fulcrum. Mr. Gregg, Nick, and the children were gathered about the lever, each one pushing down, and the stone began to move, as the top end of the lever came down, much to the delight of Jessie and George, who kept shouting, "There she goes! Up she goes!" Finally the great stone turned over on the plank, and was moved to near the centre. Now came the labour of getting the monster down to the bank. This was made easier by raising the ends of the upper planks under the stone and inserting another roller, five or six feet from the end. The planks holding the stone were now resting on rollers, as seen in Fig. 3, and it was found easy to move, but in order to get it to the bank of the river the "runway," or lower planks, had to be laid down that distance; this would take too many planks, so it was decided to lay only a second length on the ground, and then when the load had travelled to this length, the plank behind the stone should be carried forward and laid down again. This was continued until the load was slid into the water. Mr. Gregg called the children and told them to push against the stone, and they all were filled with wonder to see this great stone move along so easily on the rollers.

Fig. 3. Moving rock on rollers

Fred and Nick got more rollers to put between the planks as the stone was pushed forward, for, of course, these were continually coming out at the rear end of the loaded planks. The rollers had also to be watched and kept square across the plank or they would slide, making it hard to move the load.

When the river bank was reached, Fred and Nick made a rough slide of old timber down to its side from the trench. Getting the lever properly adjusted under the planks and stone, the latter was turned over on the slide, when it plunged into the river with a great splash, causing the water to fly and sprinkle each one of the workers, much to the delight of George, who thought it fine fun to see his father, Fred, and Nick get a wetting.

It was decided that the stone as it lay in the water should form the end of the pier for the boat, as it was nicely situated and the proper distance out, being about a foot out of the water at high tide. The other stones were easily removed from the trench by Fred and his man, and were either rolled or wheeled down to the river, where Nick built them as well as he could on both sides of the big rock, leaving a hollow space between the walls, to be filled in afterward with small stones, mortar, and broken bricks, for the making of a good, strong boat pier.

Mr. Gregg then took out his note-book and pencil, and figured out the quantity of cement, sand, and gravel required to complete the cement work. He found there was good sand, clean and sharp, on one corner of the new lot. A big pile of gravel and broken stones out on the street had been left over from the building of a two-story concrete house nearby, so he concluded to buy it, if not too dear.

Measuring the trench, he found it to be 300 feet long, by 4 feet wide, making a surface of 1,200 feet to be laid with cement, concrete, and gravel, or broken stones. He calculated that every 100 superficial feet of the concrete walk would require about a barrel and a third of Portland cement; and that the top dressing of cement and sand, or fine crushed stone, required another third of a barrel; which totaled up to 20 barrels, all told. The concrete to be used was to be proportioned as follows: One part of cement, two parts of good, clean sand, and five parts of gravel, or broken stones, which should be small enough to pass through a ring having a diameter of not more than two inches. This mass should be well mixed, dry, on a wooden floor or movable platform, and then wetted just enough to have stones, sand, and cement, well moistened. All should be again mixed before being placed in the trench, and it should not be thrown in place, but shovelled in gently.

Mr. Gregg ordered the cement by telephone, to be delivered at once, either in barrels or bags; and he got into communication with the owner of the gravel, and bought the whole pile. He then engaged a team of horses, wagon, and driver, to commence work the next day. By this time Nick had gone home, and the children came rushing into the house, anxious to tell their mother all the work they had done that day.

The keen appetites of the younger folks gave positive proof of their having earned their supper, by actual work, and, when the meal was over, the father invited Jessie and the boys into his little room. George was asked to take with him his portable blackboard, some chalk, and a ruler, and all marched into their father's den.

"Now," said Mr. Gregg, "I have often told you I would explain to you some things about the mechanical powers, and this seems to be the most appropriate time to begin, as you have fresh in your minds the application of the lever as we used it to-day in raising and moving the big rock. I am glad to see that Fred grasped the idea so readily, for that encourages me to let him use his own judgment while doing this job.

"The lever is known to accomplished mechanics, as 'the first mechanical power', and Archimedes said of it, if he only had one long and strong enough, together with a suitable fulcrum, he could, alone, lift the earth from its place.

"This Archimedes was a celebrated Greek philosopher and mathematician, who lived from about 287 to 212 B. C. The discovery of the law of specific gravity, which I will some day tell you about, is attributed to him. I think George can tell you something about this great man, as I saw him and Jessie the other day reading Plutarch's 'Lives,' in which he is mentioned.

Fig. 4. Principle of lever and fulcrum

"A lever may be formed of any strong, stiff material, wood, iron, steel, or similar stuff, and it may be of any length, or dimensions, according to the purpose for which it is to be used. In theory, it is supposed to have no weight, and is simply figured as a straight line having neither breadth nor thickness. In practice, however, a lever may be a handspike, a pry, a crowbar, a fire poker, a windlass bar, or any other appliance or instrument that can be used for prying. While we may not know the proper name of the little steel tool the dentist employs when preparing one's teeth to receive the filling, by cleaning out the cavities, we are safe in calling it a small lever. When your mother stirs the fire in the grate, she makes a lever of the poker, and bars of the fireplace become fulcrums. The fulcrum is the fixed point on which the lever rests when in use. The force applied is called the power and the object to be acted upon is called the weight. The spaces from the power and the weight, respectively, to the fulcrum, are called the arms of the lever. There are three different ways of using the lever, according to the relative positions of power, weight, and fulcrum. This rough sketch I am drawing on the blackboard (Fig. 4) shows the lever being used to raise one end of a heavy stone. Suppose W is a big rock, C will be the fulcrum, B