Introduction . 7 
thereby - , which although not eaual to one other , yet their Sum , that is , taken altogether , will be equal to four right Angles . For as much as the Angles ON are leflencd , the other oppofite Angles M P will be enlarged , and therefore their Sum will be as before . 
4 . If again , as in Fig . 4 , there be another Line ST drawn Parallel to X Z , then there will be four more Angles , 1 , 2 , 3 , 4 , made in all Refpedts equal to the former , MNOP ; for M is equal to 1 , N is equal to 2 , O is equal to 3 , and P is equal to 4 , as may be demonstrated at large . 
Thus far then being known of Lines , and the gles they form by thefe Pofitions one to another ; let a Triangle be formed , as in Fig . 5 , and the Side A H be extended as far as E , it is plain by Step i , 2 , 3 , that the internal Angle A , and the external Angle D taken together , are equal to two right ones : If therefore it can be proved , that the Angles B and C amount to juft the lame as the Angle D , then the Proportion will be demonilrated . To this Pur - pofe therefore , the beft Way is to try to divide D into two Angles in fome fuch Manner , if poiTible , as to be commenfurable feparately to B and C : And how this may be done , appears from Step 4 , by drawing a Line through the Vertex , as XZ , which lhall be parallel to the Bafe BC , as in Fig . 6 , and will cut the external Angle into K and L . Now by the fame Reafon that N is equal to 2 , Fig . 4 , or that there is an exaft Equality between the correfpondent Angles made by each Parallel ; for the very fame Reafon , I fay , K is equal to B in Fig . 6 , and L equal to C . If then the Sum of ALK is equal to two right Angles , as before proved , and B added to K is equal to K added to L , then the Sum of 
B 4 ABC
	        

Note to user

Dear user,

In response to current developments in the web technology used by the Goobi viewer, the software no longer supports your browser.

Please use one of the following browsers to display this page correctly.

Thank you.