338 SURVEY OF THE COAST 



it is advisable to take two series, like the simple angles in 

 the two-feet theodolite, with the telescope in a direct and in 

 a reversed position, in order to compensate the influence of 

 the verticality of the telescope ; though near the horizon this 

 influence is very small. In like manner also, the instrument 

 may be placed in the three possible positions of the legs 

 upon their cones, and such series may be taken at each po- 

 sition. 



All these changes would undoubtedly contribute to accu- 

 racy, by the compensation which they would give to the in- 

 discriminate mean of the proper number of series. It might 

 be asserted that these changes with series of only a smaller 

 number of angles would be preferable to long seiies in one 

 position. 



What has been said upon this subject relative to the two- 

 feet theodolite, refers evidently to this instrument, as it is 

 grounded upon the same principles, the multiplication of the 

 angles excepted, which, in using it according to such a sys- 

 tem, would be considered merely as a compensation for the 

 greater diameter of the instrument. I shall not enter here 

 into further details on this subject, as I never had time or op- 

 portunity to test these instruments completely. I measured 

 only some angles by way of trial, the coincidence of which 

 with those of the two-feet theodolite was very satisfactory. 



Method of Observing Vertical Jingles with the Repeating 



Theodolite. 



When the instrument has been carefully adjusted and level- 

 led in all respects, the observation of a series of zenith dis- 

 tances with it is taken nearly in the same manner as with the 

 repeating circle 5 but in respect to its order in the use of the 



