时间：2022-08-10 08:32:37
How Far Can You See?
If you stood on the shore looking across the sea to the horizon(地平线 the line appearing to separate earth from sky),you might be able to see about two and a half miles. But the higher you stood the farther you would be able to see. As the earth is curved(曲线状), the horizon would appear farther away with every increase in height above see level.
As a height of 20 feet you might see for six miles. From the top of a 300-foot cliff(悬崖)your view could extend for 23 miles,while on the summit(绝顶)of a 3500-foot mountain,it could lengthen(延长)to 80 miles. From an aircraft flying at 16000 feet you might have an uninterrupted(边续不断的)panorama(全景)for 165 miles.
If you look straight up into the sky, the distance you can see is immense(无际的). The moon is about 239000 miles away and the stars are millions of miles distant.
When Does the Brain Begin to Decline?
The brain begins to show signs of decline(衰退)after a certain proportion(比例)of the nerve cells(神经细胞)or neurons (神经细胞)of which it is formed have died.
A grown man has no more nerve cells than he did when he was born. These cells do not multiply(增加) as the body grows,as bone and skin cells do. Indeed,as a person grows older he has fewer and fewer nerve cells,because those destroyed are not replaced.
At the age of 70 or 80,as many as a quarter of the nerve cells may have died. That is the reason some old people cannot hear well or have poor memories. Yet others manage to retain their faculties(本领)and abilities until they are very old. Recent techniques have made it easier for scientists to study how our brains function(起作用),but there is still a great deal to discover.
What Happens When We Blush?
When we blush(害羞,脸红)we go red in the face, particularly in the cheeks and neck. This is because a sudden emotion(情感),such as embarrassment(窘迫), a sense of shame or pleasure,causes the very small blood vessels(血管)just below the surface of the skin to become larger. As these blood vessels,called capillaries(毛细血管),are full of blood,the skin looks red.
When people blush they usually feel hot and uncomfortable, and imagine that their red faces are very noticeable. This is not always so,because some people’s blushes do not show. It depends very much on the colour of the skin. Blushing is most noticeable on a fair-haired(金发的),pale-skinned(白皮肤的)person. Young people,particularly girls in their teens(十多岁时),are supposed to blush more easily than older people. This is true of many girls. As they grow older,they usually blush less easily,because they are embarrassed less often,and learn to cope(妥善处理)with sudden emotions.