is a mathematical instrument, consisting of two equal rulers, either of wood or metal, connected together by two slender cross bars or blades of equal length, moveable about the points of junction with the rulers. There are other forms of the instrument; some, for instance, having the two blades crooking in the middle, and fixed only at one end of them, the other two ends sliding in grooves along the two rulers, &c.

The use of this instrument is obvious. For the edge of one of the rulers being applied to any line, the other opened to any extent will be always parallel to the former; and consequently any parallels to this may be drawn by the edge of the ruler, opened to any extent.

**PARALLELS, or PLACES OF ARMS**, in a siege, are deep trenches, 15 or 18 feet wide, joining the several attacks together; and serving to place the guard of the trenches in, to be at hand to support the workmen when attacked. There are usually three in an attack: the first is about 600 yards from the covert-way, the second between 3 and 400, and the third near or on