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Aviation History
1914
1914 - 0652.PDF
LTSCHT] JUNE 19, 1914. HOW TO READ A GRAPH. SEVERAL correspondents have asked us to give an explanation of how to read the graphs that frequently accompany articles in FLIGHT, and which are in general much used for the illustration of scientific facts. A graph is a pictorial way of illustrating the relationship of one thing to another that depends upon it. For example, suppose the resistance of a certain object is known to vary as the square of the speed, it is possible to show this relationship pictorially by means of a graph. The way in which a graph is drawn is to take a piece of squared paper (Fig. 1), and to use the lower edge as a scale for measuring one of the variable quan- ance will be opposite the figure 100 on that scale. Above 10 and opposite 100 we therefore put a little circle or a dot. Similarly fer the next division en the scale, the square of 20 is 400, so we put another dot above 20 and opposite 400 on the resistance scale, and so on opposite to 900 above 30 and opposite 1,600 above 40. Through the series of dots we draw a line, which is called a " graph." It passes through all the intermediate positions 1600 1S0Q 1400 1300 ieoo 1100 1O0C 000 700 coo 50c 400 300 £?00 100 1 / 1 / / / / / / / 1 r i f 1+ # -A . ~rJ • Fig. 2. O 10 20 30 40 iO Fig. 1. tities. The vertical edge is used for measuring the other variable quantity. Let us, for the sake of example, suppose that we want to illustrate the relationship represented by a resistance that is always numerically equal to the square of the speed in miles per hour. In this case we will use the horizontal scale for the purpose of measuring the velocity, and each division shall represent, for convenience, say, 10 miles an hour. Similarly the vertical scale will be a measure of the resistance, and for convenience each division shall be, say, 100 lbs. Taking the first division on the scale of velocity, the figure is 10, and the square of 10 is 100. The point corresponding to a velocity of 10 on the scale of resist ed 2 <* 6 8 IO 12 14 16 IS SO INCIIIWION of CtyORD Fig. 3. that we did not calculate, and thus in a simple accurate manner it completely presents the relationship between the two quantities speed and resistance. Now being told that the resistance varies as the square of the speed may or may not have conveyed an adequate impression on the mind, but seeing this graph leaves absolutely no excuse for anyone to fail to recognise the extreme rapidity with which the resistance increases at high speeds. The steepness of the latter part of the curve shows that at a glance, and it is by studying the slopes of graphs that we are able to bear in mind the characteristic effects produced by various relationships that have experimentally been proved to be true. In the making of tests, graphs are of the utmost utility 652
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