T Technology. General TA-TH _Building and Engineering Group_ TA Engineering. General. Civil engineering TC Hydraulic engineering (harbours, rivers, canals) TD Sanitary and municipal engineering TE Roads and pavements TF Railroads TG Bridges and roofs TH Building construction 9111-9600 Fire prevention, fire extinction TJ-TL _Mechanical Group_ TJ Mechanical engineering TK Electric engineering and industries TL Motor vehicles. Cycles. Aeronautics TN-TR _Chemical Group_ TN Mineral industries TP Chemical technology TR Photography TS-TX _Composite Group_ TS Manufactures TT Trades TX Domestic science U Military Science. General UA Armies. Organization and distribution UB Administration UC Maintenance and transportation UD Infantry UE Cavalry UF Artillery UG Military engineering UH Minor services V Naval Science. General VA Navies. Organization and distribution VB Administration VC Maintenance VD Seamen VE Marines VF Ordnance VG Minor services VK Navigation VM Shipbuilding and marine engineering Z Bibliography and Library Science =239. Subject Classification.=--This, the most recent British scheme, is the work of the author of this manual, James Duff Brown; is a complete, homogeneous, detailed and well-indexed scheme, and is selected for notice as being generally applicable to British libraries of all kinds, although it is not likely to oust the Decimal scheme from its priority of place. It is based on the principle of placing all topics in a logical sequence; of keeping applications of theory as close as possible to the foundation theory; and of providing one place only for each important topic. The complications and intersections of human knowledge prevent anything more than an approximation to this ideal, but it has been found in actual practice to be a classification scheme which works easily and harmoniously. The following extracts from its valuable introduction will give the best view of the principles on which the system is based: “THE ORDER OF THE MAIN CLASSES.--The reasons which determined the adoption of a certain sequence of classes in this system may be briefly set forth here, instead of any argument or attempt to justify the order. The battle which has raged, and is still raging, among scientists, as to the best and most desirable order in which to arrange the great branches of human knowledge in order to produce a ‘hierarchy,’ must deter a non-scientific classifier from arguing on such a complicated and difficult topic. It will, therefore, suffice if I briefly describe the main classes in their order and give reasons why they were assigned to the places they occupy.