File:Astronomy for high schools and colleges (1881) (14577170549).jpg

Identifier: astronomyforhigh00newc (find matches)
Title: Astronomy for high schools and colleges
Year: 1881 (1880s)
Authors: Newcomb, Simon, 1835-1909 Holden, Edward Singleton, 1846-1914, joint author
Subjects: Astronomy
Publisher: New York, H. Holt and Company
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

View Book Page: Book Viewer
About This Book: Catalog Entry
View All Images: All Images From Book
Click here to view book online to see this illustration in context in a browseable online version of this book.

Text Appearing Before Image:
rod. Theright-hand one is visible, while the other is hidden be-hind the left-hand weight W. In this position it will beseen that the attraction of the weights W tends to turnthe balance in a direction opposite that of the hands of awatch. The fact is, the balance begins to turn in this di-rection, and being carried by its own momentum beyondthe point of equilibrium, comes to rest by a twist of thethread. It is then carried part of the way back to itsoriginal position, and thus makes several vibrations which DENSITY OF TUK EARTH. 193 require several minutes. At lengtli it comes to rest in aposition somewliat different from its original one. Thisposition and the times of vibration are all carefully noted.Then the table T is turned nearly end for end, so that oneweight W shall be between the observer and the right-hand ball, while the other weight is beyond the left-handball, and the observation is repeated. A series of observa-tions made in this way include attractions in alternate di-
Text Appearing After Image:
FiG. 65. rections, giving a result from which accidental errors willbe very nearly eliminated. A third method of determining the density of the earthis founded on observations of the change in the intensityof gravity as we descend below the surface into deepmines. The principles on which this method rests will beexplained presently. The most careful application of itwas made by Professor Airy in the Hai-ton Colliery, Eng- 194 ASTRONOMY. land. The results of this and the other methods are asfollows : Cavendish and Hutton, from the attraction of balls, 6 • 32Eeich, 5.58 Baily, 5-66 Maskelyne, from the attraction of Schehallien 4 • Yl Airy, from gravity in the Harton Colliery 6-56 Of these different results, that of Baily is probably thebest, and the most probable mean density of the earth isabout 5f times that of water. This is more than doublethe mean specilic gravity of the materials which composethe surface of the earth ; it follows, therefore, that the in-ner portions of the ear

Note About Images