①在量筒中倒入适量的水，将小筒放入量筒中，使其漂浮在水面上，如图甲所示，读出量筒中水面的刻度值V₁，并记录；

②将待测矿石放入小筒中，使小筒仍漂浮在水面上，如图乙所示，读出量筒中水面的刻度值V₂，并记录；

③____，读出量筒中水面的刻度值V₃，并记录；

④根据已知量和测量量，导出计算铁矿石密度的公式ρ=____．

阅读理解

      Our factories and homes burn coal and oil. Every year we have forest fires somewhere. Many scientists believe that the weather is changing.

      The earth will perhaps be warmed by about 7℃ at the South Pole(极) and the North Pole. The ice will be melt(融化). The sea level(平面) will rise by about 7 meters. As a result, water will cover cities like London, Tokyo and New York. Some ports(港口) and towns along the coasts will disappear under water.

      Some other scientists think that the dust, smoke and pollution from coal, wood and oil fires will block out(封闭) the ultraviolet rays(紫外线). Then the earth will become colder and great sheets of ice will cover Europe and North America.

      When chemicals in the air mix with rain, we have acid(酸) rain. Acid rain can bring us great harm. It destroys(破坏) forests and kills life in lakes and rivers.

      People are beginning to see how serious the problem is. They find it important to work with nature instead of against it. They are trying to save the earth in many different ways. But one of the most important things to do is to make people understand that saving the earth is saving ourselves.