Physics of the Earth - II The Figure of the Earth Bulletin of the National Research Council (1931) / Chapter Skim
Currently Skimming:
Chapter I. Introduction
Chapter I. Introduction
Pages 3-18
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 3... ...
Fron1 ancie~.~.t times the celestial bodies were suspected of having, art influence of some sort on our planet in causing the tides of the oceans, but only in very recent times has it been known that these forces were of sufficient magnitude to cause a tide : also in the supposedly rigid body of the earth. Ancient philosophers ascribed ocean tides to all courts of fantastic causes; as, for example, that the earth was an animal and the tides were caused by its breathing or by the beating of its pulse.
|
From page 4... ...
Upon discovery of the law of gravitation by Newton toward the end of the seventeenth century a rational explanation was furnished of the forces exerted by the sun and the moon in bringing about the tides. Newton showed that the tide was a natural consequence of the law of gravitation and having proved it, he left the development of the different theories to scientists following him.
|
From page 5... ...
This paragraph on earth tides is by W
|
From page 6... ...
directl~ ~s its m~ss ~d inve~sel~ ss tbe c~e of its Jist~uce Ir~m thc e~rtb. When tbe moon is IU11 01 Uew; tbe SUB ~]
|
From page 7... ...
Ibe p~tb of tbe moon in its o~it is RD eETpse; its dist~nce f~m ~e e~tb V~) iDg during tbe course oT tbe lun~r month~ ~lso C8USiDg V~TiRtions in tbe r~nge of the tide.
|
From page 8... ...
Even these simplest types vary during a lunar month, with the char~gin, declination of the moon, in the heights of consecutive high and low waters, as illustrated in Figure 2, reproducing tidal records at Portland and Manila. The upper graph for Portland is 1:nowr1 as an equatorial-form, whacks occurs when the moon is near the equator in its changing declination from north to south or vice versa.
|
From page 9... ...
Ill addition to these variations, two distinct -forms of the mixed type occur in some localities, as illustrated, for example, by the tides at Seattle, Washington, and at I-Ionolulu, I-Iawaii (Figure 3~. At Seattle, little diurnal variation occurs in the high waters but considerable variation ocurs in the low waters; while at FIonolulu the tide shows little
|
From page 10... ...
I'his cliff'erence in form is due, briefly, to different combinations of the diurnal and semi-cliurnal waves they ~~~~\ \ \ \ // 'aft \ // / ,-~~-~ Use , ~ c~ / ~ ~\ /~//__` r \\ \ lFesu/{c~r~ f ~ \ <~' ~ it ~;/~9 FIG. 4.- Idealized tide curves illustrating various combinations of diurnal and semidiurnal waves which give rise to many different forms of mixed types of tide.
|
From page 11... ...
In the lower graph the diurnal and semi-diurnal waves have phases and amplitudes the resultant of which is the " vanishing, tide "; in this type the water of the falling tide remains for several hours at a nearly fixed height at about mean sea level, then falls to the one clearly defined low water of the day. The tide in a river illustrates another type of tide.
|
From page 12... ...
id Canacla.t :~: Earth tides arc subject to the same inequalities as are oceanic tides :in regard to springs alla neaps, apogean and perigean tides. Since, as has been already noted., the pure earth tides conform to the equilibrium law, the inequalities in height are in the same ratio as the inequalities in the tide-producing forces and the change in phase of the earth tide l~eeps pace with the change in phase of the resulting force.
|
From page 13... ...
The connection of the zero of the staff with permanent bench marks on shore by means of spirit leveling fixes the elevations of these bench marks in relation to the heights of certain tidal datum planes as determined by the readings of the tide on the staff, such as mean low water, mean sea-level, etc. ~ tape gauge is a modification of a title staff.
|
From page 14... ...
The paper may be either plain or cross-section. If cross-section paper is used, the gauge pencil or pen must at all times be set at a reading on the paper corresponding to a plain tide staff reading, in order to connect the marigram curve with the tide, and the time ordinate set with the correct time; if plain paper is used, daily comparative readings must be made on a plain tide stain' to connect the datum line of the record with the tide; and in addition to the motor clock an auxiliary time clock is used with an attachment.
|
From page 15... ...
The water stage register, however, has a larger height scale for accommodating extreme fluctuations brought about by freshets and droughts. Another type of tide gauge consists of a sensitive bulb connected with a gauge, somewhat similar to a steam gauge, by means of an air-ti~ht ]
|
From page 16... ...
Horizontal pendulums are now usually established in deep cellars or mines in order to shield them as far as possible from the effects of diurnal and seasonal changes in temperature. The water level is simply a long horizontal pipe buried in the ground and filled wither water.
|
From page 17... ...
When the determination of sea level has been e2~tended over centuries, quantitative evidence will be available as to the stability of coast lines of continents by reference to permanent bench marl<-s connected with tide staffs on which this mean sea level datum has been determined. For the preservation of these tidal datum planes established by lone, series of tide observations, these bench marks should be of a permanent character.
|
From page 18... ...
143-146. (For bibliography of instruments used in observing earth tides, see Chapter V.)
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.