G80 Colonel Edmond H. Hills [May 19, 



Astronomer-Royal for Scotland, in co-operation with the late Dr. 

 Common. It marked a very decided advance upon the earlier type. 

 In two points specially, the screening; of the floating part from wind 

 disturbance, and the attachment of the eye-piece to the fixed part, 

 the designers had the idea of a movaljle instrument, which a slight 

 touch or a puff of wind would set vibrating to such an extent that no 

 observation would be possible for a minute or two, clearly before them. 

 The almucantar method of observation, meaning by this not the use 

 of a floating type of instrument, but the observing of stars crossing 

 a horizontal circle, though appropriate for the particular class of 

 observation we are here concerned with, those for determination of 

 latitude, is not absolutely the best that can be used. To reduce 

 every possible source of error to a minimum, particularly those due 

 to refraction of the atmosphere, we want lo observe stars as near the 

 zenith as possible. 



The floating principle has been applied with great success to a 

 zenith instrument in the Cookson floating zenith telescope now at 

 Greenwich, designed by the late Bryan Cookson, whose early death 

 was a great loss to astronomy. 



It is a photographic instrument, with a telescope or camera 

 tube attached to a circular float which floats in a ring-sbaped 

 trough of mercury. The angle between telescope and float can 

 be altered, so that it can be clamped to point either vertically 

 upwards or at any angle, up to about 30^, from the vertical. It is 

 used in the well-known Talcott method. A pair of stars is selected 

 which cross the meridian within a few minutes of each other at 

 nearly the same zenith distance, one north and one south of the 

 zenith. The instrument is set so as to include the first star in the 

 field, the lens is opened, and as the image of the star moves across 

 the plate it traces a fine line or trail. After the star has crossed the 

 meridian, the telescope is turned through 180"^, leaving tube and 

 float clamped in the same relative position, and the second star traces 

 out its trail. The distance between the two trails on the plate, 

 which is small if the difference of their zenith distances is small, 

 when the appropriate corrections are applied, gives the observed 

 difference of zenith distance of the two stars, and, therefore, the 

 observed position of the zenith, and hence the latitude of the 

 observer. By repeating the observation with a number of pairs of 

 stars a very precise determination of the latitude is made. 



Recently a zenith telescope, designed not on the floating but on 

 the hanging principle, finding the vertical line by virtue of its free 

 suspension in a gimbal ring, has been constructed, and would have been 

 at work by now had it not been for the interruption caused by the 

 war. Though it has thus not yet been tested by practical experience, 

 a few words on it may not be out of place. The method of observa- 

 tion will be the same as I have just described, except that there is 

 no arrangement for clamping the instrument at an inclination to the 



