1l2. The most direet vay to attenpt to negate the threat against strategicveapons would be to introduce a passive defence in the for; of lrardening. Thenissile silos in the soviet union and the united states have successively beenh€Jdened to leveIs estinated. to be above 100 atmospheres, which is presumabr-ycfose to the practical linoit for these installations. Certain connand and. control-posts may have hardness in the 200 atnosphere ( alnost I,OOO pSI) range. Hovever,a'y sueh instal-lation woul-d be vuLnerable to a direct hit ,nless it vas so deepthat the disturbance from a crater irnmediately above wourd not provide enough Ground n4otion or faul-t ing to damage the instalration. Depths o-f the order of2 km. are probably required, but no instal"fations or plans are knornn whichutiLize such super-hardening. 113. To nuuify or substantially invalidate the nuclear threat against civiliantargets by the provision of adequate shelters fox the popur-ation and" the hardeningof industrial structures is in practice inpossibJ-e, This protlem is more fur_lytreated in chapter IV. l-14. The problem of active defence against nuc],ear weapons primarily d.evofves uponthe detection of such weapons, their tocation to a.n accuracy which allows adef^ndlng reapon tc be directed against the attacking weapon, a',d the provision ofad.equate numbers of defending weapons to handle an attack. 115' The principal technique to counter these development s is to nake systemsmobile. This can be e:{pected to be a major aspect in development of bailisticmissile systems in the future. l-l-6. As has been indicated- earlier, the SALT r rreaty betveen the united. states andthe.USSR effeciively bans, indefinitely, the extensive deploynent of anantiba istic nissile system. fn addition, it specifical-:-y ir,rr" nobil_e ABM"including those deployed in space.A,/ 35 / 392 Engfish ANNCX Page \8 117. On the other hand, the technical fac-Lors invofved have in flarr.y cases combi.nedto make the use of nuclear veapons for defence less effective than the use of
**Analysis**
The text appears to be discussing strategic defense systems, specifically the development of passive and active defense mechanisms against nuclear weapons. The language used is technical and focused on military concepts, such as missile silos, hardness, and ABM systems. The text also mentions international agreements, like the SALT treaty, and the limitations they impose on the development of certain defense systems. Given the focus on military defense and technology, it is likely that the text falls under a category related to science and technology.

**Category**
Science and Technology