gives positive indication of premature ignition due to excessive spark advance.

At the other hand, if the spark lever is not as far forward as it should be, the explosion may be late because of the "retarded spark." If the spark occurs late in the cycle, the charge is not fired until the piston has reached its highest point and after it has completed a small portion of its next downward movement. As the point of maximum compression is passed and the piston moves down in the cylinder, the size of the combustion chamber augments and the gas begins to expand again before it ignites. Owing to the moderate compression the power resulting from explosions is less than would be the case with a higher degree of compression. To secure power it is necessary to supply more gas to the cylinders. Driving with a retarded spark produces heating of the motor and is wasteful of fuel.

For ordinary running the spark lever is usually placed about midway of its travel on the sector, and as a general rule an engine with magneto ignition does not require the frequent manipulation of the spark necessary when current is produced by a battery. As the engine speed increases the current produced by the magneto is proportionately augmented, and the spark lever need not be advanced from the center position except under conditions which permit of exceedingly high speeds.

The diagram presented at Fig. 26 was furnished by the Cadillac Motor Car Company to owners of its four-cylinder cars, and shows the position of the spark and throttle levers to obtain various engine speeds when the car is on the direct drive. At five miles per hour the throttle is practically closed and the spark lever has been advanced about a quarter of the way down the segment. To obtain a speed of eight miles per hour the spark lever is moved to the point on the steering wheel sector indicated by the letter C. The throttle lever is not disturbed. Moving the spark lever about two-thirds of the way on the sector will increase the speed of the car to nine miles per hour. From this point speed ratios are augmented by moving the throttle lever and the car speed increases progressively as the amount of gas supplied the engine is augmented. For higher

speeds than twenty-five miles per hour the spark and throttle levers are moved toward the end of the sector and it is good practice to advance both in conjunction beyond this point.

How Greatest Fuel Economy Is Secured.-Summing up, it will be evident that the greatest economy of fuel consumption will result when the car is driven with as little throttle opening as possible and with the greatest spark advance the motor speed will allow. To obtain maximum power, as in hill climbing on the direct drive, the spark lever should never be advanced beyond center and the throttle should be opened as wide as possible. For extreme high speeds the throttle should be advanced to a point about midway of its travel before the spark is advanced beyond that point. If this does not give the required increase in speed, the spark lever should be advanced as far as possible and the amount of gas increased, by moving the throttle lever from its central position to the extreme position on the sector. Control lever placing varies on nearly all cars, but the most common position is on top of the steering column, where they are convenient to operate and very accessible. In some cars the spark and throttle levers may be placed under the steering wheel and on one side of the steering post, one being located above the other. In other vehicles, they are disposed under the wheel and on opposite sides of the steering post. Some designers do not furnish variable spark when a magneto is used for ignition. The magneto contact breaker is advanced to the point where the best operation under average conditions is attained, and motor speed regulation is entirely by using the throttle lever or accelerator. Many battery ignition systems have automatic spark advance mechanism in the timer-distributor.

Controlling Cars with Friction Transmission.-After the engine has been started the next point is to put the automobile in motion. The means for obtaining the various speed ratios will determine the steps that should follow. When a friction or old style planetary transmission is installed the control is very simple and a single hand lever suffices to furnish all desired speed ratios. One hand lever at the side of the car serves to move the drive member to its various positions on the face of

the driving member. One foot pedal is employed to bring the friction disks together and establish driving contact between them when the proper speed position has been selected with the hand lever. The other pedal is used to apply a running brake at the rear wheels. Motor speed is regulated by spark and throt tle levers on the steering wheel. Friction drive cars are rare at the present time, only one make, the Metz, being used extensively.

With this form of control the friction pedal is released before the engine is started, and as this breaks the driving connection between the friction disks the engine can turn without moving the vehicle. After the motor is started in the manner previously indicated, the speed changing lever is placed at a position about midway in its travel. This gives one of the lowest speed ratios. To start the car the friction pedal is pressed with the left foot until sufficient pressure exists to cause the driving member to turn the driven wheels and transmit the engine power to the rear wheels. After a certain degree of headway has been attained, the friction pedal is allowed to return to its free position and the hand lever is pushed forward a few inches to give a slightly higher speed. The friction pedal is again depressed and when sufficient pressure is exerted the car will move forward at a higher speed. The farther forward the handle is placed the higher the vehicle speeds, and if the handle is brought back beyond a central position a reverse motion is obtained.

The friction pedal may be locked at any desired point by tilting the foot pad up by raising the heel. When it is desired to stop the car the friction pedal is released by bearing down on the lower portion of the foot pad, which loosens the ratchet lock, and then pushing on the brake pedal. It is important that the friction pedal be applied gradually and that it is not pressed down any farther than is necessary to drive the car. The amount of pressure will depend on the road conditions, and the lighter the degree of pressure the less wear will take place on the friction wheel fiber ring. When on a hill, or in sand, the friction pedal will have to be pushed up harder than when the car is driven on a level highway with a good surface.

Before the hand lever is changed from one position to another the friction pedals should always be released. An emergency braking effect may be obtained by pushing the hand lever in reverse position and applying the friction pedal if the car is going forward, or vice versa, if the car is traveling in a reverse direction. One of the advantages of the friction transmission is that it is difficult to injure it by careless handling because there are not gears to be stripped, as none are used. The transmission is practically noiseless and speed changes are effected easily.

Planetary Gearset Control.-One of the advantages of the planetary gearset, when applied in the two speed forward and reverse forms as in early Maxwell cars, is that the method of obtaining the various speed ratios is very simple and easily understood. The speed changes are obtained by a single hand lever and the hub brakes are applied by the usual form of pedal. Five positions of the handle give two neutral points, one reverse motion and two forward speeds. Ordinarily the lever is in an approximately vertical position and is at the neutral point between the reverse and slow speed. When pulled back from this position a reverse motion is obtained. If pushed forward the slow speed gears are put into action. Moving the hand lever from the slow speed position forward gives the second neutral point, while the high speed or direct drive is obtained by pushing the lever to the extreme forward position. The lever must be held in reverse position but can be locked into low and high speeds.

When running the car under conditions where it is not necessary to go into the reverse the lever may be pulled from the high speed position to the neutral point between high and slow speeds. If the car is stopped it can be easily started forward again by pulling the handle back into slow speed from neutral position and then forward to engage the direct drive. If the handle is pulled way back out of high speed into neutral position between slow and reverse, either of these ratios may be easily obtained. A point necessary to consider when operating a planetary transmission is that the slow and reverse speed must be

applied gradually and that the engine be speeded up pretty well before either reverse or slow speed bands are tightened. After the car has attained a certain degree of momentum on the low speed the lever should be put forward into the high speed position gradually in order to avoid the sudden jump which always obtains when changing from the low to the high speed of a twospeed car. This jump is caused by a sudden acceleration due to the higher gearing, provided by the direct drive position which is much higher than the maximum speed permitted by the slow speed gears.

Operating Sliding Gearsets.-Two sliding gear systems are fitted to motor cars, but at the present time the progressive system of control has been almost entirely superseded by the selective system. The principles of operation are practically the same as relate to clutch operation and gear engagement, but in the progressive system it is necessary to move the gear-shift lever from one end of a segment to the other to obtain the range of speed. In the selective system a gate segment or cane shift is utilized and the hand lever is moved only short distances to select the speed required. The progressive system is practically obsolete.

In operating a car with the selective method of control it is necessary that the gear-shift lever be in a neutral point if the clutch is engaged before starting the engine. After the motor has been started and is running at the proper speed, and it is desired to start the car, the first step is to release the emergency brake lever and depress the clutch pedal so that the driving connection between the engine and gearset is interrupted. With the clutch pedal depressed fully the hand lever is pushed into the slot which will give the slowest speed; then the clutch is allowed to engage slowly and the start forward is made on the lowest speed. After a certain degree of momentum has been attained the clutch pedal is again depressed and the speed lever shifted into the next higher speed ratio. The velocity of the car is thus gradually increased by moving the lever in steps from the lowest to the highest ratio. With any form of sliding gear transmission it is imperative that the clutch be released every