varied from 

 10 degrees 15 minutes east to 6 degrees west. 

 In Dakota the average variation is 12 degrees 

 30 minutes east, in Minnesota 11 degrees east, 

 while in Montana it is 20 degrees east. In a 

 work on Government Surveys it is stated that 

 " the needle does not point due north except 

 in a few localities, and at noplace does it con- 

 tinue to point with a given angular distance 

 from the north for any stated length of time. 

 | It changes secularly, annually, diurnally and 

 hourly, and is, further, subject to fluctuations 

 reducible to no method of tabulation. " In the 

 vicinity of iron or magnetic sands, the needle 

 is deflected toward the material attracting it. 



Compressed-Air Engines. The ar- 

 rangements of atmospheric engines is largely 

 identical with that of non-condensing steam- 

 engines, and they are used very generally both 

 in the United States and Europe in the con- 

 struction of tunnels, their great advantage be- 

 ing that in place of escaping heat and steam, 

 which would seriously vitiate the close air in 

 the shaft, the working of the engine gives out 

 pure cold air, serving also the purpose of ven- 

 tilation. An engine worked by compressed 

 air, however, can never be a prime motor in 

 itself, since the air which propels it must be 

 compressed by another power either steam, 

 electricity, falling water, or animal force. 

 There are several ways of applying this com- 

 pressed air. One is to fill with it a large, 

 strong cylinder or reservoir, and use it to work 

 a piston in the same way that steam is used. 

 Another is to conduct the air from the prime 

 motor in tubes to several smaller engines. In 

 the construction of the Mont Cenis Tunnel the 

 hydraulic power of a cataract near the entrance 

 of a tunnel was used as a prime motor to com- 

 press air in reservoirs, whence it was con- 

 ducted by flexible tubes to work the rock-bor- 

 ing machines. When this boring is done by 

 percussion of steel drills, the atmospheric 

 pressure moves a piston connected with them. 

 When the boring is performed by rotation, as 



