CHAP. XIV.] CHRONOMETRIC MERIDIAN DISTANCES 349 



Where M^ is meridian distance A to B, 



Ai is Error at B, 



A is Error at mean epoch at A, 



a is rate of departm-e, 



b is difference of rates of departure and arrival, 



t is interval between mean epochs of rating at A 

 and K, 



T is interval between mean epoch at A and observa- 

 tions at B. 



It is to this case that om' observations on p. 343 refer, to the 

 effect that the data for calculating the position of B, as inter- 

 polated between A and K, may be also transmitted home. 



A very good way of measuring meridian distance for the Use of 

 scale of a chart, when the actual distance between the stations 

 is not too far, is by rockets. Parties landed at either end of the 

 base whose difference of longitude is to be measured ascertain 

 the Error of their pocket-chronometers. The ship, midway 

 between the two, fires rockets vertically, and the bursting of 

 these, an instantaneous phenomenon, is noted by the watches 

 at either end. 



An ordinary service signal rocket can be depended on to 

 mount 1,200 feet, and should reach 1,600. The bursting, if it 

 occurs, as it should, at the liighest point, will therefore be 

 visible nearly 40 miles on either side, which will permit a base 

 of 75 miles to be measured under very favourable circumstances 

 of dark night and clear atmosphere, when the stations are east 

 and west of one another. 



Rockets will not often, however, be seen this full distance ; 

 the balls of fire, released on bursting, are scarcely bright 

 enough ; and supposing the observers to be at the sea-level, 

 the burst of the rockets will only just be above the horizon, 

 in which position atmospheric disturbances are greatest, and 

 may disperse the rays of light before they can reach the 

 observer. Ascending a hill, therefore, will greatly assist clear 

 vision, and the use of a pair of field-glasses will do wonders. 

 Twenty-five miles, on either side, should be measured in this 

 way without any great difficulty. 



It is important, in transmitting to the Hydrographic Office Transmit- 

 the results of a Meridian Distance, that sufficient information ^g^^j^g 



