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EQUATORIAL ADJUSTMENTS.

A traveller who journeys in a southerly direction will notice as he proceeds that stars, which in England appeared close to the southern horizon, gradually increase their altitude, whereas the polar star will appear to get lower and lower, until at last, as he crosses the equator, it will disappear from view. If the traveller were to take an equatorial stand with him, he would have to gradually lower the elevation of the polar axis to keep his instrument in correct adjustment. It will thus be seen that the correct elevation of the polar axis to the latitude of the place of observation is a most important adjustment. As a rule large equatorial stands are set by their makers to the latitude of London 510, 28', 38", and afterwards adjusted by a strong foot screw as required. But when one of this kind of telescope is ordered, the maker would be apprised of its destination, in order that he may be able to set the instrument as nearly as he can to the latitude of the place.

An equatorial stand should be placed on the firmest foundation obtainable; any neglect of this precaution will be the cause of constant errors of observation and vibration of image. The pillar on which the instrument stands should be let some distance into the ground, and, when practicable, rest there on a bed of concrete. The upper surface of the pillar should be as level as possible.

I—To place the Polar Axis in the Meridian.

The polar axis being approximately directed to the pole, find out from an almanac the time any bright star of some altitude is on the meridian, due south.

Revolve the upper disc till the bubble of the level is perfectly central, and when this is done firmly clamp the telescope in that position, leaving the declination movement free. A few minutes before the time of transit point the telescope to the star as nearly as possible without unclamping, and shift the iron foot until the star is exactly in the centre of the field, using a low power. Now, in order to follow the apparent motion, gently continue to push the iron foot until the moment of transit, when immediately stop. An assistant should keep his eye at the eye-piece to regulate by word of mouth the imparted motion, and so keep the star perfectly central in the field, The mean time of transit may be obtained from an accurate watch, but it is far better to take the time from a sidereal clock on all occasions.

II.—To correct the Declination Vernier.

In a Berthon equatorial this has been originally done thus :—The upper disc and all it carries having been removed, the lower disc was accurately levelled, the stand being tilted to allow of this being

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