﻿II. 



A Synopsis of the different Cases that may happen in 

 deducing the Longitude of one Place from another ■, by 

 Means of Arnold's Chronometers, and of finding the 

 Rates when the Difference of Longitude is given. 



EV MR. REUBEN BURROW. 



IT was formerly the custom to give rules for calcula- 

 tion, without any investigation of their principles; 

 but the contrary method has so much taken place of 

 late, that those who are not acquainted with the theory 

 of a subject are seldom in a capacity of calculating at 

 all ; and those who are acquainted with it, must either 

 lose time by recurring thereto continually, or run the 

 hazard of often making mistakes. Indeed, the use of 

 practical rules is so obvious, that Newton has often 

 given them when he has omitted their demonstrations ; 

 and the want of them has been noted by Bacon among 

 the deficiencies of learning. The Hindoos were so 

 particularly attentive in that respect, that they usually 

 gave two rules for the same operation ; one couched 

 in the shortest terms possible, and often in verse, for 

 the ease of the memory ; and the other more at length, 

 as an explanation. It therefore is much to be wished 

 that authors would revert to the ancient custom so far, 

 as to pay some attention to the reduction of their 

 knowledge to practice ; that people may not be under 

 the necessity of investigating rules at the time they 

 want to use them. 



The following is one rule, out of a great number, 

 that I drew up for my owrr use, in determining the 



