【正文】 ice, or their mass may remain in equilibrium. Scientists divide each glacier into upper and lower sections termed the accumulation area, where snowfall exceeds melting during a year。 根據(jù)歷史： 古典行星：水星，金星，火星，木星和土星。 在火星和木星之間的小行星帶組成了區(qū)別內(nèi)層行星和外層行星的標(biāo)志。 根據(jù)大小 : 小行星：水星，金星，地球，火星和冥王星。 更有趣的是，有些恒星不僅是朝著地球發(fā)出的光能直接到達(dá)地球，它朝其它方向發(fā)射的光也可能被附近的黑洞的強(qiáng)引力折射而能到達(dá)地球。這是一個(gè)最基本的常識(shí)。而根據(jù)科學(xué)家的計(jì)算，中子星的總質(zhì)量不能大于三倍太陽(yáng)的質(zhì)量。說(shuō)它“黑”，是指它就像宇宙中的無(wú)底洞，任何物質(zhì)一旦掉進(jìn)去，“似乎”就再不能逃出。增長(zhǎng)速度是非?？斓摹＿@是因?yàn)槿梭w內(nèi)的汞含量還沒(méi)有達(dá)到臨界值，一旦到達(dá)臨界值，就很難進(jìn)行治療了。這是因?yàn)楣卸臼且环N神經(jīng)中毒，可以造成全身性的神經(jīng)損害。大量吸入汞蒸汽會(huì)出現(xiàn)急性汞中毒，其癥候?yàn)楦窝?、腎炎、蛋白尿和尿毒癥等。甲基汞可溶于水，因此又從底泥回到水中。 人類活動(dòng)造成水體汞污染，主要來(lái)自氯堿、塑料、電池、電子等工業(yè)排放的廢水。 托福聽(tīng)力背景知識(shí) (二 ) 考古 中的 relative dating 和 absolute dating： Dating Methods in Archaeology If there is one fundamental issue on which most archaeologists can generally agree, it is the importance of chronology in studying the past. Although one of the most important contributions that archaeology can make is the study of cultures over long time spans, control of the time dimension is crucial in almost all kinds of archaeological research. In studying the archaeological record, the archaeologist needs to differentiate those materials that are contemporary and those that reflect the passage of time. Given the importance of establishing the temporal relationships of archaeological remains, it is not surprising that until the introduction of dating techniques from the physical sciences (., chemistry and nuclear physics), issues of chronology dominated archaeology. Archaeologists can now access a wide variety of techniques to estimate the age of archaeological remains, and can now turn their attention to issues other than chronology. The dating methods used by archaeologists vary considerably in precision and the nature of the material actually dated. Age determinations for archaeological materials may be direct or indirect. Direct dates are derived from the actual artifact, feature or ecofact to determine the age of the material. Indirect dates are based on material associated with the archaeological item of interest. For example, ash in an ancient hearth can be dated by the radiocarbon dating method (discussed below) and other material, such as pottery, stone tools, and fragments of animal bone associated with the hearth, can then be indirectly assigned the same age. This assignment, however, assumes that there is sufficient evidence indicating that both the hearth and the artifacts reflect the same contemporaneous event, thus allowing the extension of the direct date to the other related material. Dates may be relative or absolute in their scale of measurement. Relative dating simply orders remains into a ranked chronological sequence, where the age of each is relative (., earlier than later than, or contemporary) to the others. The preliminary steps in most archaeological research usually employ relative dating methods to order artifacts, features and sites into temporal sequences. Absolute dating provides a specific calendar age estimate (., 1000 years ago), thus allowing determination of the actual amount of time difference between absolute dates. Although absolute dates provide a calendar age, they vary in their precision, as many have an uncertainty factor attached that indicates a range of time rather than a specific date. Although dating methods are used to determine the age of objects and events, these dates are ordered to establish a chronology that provides a temporal framework in which the archaeological material can be placed. This chronologybuilding process is the foundation for archaeological studies. In this chapter, we examine some of the major techniques that have been developed to date the archaeological past and material from the archaeological record. 托福聽(tīng)力背景知識(shí) (三 ) 汞污染 汞是在常溫下唯一呈液態(tài)的金屬元素。 然而， 3000 多年之后，一個(gè)叫海因里希?施里曼的德國(guó)商人，就是憑著《伊利亞特》和《奧德賽》的指點(diǎn)，在小亞細(xì)亞西北端等地獲得了驚人的發(fā)現(xiàn) —— 挖掘出湮沒(méi) 3000 多年的特洛伊古城等大量歷史遺跡，于是得出震驚世界的結(jié)論：“荷馬史詩(shī)”就是一部希臘史前文明史，把古希臘文明史向前推進(jìn)了 810 個(gè)世紀(jì)。一身流浪漢打扮的奧德修斯來(lái)到宮殿，只有他的愛(ài)犬阿果斯認(rèn)出昔日的主人，但高聲大叫后卻斷了氣。憤怒的 太陽(yáng)神請(qǐng)求天神宙斯懲罰。歐呂卡洛斯飛身返回船上向奧德修斯報(bào)告。當(dāng)他們舉目可見(jiàn)家鄉(xiāng)的海岸時(shí)，一直認(rèn)為風(fēng)袋里裝有財(cái)寶的幾個(gè)同伴，趁奧德修斯睡覺(jué)時(shí)，打開了風(fēng)袋，結(jié)果吹起逆風(fēng)，又把他們吹回風(fēng)神島。天神掀起了一陣大風(fēng)暴，大多數(shù)戰(zhàn)船沉沒(méi)了，剩下的少數(shù)人，由奧德修斯帶領(lǐng)，在大海上漂泊。希臘軍隊(duì)佯敗，棄一匹巨大的木馬而逃。《伊利亞特》寫到此就結(jié)束了。當(dāng)時(shí)，希臘聯(lián)軍的統(tǒng)帥阿加米農(nóng)奪走了主將阿基里斯心愛(ài)的女俘虜布里塞斯，屢建奇功 的阿基里斯惱怒萬(wàn)分而拒絕出戰(zhàn)，使希臘聯(lián)軍陷入困境，阿加米農(nóng)、奧德修斯等將領(lǐng)都受了傷。海倫見(jiàn)了這位英俊的王子，也感到滿心歡喜。結(jié)果，帕里斯把金蘋果判給了阿芙羅狄蒂。他生于愛(ài)奧尼亞（今土耳其西海岸），生存年代約是公元前 8 世紀(jì)的后半葉。他邀請(qǐng)了奧林匹斯山上的眾神出席，卻把專管爭(zhēng) 吵的女神厄里斯給忘記了。一天，斯巴達(dá)王宮來(lái)了兩位尊貴的客人，他就是特洛伊國(guó)王普里阿摩斯和他的二兒子帕里斯。于是，以阿加米農(nóng)為統(tǒng)帥，組織了 10 萬(wàn)希臘聯(lián)系，乘坐 1013 艘戰(zhàn)船，越過(guò)愛(ài)琴海攻打特洛伊城。特洛伊人以為是希臘英雄阿基里斯 本人出戰(zhàn)了，紛紛不戰(zhàn)而退。傳說(shuō)在阿基里斯出生后，被母親提著腳跟浸在冥河水中，所以他全身刀槍不入，只有腳跟因被母親握著沒(méi)有被冥河水浸過(guò)，所以留下一個(gè)致命弱點(diǎn)。老國(guó)王和大多數(shù)男人被殺死，婦女和兒童被出賣為奴，海倫又被帶回希臘，持續(xù) 10 年之久的戰(zhàn)爭(zhēng)終于結(jié)束。奧德修斯便帶領(lǐng)其余的人回船后繼續(xù)向北航行，不久 來(lái)到了獨(dú)眼巨人族的領(lǐng)地。奧德修斯率其余同伴登上唯一未遭破壞的船逃回海上，漂到女巫塞西居住的艾艾伊島。之后，奧德修斯在島上停留了一年，還與塞西生下一子特勒格諾斯。奧德修斯乘坐木筏，終于回到已闊別 20 年的家鄉(xiāng)伊薩卡島。世人了解歐洲文明的起源，總是從了解古希臘文明開始的?！秺W德賽》主要舞臺(tái)的地中海中部和東部海域的海底今天仍然長(zhǎng)眠著許多古代的商船和古物。隨著自然的演化，環(huán)境的各個(gè)因素中都可能含有汞，形成汞的天然本底。大氣中氣態(tài)和顆粒態(tài)的汞隨風(fēng)飄散，一部分通過(guò)濕沉降或干沉降落到地面或水體中。 汞雖然是一種累積性毒物，但人體對(duì)汞具有一定的排泄能力。 由于汞的毒性強(qiáng)，產(chǎn)生中毒的劑量就小，因此我國(guó)飲水、農(nóng)田灌溉，都要求汞的含量不得超過(guò) 毫克／升，漁業(yè)用水要求汞不得超過(guò) 毫克／升。原本正 常 的貓，走路變得搖搖晃晃，就像跳舞一樣。經(jīng)過(guò)化驗(yàn)，發(fā)現(xiàn)這些人體內(nèi)的汞比一般人都高，病因就是 這些人愛(ài)吃海魚。 根據(jù)廣義相對(duì)論，引力場(chǎng)將使時(shí)空彎曲。 我們?cè)?jīng)比較詳細(xì)地介紹了白矮星和中子星形成的過(guò)程。而當(dāng)它的半徑一旦收縮到一定程度 (史瓦西半徑 )，正象我們上面介紹的那樣，巨大的引力就使得即使光也無(wú)法向外射出，從而切斷了恒星與外界的一切聯(lián)系 —— “黑洞”誕生了。形象地講，好象光本來(lái)是要走直線的，只不過(guò)強(qiáng)大的引力把它拉得偏離了原來(lái)的方向。不過(guò)，這些當(dāng)代天體物理學(xué)的最新成果不是在這里三言兩語(yǔ)能說(shuō)清楚的。 巨行星的直徑大于 48000 公里。 地內(nèi)行 星看起來(lái)的如同地球上看有時(shí)不完整的月亮。 行星系統(tǒng)如何與其它星系共處？已有了木星般大小的在附近軌道運(yùn)動(dòng) 的對(duì)象的恒星的極好的證據(jù)。s mass balance. To find mass balance, scientists measure the area of each region and observe amounts of accumulation and ablation relative to preset stakes. After density measurements are made they may calculate how much water has been added or lost to the glacier. After a series of positive mass balance years, the glacier may respond to the increased thi