What was used as a warning system in the old north church

The answer is a lantern.

Explanation:

Old North Church was located in  the North End, Boston and it is considered a mission regarding the Episcopal Diocese of Massachusetts. The lanterns were used in the Old North Church to warn Charlestown patriots that were across the Charles River concerning the different British Army's movements.

This method in which the lantern was used was important because it allowed informing the back-up riders that were in Charlestown regarding the British movements. These back-up riders' purpose was to warn Lexington and Concord in the case that Revere and Dawes were arrested.  

no , per capita income is not a true measure of the development because: -it only tells us about average income not how income is distributed among the people.it only give us an idea of the economic aspect.it is only at the qualitative basis.moreover, income is not only the way of development other factors are also as much as important as income.