elasticity, is endeavoring to force the mercury up the tube. The mercury in the tube will be found to stand about 30 inches higher than the level in the basin, varying slightly, according to the state of the atmosphere.

The scale of a barometer-gauge may be explained as follows: As 30 inches of mercury press down with the same force as the atmosphere, say 15 lbs. per square inch, two inches of mercury correspond to one pound of pressure, and a scale of inches measured from the mercury in the cup upwards must be fixed near the glass tube. As the vacuum, while the engine is working, may be supposed to be good, the scale need only be marked to a few inches below 30 inches, every fall of two denoting one pound of pressure in the condenser.

The sources of error, in estimating the vacuum by this gauge, arise from the following two facts: That the pressure of the atmosphere, or the mercury in the cup, is liable to change. That the gradations on the scale are marked, on the supposition that the level of the mercury is stationary; because it is from this level that the scale commences. Therefore a fixed scale must be erroneous, on account of the sinking of the mercury in the cup as it rises in the tube.

The first source of error may be corrected by observing the actual height of a weather barometer, and subtracting it from the height as shown by the gauge. This will be correct, if a tube of a standard diameter is used. This error may be corrected by a short gauge, similar to what a weather barometer would be if it were enclosed in a space, communicating with the condenser. In that event, before a vacuum is created, the mercury would stand as high in the glass tube as in the weather barometer. On creating a vacuum, thus taking off the pressure from the mercury in the cistern, the mercury would fall in the tube. In this instrument, the less the height of the mercury the better the vacuum.

The second source of error may be obviated by having a movable instead of a fixed scale, so that its lower end might always be kept in contact with the surface of the mercury in the cup.

A siphon-gauge, such as has been spoken of, may be used as a vacuum-gauge. When so used, it is necessary to connect the long leg with the condenser, placing a stick in the short leg. In this case the scale would require to be graduated directly contrary to that for steam. The state of the atmosphere will affect the gauge. The pressure in the steam-boiler may be ascertained by the temperature, by the safety-valve, or by the steam-gauge.

The Mariner's Compass.

The object of the mariner's compass is to enable travellers to steer their course with certainty from one location to another. The needle is understood to point to the north, and the other points, east, west, etc., are easily found. In certain parts of the world, however, the needle does not point to the north, but is drawn to the right or left of true north. This is called the variation of the compass, and must be known accurately by the navigator, in order to correct and steer the right course. For instance, in crossing the Atlantic Ocean, the variation of the compass amounts in sailing vessels to 2 or 2 points westerly, and the course steered must be corrected accordingly. If a due east course is desired, the vessel must be steered 24 or 23 points south.

Off the Cape of Good Hope, the variation of the compass in ships bound to India or Australia is 2 points easterly, and, in order to make a due east course, it is necessary to steer 23 to the north, or left of her course; while towards the equator there is hardly any perceptible variation of the compass at all. The best means of finding out how much the compass varies in different parts of the world is by observations of the sun taken with the compass, and the difference between the true and magnetic compass is the variation, which must be applied as a correction to the course. steered. In iron ships or steamers, the deviation must be considered as well as the variation. This is due to the local attraction caused by the iron, and must be carefully understood before steamers or iron ships go to sea. Before a vessel proceeds on her first voyage, the compass must be carefully swung and magnets fixed to the deck.

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Technical Terms and Definitions Used in Navigation.

Apparent altitude. The apparent altitude is the observed altitude, corrected for the indicated error of the instrument, and dip of the horizon.

Meridian altitude. The meridian altitude is the highest altitude a celestial object attains on the meridian of the observer.

Observed altitude. The observed altitude is the altitude of a celestial object above the horizon measured by a sextant or quadrant.

True altitude. The true altitude is the apparent altitude corrected for refraction and parallax.

Amplitude. The amplitude is the arch of the horizon contained between the centre of the celestial object, when rising or setting, and the east or west points of the horizon, measured from the east when rising, and from the west when setting.

Azimuth. An azimuth is the angle at the zenith contained between the vertical circle passing through the centre of the celestial object, and of the meridian of the place.

Course. The course is the direction steered by compass.

Magnetic course.—The magnetic course is the compass course corrected for deviation of the compass.

True course. The true course is the compass course corrected for variation and deviation of the compass.

Course made good. The course made good is the compass course corrected for deviation, variation, leeway, and set of the current, and is the ship's real track on the ocean.