57. Q.—­Has not M. Morin, in France, made some very complete experiments to determine the friction of surfaces of different kinds sliding upon one another?

A.—­He has; but the result does not differ materially from what is stated above, though, upon the whole, M. Morin, found the resistance due to friction to be somewhat greater than it has been found to be by various other engineers.  When the surfaces were merely wiped with a greasy cloth, but had no film of lubricating material interposed, the friction of brass upon cast iron he found to be .107, or about 1/10th of the load, which was also the friction of cast iron upon oak.  But when a film of lubricating material was interposed, he found that the friction was the same whether the surfaces were wood on metal, wood on wood, metal on wood, or metal on metal; and the amount of the friction in such case depended chiefly on the nature of the unguent.  With a mixture of hog’s lard and olive oil interposed between the surfaces, the friction was usually from 1/12th to 1/14th of the load, but in some cases it was only 1/20th of the load.

58. Q.—­May water be made to serve for purposes of lubrication?

A.—­Yes, water will answer very well if the surface be very large relatively with the pressure; and in screw vessels where the propeller shaft passes through a long pipe at the stern, the stuffing box is purposely made a little leaky.  The small leakage of water into the vessel which is thus occasioned, keeps the screw shaft in this situation always wet, and this is all the lubrication which this bearing requires or obtains.

59. Q.—­What is the utmost pressure which may be employed without heating when oil is the lubricating material?

A.—­That will depend upon the velocity.  When the pressure exceeds 800 lbs. per square inch, however, upon the section of the bearing in a direction parallel with the axis, then the oil will be forced out and the bearing will necessarily heat.

60. Q.—­But, with, a given velocity, can you tell the limit of pressure which will be safe in practice; or with a given pressure, can you tell the limit of velocity?

A.—­Yes; that may be done by the following empirical rule, which has been derived from observations made upon bearings of different sizes and moving with different velocities.  Divide the number 70,000 by the velocity of the surface of the bearing in feet per minute.  The quotient will be the number of pounds per square inch of section in the line of the axis that may be put upon the bearing.  Or, if we divide 70,000 by the number of pounds per square inch of section, then the quotient will be the velocity in feet per minute at which the circumference of the bearing may work.

61. Q.—­The number of square inches upon which the pressure is reckoned, is not the circumference of the bearing multiplied by its length, but the diameter of the bearing multiplied by its length?