78 LECTURES TO SCIENCE TEACHERS. 



But on no subject of civil engineering is there any complete 

 series of models except the one I have alluded to. 



I shall now call your attention to the collection of 

 drawing instruments, which are, as you see, principally of 

 the ordinary mechanical construction ; but there are a few 

 other instruments which I shall briefly explain. Before 

 I do so, I think I should remind you especially those of 

 you who are students in drawing of the absolute necessity 

 for good instruments. Without them it is impossible to 

 acquire real neatness of execution ; and I have generally 

 found that the use of inferior instruments leads beginners, 

 at least into habits of slovenly execution, and in all cases 

 greatly retards their advancement. Among the cases of 

 ordinary instruments I have selected some boxes of English 

 and some of foreign makers. These which I have before 

 me are exhibited by Mr. Stanley, and with most of them 

 you are familiar. Among the foreign makers Tacher has 

 sent a very good collection. Besides these instruments in 

 cases there are a variety for special purposes. Here are, on 

 a board, some of Stanley's protractors. First there is the 

 ordinary protractor, which, as you know, is used for setting 

 off angles in all kinds of drawing, more especially for 

 surveying, where it is necessary that these angles should 

 be very carefully measured. Here is one of a peculiar 

 kind, with three arms, commonly called a station pointer, 

 for fixing the position of a point from three given points. 

 Two of the arms are fitted with verniers for obtaining very 

 accurate angles. In delicate work, such as laying down 

 a long base line, a protractor is not sufficiently accu- 

 rate, and recourse is had to other methods of laying 

 down angles, as for instance setting out tangents with 

 the assistance of a table of logarithms. There are some 

 other special instruments, such as the pantograph, the 

 eidograph, the elliptograph, the conchoidograph, and heli- 

 cograph, an instrument for drawing spirals. This latter 

 is a very elegant instrument, and might be used for 

 drawing Ionic columns. There is a little wheel in the 

 centre, which may be set to any required degree of 

 obliquity, and when thus slightly turned to one side it 

 forces round by a rack motion the whole instrument, 

 which is only fixed at one point, and gradually the tracing 

 point recedes from the centre, describing the spiral. 



