When we say that any one is strong, we l"can that he is capable of enduring physical exertion and fatigue longer than the average run of men. Many experimenters have endeavored to find an average on which to base observations as to the strength of mon; bnt many allowances have to be made for climate, temperature, food, age, and othel accidental intcrferences. It has, however, been determined that the best method of arriving at a true idea of the strength of men, is to observe the average effect produced by a laborer who continues his exertions several successive days. What is called a " dynamic unit" is then fixed upon as a standard by which to compare results, and this is 1,DOO lbs. avoirdupois carried one foot in one minute. According to M. Coulomb, a man traveling on a level road may walk 30 miles per day, or 264 feet per minute, and continue his exertion f orten hours a day. Taking the man's weight at 150 lbs., he has expended 23,760 dynamic units, or, in other words, has used the amount of strength that would have been required to transport 23,760,000 pounds one foot in one minute. If instead of walldng on a level road he ascends a staircase, the velocity is reduced to 26-4 feet per minute, and he can only work eight hours per day. In this case he has only expended 1,901 dynamic units. A person carrying a weight of 90 lbs. along a le\-el road, travels at the rate of a mile and a half an hour, or 132 feet per minute, and continues his exertion scven hours per day. The useful effect is, consequently, 4,989 dynamic units. The force which a man exerts in dragging a load has been variously estimated. Schulze ays that a man can exert a pressure of 107 lbs. for a short time, and that a man may walk at the rate of from 4 to CJ miles per hour. The most advantageous method of employing human strength is in rowing a boat. or course there are numerous exceptions to these rules of strength we have given, as they can but be an approximation to the average ; but in investigations of this kind we must be content with such results.