OF FORMS IN GENERAL. 



15 



. 28. SIMILAR SOLID ANGLES. 



Solid angles are similar when formed of an equal number 

 of plane angles, of which the corresponding ones are similar. 



Similar solid angles may be wholly made up of similar plane an- 

 gles; or there may exist a degree of dissimilarity among them, pro- 

 vided this dissimilarity corresponds in both : in the former case 

 they are said to be equiangular, and in the latter unequiangular. 



A solid angle, formed of three, four, five, &c. faces, is said to be 

 a solid angle of three faces, a solid angle of four faces, &c.* 



* Elementary definitions. A triangle is a plane figure contained 

 within three sides. When the sides are equal it is called an equilateral 

 triangle. Fig. 2. The angles of an equilateral triangle are equal. A 

 triangle having but two equal sides is called an isosceles triangle. Fig. 

 3, and 4. In Fig. 3, the two equal sides contain an angle less than 90 ; 

 and in Fig. 4, an angle greater than 90 ; Fig. 3, is therefor. ecalled an 

 acute triangle, and Fig. 4, an obtuse triangle. The unequal side b c, 

 is termed the base of the triangle. The angles a b c and a c b, which 

 are adjacent to the base, are equal to one another. A triangle with all 

 its sides unequal is termed a scalene triangle ; Fig. 5, in which, all the 

 -angles also, are unequal. 



Fig. 2. Fig. 3. Fig. 4. Fig. 5. 



A square has four equal sides. Fig, 6. Its angles are right angles. 

 A rectangle has its opposite sides, only, equal ; its adjacent sides .ir 

 unequal. Fig, 7. Like the square its angles, are all right angles. 



Fig. 6. Fig. 7. Fig. 8. 



A rhomb has four equal sides. Fig. 8. Two of its angles, as a and c are 

 obtuse ; the other two, d and 6, aj-e acute. 



