30

NEW EXPERIMENTS

TRACT 34.

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each ball should fall in the water, as well as note down thetime emploved-by the ball in flying through the air, fromthe visible discharge of the gun to the plunge of the ball inthe water. The method of determining the place of thefall was this : Two parties of observers, consisting of threeor four steady and intelligent young gentlemen cadetsineachparty, having taken their stands at d and f, or e and G,according to the expected length of the range, carefullywatched the discharge of every ball from the gun at a ;then tracing it, as it were, through the air by the loudwhizzing noise it made in its flight, their eye was preparedand directed gradually towards the place of the fall, whichthey seldom missed of observing. Then immediately onperceivingtheplunge, some of them noted the time of flight,by a good stop watch, while others observed some remarka-ble land object on the opposite coast, and directly in a linewith the place in the water where the ball fell. This done,they directed the telescope or sights of an instrument, suchas a theodolite or plain table, to that object, and noted theposition of it. This being done b} r each party of observers,and the line of position from each station drawn on the plan,afterwards at leisure, the intersection of those lines gavevery exactly the place of the fall, and consequently itsdistance from the gun. In this manner then were determinedall the ranges and times of flight registered in the followingexperiments; those places being left blank where the obser-vation was either doubtful, or not made at all. The timesof flight were also sometimes observed at the gun itself,where the plunge of the ball could often be perceived.

In this map of the river in plate i, the dotted line oneach side of the river, denotes low-water mark; the firstblack line next without it denotes high-water mark ; andthe other, or outermost line, is the land bank which hasbeen raised in former ages, with immense labour, fromGreenwich for many miles below, to prevent the waters ofthe river from overflowing the adjacent fields, which it woulddo every tide, as they lie low and are otherwise very marshy.