and thus present a new point to the wheel. Keeping the wheel true is important for the operator to observe, particularly so when he comes to make a final finish. The wheel should be traversed by the diamond at a uniform speed, rather slowly in order to give it time to cut away the particles. If it is desired to do rapid cutting, it will be found proper to pass the wheel by the diamond more rapidly thus making a rougher face on the wheel.

The number of times that the face of a grinding wheel has to be trued depends entirely on the character of the work being finished and the kind of wheel used. There are some wheels that wear away rapidly enough so that little truing is necessary. There are also cases where a harder wheel is desirable and a hard wheel necessarily requires more truing than a soft one. Where pieces are rather large and considerable stock has to be removed, it may be necessary to true the wheel each time a piece receives its finishing Where the stock to be removed is not more than 4-inch diameter it is advisable to finish in one operation, but when there is as much as 32-inch diameter to be removed it is good practice to grind it in two operations. As stated above, it is desirable generally to present a sharp point of the diamond to the wheel in truing, but there are times when the smooth surface is preferable, particularly when it comes to producing a very fine finish; the flat surface of the diamond will tend somewhat to glaze the wheel and thus produce a better finish. A coarse wheel properly trued will produce a good finish.

The amount of wear the wheel is subjected to depends upon the operator in many cases. Never bring an emery, corborundum or other abrasive wheel suddenly against the work or the work abruptly to the wheel. The feed should be gradual, so that the sparking will start almost imperceptibly. Grinding is not intended to be a roughing process but is a method of finishing in most cases so careful manipulation of the feed control is required to prevent the wheel from "digging" in.

Speed for Wheels.-The table below designates number of revolutions per minute for specified diameters of wheels, to cause them to run at the respective periphery rates of 4,000, 5,000 and 6,000 feet per minute.

The medium of 5,000 feet is usually employed in ordinary work. but in specific cases it is sometimes desirable to run them at a lower or higher rate according to requirements. We recommend a number of revolutions equivalent to a surface speed of 5,500 feet. This does not indicate that they cannot be run at a higher or lower speed, but that it is a good average speed to produce good results. To allow an ample margin of safety it is recommended that wheels should not be run at a surface speed exceeding 6,000 feet. Every shop should have a speed indicator in order that the speed of its grinding machinery may be known.

Grading of Landis Grinding Wheels.

NUMBERS

The grains are numbered according to the number of meshes per lineal inch of the sieve through which they have passed. For example, No. 30 is a grain that will pass through a sieve having thirty meshes to the inch, but will not pass through a sieve having

irty-six meshes. The fineness, or number, of the emery or corunum used in making a wheel determines the "number" of the -heel.

The grains (and similarly the wheels) are numbered as follows: ), 12, 16, 20, 24, 30, 36, 46, 54, 60, 70, 80, 90, 100, 120, 150. In is list the lower numbers indicate the coarser grains, the higher umbers, the finer ones.

When ordering wheels, be sure to specify diameter, shape, thickess, size of center holes, the grade and grain or description of aterial to be ground and speed proposed to run the wheels. If ossible, give shape number.

For grinding hardened steel and cast-iron, wheels made by what known as the silicate process give very good results, but the itrified wheel in our experience is the better for general use.

A soft wheel is less apt to change the temperature of the work r become glazed.

A wheel is most efficient when just soft enough not to glaze and ard enough not to wear away rapidly.

Use a fine grained wheel for finish, a coarse wheel to remove tock. For general grinding a 24 combination grain wheel gives xcellent results.

A good practice is to have several grades of wheels on hand est adapted for your different classes of work.

Always keep a spare wheel or two on hand for emergencies.

CHAPTER XII

USEFUL INFORMATION FOR AUTO REPAIRMEN.

Anti-Freezing Cooling Solutions-Substances Usually Combined with WaterAdvantages of Different Solutions-Electrolytic Action Not DesirableThe Best Mixture-Extinguishing Fires in Volatile Liquids—First Aid to the Injured-Schaefer Method of Artificial Respiration-The Repair Shop Medicine Chest-Remedy for Burns, Cuts and Abrasions-Wounds and Painful Injuries-Home-made Aseptic Gauze.

Anti-Freezing Cooling Mediums.-To lower the freezing point of water it is possible to add various substances and the proportions added determine the point at which the solution will congeal. Among the materials commonly used may be mentioned common salt, alcohol, glycerine and calcium chloride. The alkaline solutions produce a distinct electrical action wherever two dissimilar metals are used together in the cooling system, such as the brass tubing of a radiator and the solder used at the joints; the cast iron water jacket and the aluminum or brass plates used to close the core print holes; the aluminum pump casing and steel or bronze impeller, and at many other points which will vary with the design of the car and the materials of the components. The alcohol solutions evaporate very quickly, the glycerine solution affects the rubber hose, and the salt solutions leave an incrustation as the water evaporates. It is reasonable to expect electrolytic action when metals of different potential are used together in any alkaline solution, which are electrolytes of high value. Taking it all in all, the selection of the best solution involves a consideration of many facts and various requirements must be considered in the selection of that most suitable. Considering the qualities of such a compound it will be seen that no one will combine all the desirable

features, so in selecting the solutions the following should be kept. in mind: To begin with, and it is a highly important consideration, the solution used should have no corrosive action, nor should its use prove deleterious on the metals or rubber used in the circulating system. It must be easily dissolved in or combined with water, must be reasonably cheap, and not subject to rapid waste by evaporation, and should not be of such character that it will deposit sediment or foreign matter in the jackets, pipes or radiator water spaces. Its boiling point should be as high, if not higher than, that of water, and it should not congeal at temperatures ordinarily met with where it is used.

Substances Usually Combined with Water.-Alcohol is prepared by destructive distillation of various vegetable substances which contain starch or sugar, such as potatoes, beets and numerous grains and fruits. Any starchy material will serve for the production of alcohol and the real question is one of cost, which varies with the locality in which the manufacture is carried on. It mixes readily with water, and does not congeal at any known temperature, though its boiling point is about 175 degrees Fahrenheit, and above this point evaporation is rapid. It is a very volatile liquid and will evaporate at very moderate temperatures. The alcohol generally used is denatured by the addition of a substance which renders it unfit for drinking purposes and because of the recent removal of the government tax it may be obtained for about 60 cents a gallon.

Glycerine is obtained as a by-product in the saponification of fats in soap and candle making, and is an oily substance which will vary in color from reddish brown when crude to a colorless liquid when pure. Crude glycerine sometimes contains free acids in small quantities though it may be purified and the color removed when it is to be used for certain purposes. This substance has a much higher boiling point than water, ebullition taking place at a temperature of 554 degrees Fahrenheit. Glycerine when pure is a sweet, colorless liquid and is mixable with water and alcohol in any proportion. It is most largely used in the manufacture of nitroglycerine, though utilized to some extent in pharmacy, soapmaking, filling instruments which require a liquid seal and which