By no means the least arduous of the many labours of Parliament are those which are undertaken by legislators serving upon the various Committees, of which the public knows so little, but whose work is very necessary to the carrying on of public business. The appointment of Select Committees in both Houses has been practised ever since the earliest days of Parliament. The duties of these subsidiary bodies, which may be appointed for any purpose, are prescribed by the terms of the reference: they may collect facts for future legislation, investigate conduct, or examine the terms of a Bill referred to them, thus saving the time of the House. To them go all opposed Private Bills, when counsel appear to argue the merits of clashing interests. The system of Committees perhaps originated in the conferences held in former times by the two Houses in the Painted Chamber. There are records of small deliberative bodies, somewhat similar in character to the modern Committees, in the middle of the sixteenth century. By the time Queen Elizabeth came to the throne such Committees were common, and were usually composed of members of one or other House. Select Committees did not exist until the eighteenth century, and were originally semi-judicial in character. All members of the House of Commons are subject to be called upon to serve on Select Committees, being chosen for the purpose by a Committee of Selection, and the work thus done outside the actual Chamber adds considerably to the daily labours of politicians. No member may refuse to serve, if called upon to do so, and when, in 1846, Mr. Smith O'Brien declined to sit on an English Railway Committee, he was confined in the Clock Tower in the custody of the Sergeant-at-Arms. The whole House can also resolve itself at any time into a Committee, when its function becomes one of "deliberation rather than inquiry."[378] Every Public Bill not referred to a Grand Committee must be considered in a Committee of the Whole House, and, indeed, the greater part of each session is occupied by this stage of legislation. The Committee of Supply and the Committee of Ways and Means are both "Committees of the Whole House," and are appointed to discuss the financial projects of the Government, the one to supervise expenditure, the other to devise taxation. [378] May, p. 430. A Committee of the Whole House differs in no respect from the House itself, save that it is presided over by a chairman in place of the Speaker, and that the mace is removed from the Table. There are also some changes in the procedure of debate, as, for example, the cancelling of the rule forbidding a member to speak twice on the same question.