29.5 - 26 = 3.5 ÷ 2 = 1.75
Answer, 175⁄100 lbs. per square inch.
A dial vacuum gauge of the Bourdon construction is similar to the Bourdon steam gauge, that is used upon the boiler, except that the inside of the elliptical tube is in communication with the condenser and the atmospheric pressure bends the tube into a curve of smaller radius (instead of to a larger one, as in the case of the steam gauge).
Obviously, therefore, the zero of the dial vacuum gauge is atmospheric pressure.
Suppose the dial vacuum gauge shows 10 lbs., the steam gauge 120 lbs., and the barometer 15 lbs., and we may find the total pressure or pressure above vacuum of the steam in the boiler is as follows:
| One-half Pressure by steam gauge = | 60 | lbs. |
| A perfect vacuum = | 15 | lbs. |
| Total pressure supposing condenser had a perfect vacuum = | 75 | lbs. |
To make the correction necessary because there is not a perfect vacuum in the condenser, we then proceed as follows:
| Barometer 30 inches of mercury = | 15 | lbs. | per | sq. | in. |
| Dial vacuum gauge = | 10 | „ | „ | „ | „ |
| Actual pressure in condenser = | 5 | „ | „ | „ | „ |
Then
| Total pressure supposing condenser had a perfect vacuum = | 75 |
| Actual pressure in condenser = | 5 |
| Actual pressure of the steam = | 70 |
Racing means a sudden acceleration of the engine speed, and occurs when the propeller is not fully immersed in the sea, as by reason of the pitching of the ship. Racing augments the strain on the working gear of the pumps, and is likely to lead to accident. It is obviated by the use of a governor or by partly shutting off the steam by hand.