CONSTRUCTION OF A STANDARD BAROMETER. 447 



It was originally mounted on an independent support; but this was 

 found to be too unsteady for exact observation. It was accordingly 

 removed from its support and mounted between brackets attached to 

 the quadrant wall. The scale of this instrument has been compared 

 with the Kew standard scale, both in the horizontal and vertical 

 positions; in the former by observation of both scales by fixed micro- 

 meter microscopes, and in the latter position in measuring by the 

 cathetometer the divisions of the standard scale, placed vertically at 

 a distance of five feet. In the horizontal position there appeared to 

 be no appreciable error in the graduation of the cathetometer; but 

 when vertical, its scale was found to be somewhat too long, the mea- 

 surement of a length of 30 inches requiring a correction of + 0.003 

 inch. Besides this discrepancy, which is probably due to irregular 

 flexure of the bar and to imperfect fitting of the sliding frame which 

 carries the telescope and level, the manipulation of the instrument is 

 exceedingly inconvenient and troublesome, and requires much care 

 and patience. It is believed, however, that when the requisite care 

 and time are bestowed, the measurements, after allowing for the cor- 

 rection mentioned, are accurate. 



A new cathetometer is at present being constructed by Mr. Beck- 

 ley, at the observatory, which promises greater accuracy and conve- 

 nience. This instrument is very nearly completed, and will be 

 described in a subsequent communication. 



