OBL 
sae it from all the other an are parts, that are 
added to enrich the harmon that if retrenched, 
a piece will be mutilated. Thofe oe perform the ripieno 
parts, may ftop whenever they pleafe, and the piece never- 
thelefs will go on; but the performer to whom an obligato 
part is ae cannot ftop a moment without being miffed. 
E, in Law, is the party to whom an obliga. 
tion, or bond is made 
OBLIGOR, the party who enters into or executes an 
obligation, or bon 
UATION, in Catoptrics. 
tion is a right line drawn perpendicular to a 
point of incidence, or reflection of a ray. 
UE, in Geometry, ei aflant, indire&t, or 
that deviates from the perpen 
Os ique Angle. 
Onuiaue- Angled oe is that whole angles are oblique; 
t. é. either obtufe, o 
OBLIQuE Arches. Shae pally roads run oblique to the 
courfe of any river, rivulet, drain, or canal, neceflary to 
be paffed over by a bridge, the direftion of the former is 
generally varied {9 as to be reCtangular to the courfe of the 
fatter ; unlefsin {mall ftreams, over which, when their courfe 
is not made to fuit the road, there are eel inftances of 
Cathetus of aie 
mirror, in 
eae 
oO 
1e) 
m the oppofite abutment, sales ois 
may be derived from the interlapping or ya joint of 
the bricks or ft e arch; and from the good- 
nefs of the mor tar, tending toc ement them ae one mals: 
has crac ced or given w hefe circumftances pete pre- 
vented cautious builders from adopting this method; and 
ed them, in a few inftances, to build the arch {quare to 
run the parapets oblique, to coincide 
3 leaving Tease triangles of the arch 
on the outfide, which i a difagreeable appearance, and has 
feldom been ufed; therefore, in ae unlefs the courfes 
of ftreams or canals were made rectangular to the road, the 
line of the latter has ibeen tics fo as to admit of a dire& 
pafflage over the water, which upon high roads, when not 
curved for a confiderable diftance, is inconvenient if not 
ea 3 and d particularly fo to travellers 1 in the night time ; 
all the; 
7 
itn all tneir 
im serfections, occafionally had recourfe to; and the writer of 
thisarticle has never heard of any alteration in their form prior 
to the year 1787. At this time he had the direGtion of the 
county of pe sere a = from the Grand Canal of 
Ireland to the to 
ain the courfe of conduéting the work, feveral of the direCtors 
sas a our author was led to confider enue the pia 
imperfeét method could not be fet afide, by the fubfti- 
OBL 
the cpale pee ee 
tion of fig. 8; and as the 
t 
likewife it muft be twifted in its fommering, which, although 
not infuperable difficulties, are fo in fuch a d 
bined with the indented form of the impo 
ot to c 
quoins aad their neceflary impofts, in the forming of which 
intelligent {tone-cutters will be i role as will appear from 
int 
os and € extrados of 
being reduced in p ; 
ch voittoir on that fide of the pier where its face 
forms an acute angle with its abutment, m 
tufe angle with its foffit, eer in their ch to 
the crown of the arch, and thenceforward beco omin g acute, 
5 
n 
abutment ; therefore the different fides of the 
muft form different Se of elevation or depreffion from 
the rectangle with its hea 
geometric mode of formin ing each voiffoir would be 
sy wari as will appear from the rales diagrams, viz. 
ie the line , and de, in the 4s include a 
of the arch at each place vefpedtively 3; wand @ wi 
fhew the crowns of their foffits. The line ¢¢, esi at 
right angles with the face of the arch, and with its re{pec- 
tive extremities equid:ftant from the points z and v, will re- 
prefent a joint on the foffit, which muft neceflarily be hori- 
zontal at its extremities ; becaufe equidiftant from the crown 
refpettive arc, it will 
the fame oo will aC 
Na 
