82 LECTURES TO SCIENCE TEACHERS. 



draw these, make the models themselves. For instance, 

 one figure is what is called a Marseilles arch, and 

 here is a model in exactly the same form, cut out in 

 plaster ; in fact, it is executed by the students from the 

 drawing. There are several other examples of the same 

 kind. Here, again, is a drawing in perspective of a groined 

 arch, and a model made of it, by the students, showing 

 how admirably they are taught. There are, besides, a 

 variety of other examples, so that by the end of the 

 second year the student becomes perfectly familiar with 

 all this class of drawing. 



We should remember, with regard to perspective, that in 

 these countries it is, no doubt, taught in its truest 

 manner, but it is rather complicated it is given its 

 fullest development whereas in these schools in France, 

 there is a very simple system of perspective taught, based 

 upon Descriptive Geometry, and known as that of Monge, 

 one of the founders of the Polytechnic School in France. 

 Although it is hardly necessary to explain this method to 

 you, still, as it is an excellent way of teaching beginners, I 

 will say one word upon it. You draw a ground line, X Y, 

 and a line perpendicular to it, supposed to represent 

 the picture plane. Now, let an object, say a cube, be 

 standing on the ground, and assume S, S' as the position of 

 the observer's eye. By drawing lines from S and S' to the 

 plan and elevation of the cube, and determining their 

 intersections with the picture plane, we obtain, rotating 

 this last in plan round o, the true perspective of the cube, 

 the sight point being opposite the point S". The vanishing 

 points are similarly determined, if necessary. 



Now, to return to the collection of models before us. 

 The Loan Exhibition contains no models whatever of 

 pure descriptive geometry; and as it is very often 

 difficult to render the position of lines and planes in 

 space apparent to the minds of beginners, I have found 

 a small collection, which I bought in Paris some years 

 ago, very convenient for this purpose. It is of a very 

 simple kind, made by Rouvet, of the Quai de 1'Horloge. 

 There is a book pretty well known by those who have 

 studied this subject, called Lefebure de Fourcy's Elementary 

 Treatise on Descriptive Geometry, which is one of the best 

 of its kind ; and this collection of models is made to 



