RULE, iS generally used in taking dimen sions, and in casting up the contents of timber and artificers' work.

It consists of two equal pieces of box, each one foot in length, connected together by a folding joint : in one of these equal pieces there is a slider, and four lines marked at the right hand, A, C, u; two of these lines, 11, c, are upon the slider, and the other two, A, D, upon the rule. Three of these lines, viz., A, D, c, are called double lines, because they proceed from one to ten twice over in the length ; these three lines are all exactly alike, both in numbers and division. They are numbered from the left hand towards the right, 1, 2, 3, 4, 5, 6, 7, 8, 9, 1, which stands in the middle ; the numbers then go on again to 10, which stands at the right hand end of the rule. These numbers have no determinate value of their own, but depend upon the value you set on the unit at the left hand of this part of the rule ; thus if you call it 1, the 1 in the middle will be 10, the other figures which follow will be 20, 30, Sc., and the 10 at the right band end will be 100. 11 the first, or left-hand unit be called 10, the middle 1 will be 100, and the following figures will be 200, 300, 400, &c., and the 10 at the right-hand end will be 1000 ; and thus, whatever be the value of the first unit, the second unit in the middle is always ten times greater ; and whatever is the value of the first and second unit, the following numbers to the right denote so many times that value as the number expresses.

The fourth line, called the girt line, is a single line, proceeding from 4 to 40. Upon it are marked w o at 17 15, and A G at 18.95, the wine and ale gunge points, to make it serve the purpose of a guaging-rule.

The use of the double lines, A and a, is for working the rule of proportion, and finding the areas of plain figures. And the use of the girt line D, and the other double line c, is for measuring of timber. On the other part of this side of

the rule, there is a table of the value of a load, or 50 cubic feet, of timber, at all prices, from sixpence to twenty-four pence, or two shillings, per foot.

On the other side of the rule are several plane scales, divided into 12th parts, marked inch, &e., signifying that the inch, inch, Sc., are each divided into 12 parts.

These scales are useful for planning dimensions that are taken in feet and inches. The edge of the rule is divided into inches, and each of these inches into eight parts, repre senting half inches, quarter inches, and half quarters.

In this description, the rule is supposed to be folded ; let it now be opened, and pull out the slider, you will find the lisek of it divided like the edge of the rule, so that altogether it will measure one yard, or three feet, in length. The slide is very useful in taking inside dimensions for any length not less than one foot, nor greater than three feet.

Some rules have other scales and tables upon them ; as a table of board measure, one of timber measure, a line for showing what length for any breadth will make a foot square, also a line showing what length for any thickness will make a solid foot. The former line serves to complete the table of board measure, and the latter the table of timber measure.

The thickness of the ride is generally about a quarter of an inch ; this face is divided into inches and tenths, and num bered, when the rule is opened, from the right-hand towards the left, 10, 20, 30, Sze., to 100, which falls upon the joint. The other half is numbered in the same manner, and the same way. The scales serve for taking dimensions in feet, tenths, and hundredths of a foot, when the contents arc found by decimals.