1869.] Tlie Lambeth Ohservatory. 351 



from the lever-weight as a retarding influence. The weight of the 

 lever hangs, for this purpose, perpendicularly under a suspending 

 pivot, and therefore in repose and out of use, when the gyration of 

 the governor is not too fast and the drag is not wanted. It only 

 comes into effective operation when it is drawn out sideways by the 

 too rapidly revolving governor acting upon a tail-like lever arm 

 above. 



The zenith sector, which is to give the latitude of the principal 

 stations, is of the same optical and measuring power as the zenith 

 sector of the Eoyal Observatory, but on account of its peculiar con- 

 struction, weighs only 595^ lbs., while the Grreenwich sector weighs 

 1140 lbs. It consists essentially of a vertical pillar, carrying a 

 transverse axis, which has, in place of a wheel, two crossing sets of 

 radii, of which one pair is in reahty the telescope, and the other 

 pair the arm of the sectors, read by four microscopes, and each 

 including an arc of 45 degrees. The sectors are free from all 

 mechanical strain, and only looked at by the microscopes. In 

 observing, the position of a star nearly on the meridian and within 

 15 degrees of the zenith is noted, and the graduations of the 

 sectors are read off. The instrument is then turned half round on 

 its' vertical axis, the star brought into the telescope field again, and 

 the sectors once more read off. The angular interval of the arc 

 comprised between the two readings is then necessarily twice the 

 zenith distance of the star ; and if the star is a standard star with 

 known and determined polar distance, this at once becomes an 

 indication of the latitude of the place whence the star is observed. 

 The zenith distance of the star gives the height of the pole of the 

 heavens from the horizon, and this involves the angular distance of 

 the place from the earth's pole ; or, in other words, its latitude. 

 The star is referred to its exact place on the meridian, as it has to 

 be observed in succession immediately before and immediately after 

 its culmination to allow for the reversal of the instrument, by a 

 micrometer whe adjusted in the field of vision. A series of very 

 beautiful and perfect mechanical expedients are adopted to combine 

 in this instrument the two oj)posite excellences of lightness and 

 firmness ; in other words, to enable the instrument to be readily 

 transported from station to station, and yet to make it of high 

 astronomical value when in use. The entire series of these instru- 

 ments has been in hand and under examination during seven years, 

 and within that period they have undergone continual and fre- 

 quent modifications under the suggestions of the tests applied, and 

 are therefore in a fair way of presenting in the end a measure of 

 excellence which could not have been attained without the exact 

 and refined appliances of scrutiny which the establishment of this 

 observatory affords. Indeed, no better witnesses than these instru- 

 ments could be called in support of the argument urged by Colonel 



