# Man's physical universe

214 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME

of Automotive Engineers in America. The usual method of measuring

viscosities is to determine the time required for a liquid to flow

through a given tube or orifice at a standard temperature.

A lubricating oil needs to have a sufficient viscosity to maintain a

film of oil between the lubricated surfaces. Too low a viscosity results

in inadequate lubrication, while too high a viscosity results in needless

loss of power due to friction in the oil film itself. Oils used in cold

weather should be of lower viscosity than those used in the summer

because of the fact that the viscosity increases at lower temperatures,

sometimes to the extent that cars cannot even be started.

The viscosity of the body fluids normally remains fairly constant.

When a person has a fever, the viscosity of the blood is less, and, other

things being equal, it is therefore easier for the heart to pump the blood

through the circulatory system.

The viscosity of a liquid has its counterpart in that of gases. Both

gases and liquids offer resistance to the passage of solids through them.

This resistance, or friction, increases as the velocity of the moving

solid increases. It is only within recent years that attention has been

paid to the question of decreasing the resistance of the air or water

on a fast-moving body. Engineers first became interested in the problem

when they sought to increase the velocity of steamships and airplanes.

Modern airplanes have been designed to offer the minimum

resistance to air. More recently this same principle of streamlining

has been applied to railroad trains and automobiles.

Streamlining of bicycles, toy wagons, teakettles, and houses is, of

course, merely a fashionable trend in design.

Engineering tests with a modern car have shown the effect of different

rates of speed on the gasoline mileage:

20 miles per hour

30 miles per hour

40 miles per hour

50 miles per hour

60 miles per hour

70 miles per hour

21.7 miles per gallon

19.9 miles per gallon

18.0 miles per gallon

16.0 miles per gallon

13.8 miles per gallon

11.4 miles per gallon

A portion, at least, of this decrease in efficiency is due to the increased

resistance offered by the air at higher speeds. The resistance offered by

liquids is, of course, greater than that of gases, so that steamships have

long been so designed as to ofi'er the least underwater resistance.

The resistance to the flow of liquids is well illustrated in a waterdistribution

system.

The height to which water will rise when in motion depends on the

resistance offered by pipes of different diameters.

The velocity of the

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