Philosophical Transactions, 30f5 



small lantern placed behind its eye-glass, with oiled paper inter- 

 posed. 



The object of the author in this arrangement being to ascertain 

 the limits of variability in the position assumed by the collimator, 

 it was deranged purposely in a variety of ways, by removing and 

 replacing the float, or carrying the whole instrument from its 

 place, and every method he could think of used that could fairly 

 introduce error. His preliminary trials were made with a wooden 

 float, but this was soon laid aside after ascertaining that the 

 greatest single error committed in using it did not exceed 2".58 

 in the position of the horizontal point. Other floats were then 

 tried, and it was found that the increase of their length, and 

 browning their surfaces with nitric acid, produced material ad- 

 vantages. In 151 single results thus experimentally obtained, 28 

 only were found to give errors in the determination of the hori- 

 zontal point exceeding l"., and only two amounting to 2". ; but 

 if the means of every successive five be taken, and the experi- 

 ments with the wooden float rejected, the greatest error did not 

 exceed 0".4, and even here the influence of a constant source of 

 error depending on the support of the micrometer employed was 

 apparent. 



The author then describes at length the mode of using the col- 

 limator, and of observing with it. The instrument hitherto de- 

 scribed may be called the horizontal collimator ; but he then pro- 

 ceeds to describe a vertical collimator^ in which the telescope is 

 fixed perpendicularly to the float, and placed immediately under 

 the axis of the circle. By this arrangement the necessity of trans- 

 porting it from one side of the observatory to the other is avoided, 

 the reverse observation being made by merely turning the float 

 half round in azimuth. 



It is not necessary that the telescope of the collimator should 

 have a tube, nor does the author appear to regard its length as of 

 any importance, it being merely the direction of its axis which is 

 the subject of examination, and the accuracy of this examination 

 will depend on the length and power of the telescope of the circle 

 to be collimated. The adjustment of the cross-wires in the exact 

 sidereal focus of its object-glass is, however, a point of the 

 highest importance. 



The author next points out an important advantage which this 

 instrument presents, viz.^ that of enabling the observer, by va- 

 rying the inclination of his float, to dete(^t erroneous divisions of 

 his circle, by bringing different parts of its arc into use. 



After which he proceeds to describe an application of his float- 

 ing collimator as a permanent verification of the verticality of a 

 zenith tube ; and considers that, by its use, the error, if any, in 

 the zenith distance of a star, will be ultimately referred to inac- 



