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divided nut d, and when such an expansion does take place, its length has been so arranged that it carries upwards the pendulum ball, b b, exactly as much as the spring/, the rod a, and screw c, would tend by their united expansion to lower it, and thus, by preserving the centre of oscillation always at the same distance from the suspending pin g, the Clock is uninfluenced by changes of temperature; for it need scarcely be mentioned that if a decrease of temperature occurred, the very reverse would take place, the spring/, rod a, and screw e, would contract, tending to carry upward the bob b b, but this would be counteracted by the zinc bar, k, shortening just as much, and thus preventing any variation of the centre of oscillation.

HINTS ON OBSERVATION.

Always select the time and circumstances most suitable to the particular observation to be made.

A dark night on which the stars appear of great brilliance is generally most suitable for the examination of faint objects.

A night on which there is a slight haze, or when there is either moonlight or twilight will often prove most valuable for viewing bright objects.

The character of the definition depending on atmospheric influences can be generally ascertained by putting a star’s image out of focus and noticing if the enlarged disc appears covered with undulating waves or perfectly quiet, the latter state is of course the better. This experiment should not be tried until the telescope has been for a few minutes exposed to the air.

The most -important element in obtaining the finest definition is correct focussing. Before observing nebulse, clusters, or difficult double stars, sharply focus on a moderately bright single star.

Should the possessor of a telescope be short or long sighted, and wish to exhibit celestial objects to his friends, it will be his wisest plan to first bring his eye to a normal focus by means of an eye-glass and then to focus with this assistance. For it is best, if possible, not to allow inexperienced friends to focus for themselves unless peculiarity of vision compels them to do so. I have often been amused at a preference shown for stars out of focus; an inexperienced eye likes to see celestial objects as large as possible, and therefore prefers the larger disc seen inside or outside the focus to the minute spot seen at that point. The focus of the eye shortens with long continued gazing.

Endeavour to cultivate the use of both eyes; it frequently happens that the habit of using only one eye prevents the employment of that most sensitive to light or planetary details.

Never overpress magnifying power. A skilled observer will see far more with a moderately low power than a tyro with the deepest

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