this line should approach within a few pounds of the line of perfect vacuum.

Ques. 694.-If the admission line inclines inward. from the perpendicular, what defect in the valve setting is indicated?

Ans.-Insufficient lead.

Ques. 695.-How is wire drawing of the steam detected by the indicator diagram?

Ans. By the downward inclination of the steam line toward the point of cut-off.

MEP 27.2lbs

M.EP. 2.6ks

FIG. 183. DIAGRAM SHOWING EFFECTS OF WIRE DRAWING THE STEAM.

Ques. 696.-What is a very necessary factor in the calculation of the horse-power of an engine as shown by a diagram taken from it?

Ans. The mean effective pressure.

Ques. 697.-How is the M. E. P. of a diagram ascertained?

First,

Ans. There are two methods commonly used. by means of ordinates, and secondly, by the use of the

planimeter.

Ques. 698.-Describe the method of finding the M. E. P. by ordinates.

Ans. The process consists in drawing any convenient number of vertical lines perpendicular to the atmospheric line across the face of the diagram, spacing them equally, with the exception of the two end spaces, which should be one-half the width of the others, for the reason that the ordinates stand for the centers of equal spaces. This

is an important matter, and should be thoroughly under

stood, because if the spaces are all made of equal width, and measurements are taken on the ordinates, the results will be incorrect, especially in the case of high initial pressure and early cut-off, following which the steam undergoes great changes. If the spaces are all made equal, the measurements will require to be taken in the middle of them, and errors are liable to occur, whereas if spaced as before described, the measurements can be made on the ordinates, which is much more convenient and will insure correct results.

Any number of ordinates can be drawn, but ten

is the most convenient and is amply sufficient, except in case the diagram is excessively long.

Ques. 699.-Having succeeded in drawing the ordinates across the face of the diagram, what is the next step?

Ans. The pressure represented by each line is measured from the exhaust line to the steam line, and so on,

FIG. 185. PLANIMETER.

along the expansion curve throughout the length of the diagram, using for this purpose the scale adapted to the spring used, and having thus obtained measurements on each line, add all together and divide the sum total by the number of lines, which will give the mean forward pressure. To obtain the mean effective pressure, deduct the back pressure, which is represented by the distance

of the exhaust line above the atmospheric line in a noncondensing engine, and in a condensing engine the back pressure is measured from the line of perfect vacuum.

Ques. 700.-What is a planimeter?

Ans.-The planimeter is an instrument which will

accurately measure the area of any plane surface, no matter how irregular the outline or boundary line is.

Ques. 701.-What is the main requirement in ascer-. taining the M. E. P. of a diagram?

Ans. The prime requisite in making power calculations from indicator diagrams is to obtain the average height or width of the diagram, supposing it were reduced to a plain parallelogram instead of the irregular figure which it is.

Ques. 702.-What advantage is gained by using the planimeter in measuring diagrams?

Ans. It shows at once the area of the diagram in square inches and decimal fractions of a square inch, and when the area is thus known it is an easy matter to obtain the average height by simply dividing the area in inches by the length of the diagram in inches. Having ascertained the average height of the diagram in inches or fractions of an inch the mean or average pressure is found by multiplying the height by the scale. Or the process may be made still more simple by first multiplying the area, as shown by the planimeter in square inches and decimals of an inch, by the scale and dividing the product by the length of the diagram in inches. The result will be the same as before, and troublesome fractions will be avoided.

Ques. 703.-Having obtained the M. E. P., as shown by the diagram, how may the horse-power developed by the engine be ascertained?

Ans. The area of the piston (minus one-half the area of rod) multiplied by the M. E. P., as shown by the dia