CHAP. I.] CHRONOMETERS 47 



To prevent butting too sharply, the turns can be counted, 

 which will warn the winder ; but if winding is done delicately, 

 this is scarcely necessary. When, however, any other but the 

 officer in charge of the chronometers winds them, he should 

 do this ; to enable liim to know the number of turns each 

 watch requires, a piece of paper with the information can be 

 pasted on each box. 



The watch has to be reversed to wind, and it must be eased 

 gently back when the operation is complete, not allowed to 

 swing back. This daily reversing of the watch is said to be 

 a good thing, as it distributes the oil in the bearings. 



Accurate comparing only comes, like most other things of 

 the kind, by practice. The comparing of watches is gone into 

 at p. 300. 



A comparison book and chronometer journal are kept ; the Records, 

 former being used to enter the comparisons at the time with 

 their checks, etc., the latter as a fair book for a permanent 

 record, and contains rates, and all data for noting the per- 

 formance of each watch. 



A maximum and a minimum thermometer are placed in the 

 chronometer-room, and the reading of their indices is taken 

 and recorded in the comparison book, at the time of comparing. 



This is of importance when it is intended to take account of 

 change of rate from changes of temperature, and, in any case, 

 will enable us to estimate how far our endeavours to maintain 

 a uniform temperature in the room are succeeding. 



MARKS. 



It is of course necessary in making a survey of any descrip- 

 tion to have fixed objects, which are first plotted on to the 

 sheet, and are technically known as " points." These vary, 

 according to the description and scale of the survey, from 

 mountain peaks, whose actual summits may be of considerable 

 area, to thin staves. 



It is a great saving of time to the nautical surveyor to find Natural 

 plenty of natural marks, as peaks, conspicuous trees, houses, Marks 

 church spires, etc., anything, in fact, which can be defined 

 and recognised from the different directions it may be necessary 

 to see them ; but it is rare to find a sufficient number of these. 



