96.It was not until the 19th century，when magnetism was shown to be the product of electricity，that serious research into the nature of permanent magnetism began.

　　直到19世纪证明了磁是电产生的那个时候，人们才开始对永久磁性的本质作认真的研究。（to be the product.……为主语补足语，说明magnetism.）

　　97.A rocket is independent of air because it contains an explosive mixture which consists both of a fuel to beburned and a supply of oxygen.

　　火箭飞行依赖空气，因为它本身装有爆炸混合物，这种混合物是由待燃烧的燃料和氧气组成的。（to be burned在句中作定语修饰fuel，有“将来”的含义。）

　　98.The first law of motion explains what is to be understood by force： it iS that which tends to change the state of rest of aboby，or of uniform motion in astraight line.

　　第一运动定律阐明了对力应该怎样理解：力就是改变物体的静止状态或匀速直线运动状态的作用。（“be+不定式”有情态动词的意思，即“可能”，“应该”，“必须”。）

　　99.In almost every cause the electric current that an ammeter measures must actually pass through theinstrument，so thismetermust beconnected inserieswith the circuit inwhich it is to measure the current.

　　几乎在所有的情况下，安培计所测量的电流实际上必然要通过该电表，因此安培计必须与待测电流的电路串联在一起。（句中“be十不定式”有表示“将要”的意思。）