The inhabited history of the Las Vegas Valley stretches to 23,000 b.c. , when much of the area was covered by a prehistoric lake. During this period, the indigenous people lived in caves, hunting the mammals that gathered at the shoreline. The landscape of the valley changed dramatically over the next 200 centuries. The glaciers feeding the lake melted away and the lake evaporated. Fossils tell an obscure story of man's slow and sporadic development. Around 3000 b.c. , native Archaic Indians began to develop a lasting hunting and gathering culture. By this time, the valley was in much the same geographic state as it exists in today, with one exceptionthe presence of artesian springs that bubbled to the surface in several areas. These springs fed a network of streams draining through the Las Vegas Wash to the Colorado River. The areas surrounding the springs were desert oases: sprawling collections of grasses, trees, and wildlife. Many springs lay in areas that would eventually become the center of the modern Las Vegas metropolis. For about 4000 years, the Archaics thrived in a culture that included many signs of early civilization. Signs of even more advancement appeared halfway through the first millennium a.d. , when the Anasazi Indians inhabited the valley. Far more progressive than the Archaics, the Anasazi utilized such formal agricultural techniques as irrigation to assist their harvest. This permitted the Anasazi to achieve a benchmark of advanced societythe ability to live in permanent shelters year-round without need to follow wildlife. Mysteriously, the Anasazi vanished from the valley around a.d.

What was a difference between the hunting and gathering practices of the Archaics and the Anasazi?

Sound waves are mechanical waves. They can travel only through matter. They cannot travel through empty space. This was demonstrated in the 1600s by a scientist named Robert Boyle. Boyle placed a ticking clock in a sealed glass jar. The clock could be heard ticking through the air and glass of the jar. Then Boyle pumped the air out of the jar. The clock was still running, but the ticking could not be heard. Thats because the sound couldnt travel without air particles to transfer the sound energy. Is there any other place where sound cannot be heard? Here is a hint: is there any other place that does not have air molecules? Thats right, in space sound cannot be transferred. So can astronauts hear each other? Yes, they can, because they are in enclosed space stations or pressurized suits.In both cases, they are surrounded by air molecules. If they left their space ship and did not use their electronics, they would not be able to hear each other. Next time you watch a science fiction movie, think about this factoid. When spaceships are destroyed by hostile alien beings, there would be no sound heard. Sound waves can travel through many kinds of matter.

How do astronauts hear each other?

Duane was hungry, but he hurried through the ample supper that was set before him, urged on by curiosity and astonishment. The only way he could account for his presence there in a ranger's camp was that MacNelly hoped to get useful information out of him. Still that would hardly have made this captain so eager. There was a mystery here, and Duane could scarcely wait for it to be solved. While eating he had bent keen eyes around him. After a first quiet scrutiny the rangers apparently paid no more attention to him. They were all veterans in service--Duane saw that--and rugged, powerful men of iron constitution. Despite the occasional joke and sally of the more youthful members, and a general conversation of camp-fire nature, Duane was not deceived about the fact that his advent had been an unusual and striking one, which had caused an undercurrent of conjecture and even consternation among them. These rangers were too well trained to appear openly curious about their captain's guest. If they had not deliberately attempted to be oblivious of his presence Duane would have concluded they thought him an ordinary visitor, somehow of use to MacNelly. As it was, Duane felt a suspense that must have been due to a hint of his identity.
In Duane's opinion, how do the Ranger's perceive him?