Earth's structure

Earth consists of three major layers or zones - a dense iron-nickel core, a surface crust and a flexible mantle in between. The thickness of earth's crust varies. Earth's crust is thickest where there are continents - the major land areas of earth - and averages about 40 kilometers in thickness. Under the oceans where it does not support huge land-masses the crust averages only about eight kilometers in thickness.

Earth's second layer called the mantle is made of mostly extremely hot material that can flow under pressure. Digging through it would be quite a task. The mantle is 2.900 kilometers thick. Temperatures vary from 870°C near the top of the mantle to 3000°C at the bottom. The closer to earth's centre, the higher the temperatures and pressure. The layer beneath the mantle is even hotter. In the outer liquid iron core which is about 2.250 kilometers thick temeperatures can reach 6.100°C.

Inside this intensely hot outer core is the inner solid iron and nickel core - the hottest part. Scientists estimate that temperatures in the inner solid core may be as high as 7.000°C.

A s s i g n m e n t s

I. Comprehension questions:

1.Of what layers does earth consist? 2. Does the thickness of earth's crust vary? 3.Where is earth's crust thickest? 4. How thick is earth's crust under oceans? 5. What is the thickness of the mantle? 6. How does the temperature of the mantle vary? 7. What is the temperature of the inner solid core? 8. What does the inner core consist of?

2. Speak about:

a) earth's crust,

b) the mantle,

c) earth's core.

Lesson 10.

Text A. Reading of horizontal angles

I. Read and memorize the following words:

agree - узгоджуватись

back sight - візування назад

balance (v) - зрівноважувати

care (n) – увага

check (n) - перевірка

clockwise - за стрілкою годинника

close (v) - замикати

closely - щільно

combine (v) - сполучати/сь/

comparison - порівняння

course - курс

desire (v) - бажати

detect - розпізнавати, знаходити

divide (v) - розділяти

eliminate — усувати

error - помилка

exactly - точно

except - за виключенням

foresight - візування вперед

lose - губити, втрачати

need (v) - потребувати

once - один раз

plate – пластина, набір

plunge -пірнати, опускати/сь/

preceding - попередній

preferably – переважно

rapid - швидкий

repetition - повторення

reset - знову встановлювати

retain - утримувати

reverse (v) – перевертати

set (v,n} - ставити,розміщати,

sight (n) - візування

slip (v) - ковзати

traverse (n) - хід, траверс

turn (v) - повертати

twice - двічі

unless - якщо ... не

zero - нуль

II. Expressions for the text comprehension:

back target - задній повзунок

clockwise direction - в напрямку_за стрілкою годинника

direct reading - безпосереднє читання

forward course - передній курс

forward target - передній повзунок

fractions of minutes - долі минут

instrument work - інструментальна робота

instrumental errors - інструментальні похибки

last course - останній курс

last measurement - останній вимір

lower clamp- нижній зажим, скоба

normal position - стандартне положення

numerous purposes - числєнні цілі

previous station - попередня станція

repetition method - метод повтору

reversed position - пeрeвeрнe положення

single reading- одиночне показання, читання, відлік

stadia measurements •- виміри далекомірною лінійкою

traverse work - траверсна, полігонометрична робота

triangulation work - тріангуляційна робота

upper clamp - верхній зажим

Reading оf horizontal angles

The usual methods of reading the angles are:

1. By direct reading.

2. By azimuth.

3. By repetition.

I. By direct reading. In this method the telescope is set on the back target with the plates at zero using the lower clamp, then without plunging the telescope is turned to the forward target using the upper clamp and the angle is read. The instrument should be graduated from 0° to 360°, preferably to the right, so that angles are measured at the clockwise direction.

The direct reading is the most natural way to measure the angle. It is used for numerous purposes but it gives only a single reading without any check except by repeating it.

2. By azimuth. In this method the telescope is reversed and set on the back target with the plates set on the azimuth of the last course as read from the previous station using the lower clamp. Then the telescope is plunged and is turned to the forward target using the upper clamp and the azimuth of the forward course is read from the plates. The instrument should be graduated from 0° to 360° to the right or clockwise. After taking the foresight the plates are left clamped and are used in this position without resetting unless they have slipped in taking the backsight at the next station.

This method is a rapid one for traverse work, especially when combined with stadia measurements for the distances. The instrument retains the fractions of the minutes which are too small to be read. This gives a traverse whose angles close exactly. With the same care in doing the instrument work and in the measurement of the distances, the results by this method are more accurate than by any of the preceding ones but not so accurate as by the repetion method. There is no check on the angle readings as they are taken only once, and if repeated the plates have to be moved and the accumulation of the fractions of the minutes is lost.

3. By repetition. In this method the telescope in the normal position is set on the back target with plates set at zero using the lower clamp. Then the telescope without plunging is turned to the forward target using the upper clamp and the angle is read from the plates and recorded. This is the first reading and the method so far is the same as that by direct reading. Then without resetting the plates or plunging the telescope a second reading is taken consisting of a sight on the forward target using the upper clamp. The plates will then show twice the original angle but this need not be read. The angle is repeated, as on the second reading, as many times as desired. The vernier is read at the end of the last measurement and this angle divided by the number of repetitions gives the angle between courses. The last angle should agree closely with the first one of the set, and the comparison of the two will detect errors in the work. By taking the first half of each set with the telescope in one position and the second half with it in the reversed position many instrumental errors are balanced and so eliminated. The instrument should be graduated from 0° to 360° to the right.

The method is always used for tnangulation work and should be used on other important work. It is recommended for traverse work that a set of four repetitions be used, two readings being taken with the telescope normal and two being taken with it reversed.

E x e r c i s e s

I. Give Ukrainian equivalents of the following internationalisms;

plate, zero, natural, course, station, traverse, combine (v), fraction, accumulation, normal, original, vernier, repetition, balance (v), tnangulation, recommend