S R 3CRRCPCSSCPCC PCS: PC R R Fi PC PC S S Sn S Sf? C S R S

Heating and Ventilating Magazine

Fig. 89. Record of the Weather in New York City for December, 1912

The heavy line indicates temperature in degrees Fahrenheit The light solid line shows wind velocity in miles per hour

The dotted line depicts relative humidity in percentage from readings taken at 8 a.m. and 8 p.m. Arrows portray the prevailing direction of the wind

Initials at the base of the chart show weather conditions as follows: S, clear; PC, partly cloudy; C, Cloudy; R, rain; Sn, snow

Fig. 90 shows an example of double co-ordinate ruling on the same sheet of paper. The scheme of using double co-ordinates is not very well known even to engineers and it seems worthy of attention here. The solid line plotted in the general form of a curve with a flat space for each month shows the total water consumption in millions of gallons per day. The total water consumption is read from the scale of the horizontal lines as for any curve plotted by

The slanting lines are drawn after the total

rectangular co-ordinates.

94 GRAPHIC METHODS

number of gallons used per day has been divided by the number of inhabitants in the district so as to obtain a figure for the average daily consumption of water per capita. As the population figure used depends upon census records it may be necessary to get the rate of growth in the population from records as much as ten years apart. In Fig. 90 it can be observed that the slanting lines showing the rate of growth of the city are straight lines, indicating probably that the census figures were used in the drawing of these lines because j-early figures could not be obtained.