In　experimental　investigation　of　the　ignition　limit　of　ethylene－air　mixture　on　illtrinsically．safe　circuits，the　author carried　out　a　series　of　spark　tests　on　the　relationship　between　the　ninimum　igniting　current　and　the　source　voltage　up　to 7000V，and　the　inductance 1 to 200mH　inclusive．For　the　electrode　material　including　no　cadmium，zinc　magnesium　and aluminium，brass　plates　on　the　market　were　used，which　were　copper　alloy　conforming　to　the　JIS　H3100and　corresponding to　the　alloy　number　C2801． As　a　result，in　case　of　the　source　voltage　between 20 and 300V，it　was　clear that　the　igniting　current　values　in　the experiment　weτe　less　than　not　only　those　for　the　II　B（ethylene）gas　in　the　IEC　STANDARD79 11by34percent　at20V　to 47percent　at300V　but　also　those　for　the　hydrogen　gas． In　addition，it　was　fomd　experimentally　that　the　minimum　igniting　current　value　in　the　inductive　circuit　of　lmH　is the same　as that in the resistive　circuit. Both the resistive circuit including only as light degree of inductance to be ignored pragtically　and　the　inductive　circuit　with　an　inductance　of　1mH　have　a　source　voltage　of 24V　and　the　minimum　igniting current　of 700mA　in　common．It　means　that，in　dc　or commercial　frequency　power，any　inductive　ciYcuit　of　not　more　than　lmH　c＆n　be　designed　and handled　in　the　same　manner　as　for　the　corresponding　resistive　circuit　for　practical　use　without　causing　any　ploblems　on explosion－proof　measures．Therefore，it　was　concluded　that　an　ignition　limit　curve　less　than　lmH　might　be　useless　from　the practicalside．

Fault　tree　analysis　has　been　applied　to　probabilistic　risk　assessment　for　large－scale　systems　including　chemicaI processing　plants　and　nuclear　power　stations、The　synthesis　of　fault　trees　is　an　extremely　complex　and　time　consuming process　in　the　stage　of　analysis，This　paper　presents　an　algorithm　for　application　of　knowledge　engineering　to　automated fault　tree　synthesis．The　algorithm　is　implemented　in　computer　programs　written　in　Prolog． The　plant＄tructure（P　and　I　diagram）is　represented　using　frames and inputted　in　computers as　the　plant　specific lnformation．Operators　which　describe　the　ba8ic　structure　of　fault　trees　for　control　loops（feedback　and　feedfQrward　type control　circuits　and　protective　systems）are　stored　in　the　generic　knowledge・base．Component　mulfmction　information　is also　stored　ln　the　form　of　mini　fau1 tree.Fault　trees　are　generated　sequentially　by　searching　the　knowledge－base．According　to　the　frames，operators　are combined to form the top structure of fault trees, and tfees uder the top structure are developed using mini faulttrees. A　fault　tree　synthesis　is　demonstrated　for　a　How　control　system　using　the　proposed　methodology．

The　acid　aqueous　vapor　which　caused　the　aci（i　rain　was　measured　with　the3imple　capsule　developed　by　us． In　this　study，the　acid　aqueous　vapors　ill　the　forests　and　streets　areas　were　measured　in　summer　with　this　capsule．The following　results　were　fomd、Temporal　distributions　of　pH　and　electric　conductlvity　of　waterdrops　obtained　from　aqueous vapors　in　forest　and　street　areas　were　found　to　be 2 peaks　form　simular to　it　of　NO₂ concentration、Therefore　the　main contribution　to　acid　materials　was　suspected　to　be　HNO₃（or　NO_x）．The　aftemoon　peak　in　pH　and　electric　conductivity distribution　in　forest　areas　was　found　to　differ　from　it　in　the　street　areas because　a mount　of　aqueous　vapor　in　the　forest area　emitted　from　the　plants　and　soil　by　shining　of　sun　light．