302 HYDROGRAPHICAL SURVEYING [chap. xiii. 



Much greater precision is obtainable by the use of the 

 chronograph than by the eye-and-ear method, or by an assistant 

 taking the time. 



It also affords the means of comparing mean-time chrono- 

 meters with each other within a very small margin of error, 

 and gives results only slightly inferior to those obtained by 

 comparing mean-time and sidereal chronometers by coinci- 

 dence of beats. 



Having compared the watches, we land to observe. 

 Care in -j^lie watches to be used on the ground should always be 



Hack- carried in their boxes, and great care must be taken not to 

 Watches, j^j.]^ them, and above all to avoid any circular motion. 

 Method of rpj^^ method of observation for time differs from that already 



Observa- 



tion. described of stars for latitude, inasmuch as we observe at 



stated altitudes, generally at every 10', setting the sextant 

 for the purpose, and noting when the contact takes place. 

 In observing with the stand, therefore, we only need to work 

 the screw of the stand leg to get the suns vertically under one 

 another. 



Preparation of the ground is not necessary, as in the case 

 of observing stars, excepting so far as selecting the spot, to 

 ensure being able to see in both the a.m. and p.m. directions. 



Observing It is well to observe both upper and lower limbs, as, though 



Limbs i^ ^^'i^^ make no difference to the result, it is good to have 

 constant practice at both opening and closing suns, and not 

 have all one way in the forenoon and the other in the after- 

 noon. If we begin by a set of upper limbs, and immediately 

 after take a set of lower, as an invariable practice, there will 

 be no confusion, and we shall soon naturally fall mto the 

 system. 



It may be here noted that with the inverting tube, the 

 movable sun (the sun reflected from index and horizon-glass) 

 is above the other, when we are observing upper limb, and 

 helow when lower limb. Also that upper limbs in the fore- 

 noon are closing suns, and in the afternoon opening suns. It 

 is necessarily vice versa for lower limb. 



Dark Eye- Always use the dark eye-pieces, of wliich there should be 



pieces to ggygj-ai gf different degrees of shade, as, if the brilhancy of 

 be used. . . 



the sun varies by passing clouds, no inherent error is mtro- 



duced by changing these, which is the case vnih. the hinged 



