about six or seven hundredths of an inch, or a little less than a tenth. The regularity of the oscillations is disturbed by land.

DESCRIPTION OF THE ANEROID..

The Aneroid is an instrument which has come into extensive use, owing to its convenient size and portability. It is especially suited for use on board small vessels, in which there is not space to suspend a barometer; and, of course, all that is stated in this manual regarding the barometer as a weather indicator, applies to the aneroid, so far as a single observer is concerned.

In the aneroid, atmospherical pressure is measured by its effect in altering the shape of a small hermetically sealed circular metallic box with corrugated top, from which almost all the air has been exhausted, and which is kept from collapsing by a powerful spring.

When atmospherical pressure rises above the amount which was recorded when the instrument was made, the top is forced inwards, and vice versa when pressure falls below that amount the top is forced outwards by the spring. These motions are transferred by a system of levers and springs to a hand which moves on the dial.

Aneroids are very sensitive, but unfortunately they do not preserve their accuracy completely. If a table of corrections be determined for any aneroid, it will be found that after a time the instrument will have undergone some change, and that the values of the corrections require alteration, so that comparison with a standard barometer is necessary. In every case of such comparison the readings of the mercurial barometer should be reduced to 32° Fahrenheit.

A serious objection to the scientific utility of these instruments is their liability to injury from rust, or to the alteration of force in the springs used in their construction.

B.........Blue sky; whether with clear or hazy atmosphere. ....Cloudy; i.e., detached opening clouds.

M..

.Misty or Hazy, so as to interrupt the view.

O.. ....Overcast; i.e., the sky covered with one impervious cloud.

Р

.Passing showers.

U.

V.

W.

.Ugly threatening appearance in the weather.

.Visibility of distant objects, whether the sky be cloudy or not.
.Wet dew.

NOTE: A bar (--) under any letter augments its signification.

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DEFINITIONS COMMON TO NAVIGATION.

1. A Sphere is a round body, every point on the surface of which is equally distant from the centre of the body.

2. A Great Circle is the circumference of any section of a sphere, whose plane bisects the sphere, and passes through its centre.

3. A Small Circle is the circumference of any section of a sphere, whose plane does not bisect the sphere, nor pass through its centre.

4. The Arc of a Circle is any part or portion of the circumference of a circle.

5. The Distance between any two points on the surface of a sphere is measured on an arc of a great circle.

6. The Poles of the earth are the ends of the axis around which it revolves.

7. The Equator is a great circle which circumscribes the earth at a uniform distance of 90° from both poles.

8. The Meridians are great circles which cut the equator at right angles, and pass through both poles.

9. The Tropics of Cancer and Capricorn are small circles which circumscribe the earth parallel to the equator, at a distance of 23° 28′ north and south respectively.

10. The Arctic and Antarctic Circles are small circles which circumscribe the earth parallel to the equator, at a distance of 23° 28′ from either pole, or 66° 32′ north and south respectively.

11. Parallels of Latitude are small circles which circumscribe the earth parallel to the equator.

12. Latitude is distance north or south from the equator measured on a meridian.

13. Longitude is the arc of the equator between the first meridian and the meridian of any given place.

14. The Ecliptic is a great circle representing the apparent annual course of the sun among the fixed

stars.

15. The Visible Horizon is a circle which limits the observer's view at sea.

16. The Sensible Horizon is a plane which touches the earth at the position of the observer.

17. The Rational Horizon is a great circle passing through the centre of the earth, whose plane is parallel to the sensible horizon.

18. The Zenith is that point which is vertically over the observer, and is 90° distant from all parts of the sensible or rational horizons.

19. Vertical Circles are great circles passing through the zenith, and cutting the horizon at right angles.

20. The Prime Vertical is the vertical circle which passes through the zenith, and cuts the horizon in the east and west points.

21. The Zenith Distance of a celestial object is the arc of a vertical circle contained between the object and the zenith of the observer.

22. The Meridian Altitude of a celestial object is its distance above the horizon, measured on the arc of a vertical circle when the object is on the meridian.

23. The Azimuth of a celestial object is the arc of the horizon between the north or south points, and a vertical circle passing through the object.

24. The Amplitude of a celestial object is the arc of the horizon between the east or west points and the centre of the object when rising or setting.

25. The Declination of a celestial object is the arc of a celestial meridian between the celestial equator or equinoctial and the object. The declination is north or south, according as the object is on the north or south side of the equinoctial.

26. The Polar Distance of a celestial object is the arc of a celestial meridian between the elevated pole and the object.

27. The Right Ascension of a celestial object is the arc of the equinoctial between the celestial meridian passing through the object, and the celestial meridian passing through the first point of Aries.

28. The Hour Angle of a celestial object is the angle at the pole, between the celestial meridian passing through the object and the meridian of the observer, measured on an arc of the equinoctial, and converted into time.

29. Dip or Depression of the horizon is the angle at the eye of the observer between the visible and sensible horizons.

30. Refraction is the deflection of a ray of light towards the centre of the earth, when passing obliquely from one transparent medium to another.

31. Parallax is the angle at the centre of a celestial object, subtended by a line joining the centre of the earth and the position of the observer.

32. Semi-diameter of a celestial object is the angle at the eye of the observer subtended by half the diameter of the object.

33. Augmentation of the Moon's Semi-diameter is the apparent increase in the semi-diameter of the moon, owing to her nearer approach to the observer as her altitude increases.