ork of the re BEFORE APPLICATION OF CEMENT-SAND MORTAR. zle ted are de subsequent manipulation or handling tends to disturb this initial crystallization, thereby weakening the entire product. The only way by which this fault could be corrected would be to have the hydration take place in such a manner that crystallization would begin only at the moment of the actual emplacement of the material on the place where it belongs. This is what the cement gun actually accomplishes. Another advantage claimed for the gun is that only the amount of water actually necessary for hydration is used, the materials being projected with sufficient force to expel all surplus water and air, and the resulting product is denser, more homogeneous, and, consequently, more water-proof than anything yet attained by hand or machine processes. The labor involved is also said to be considerably less, while the saving in time is so great that the total cost of making and applying any plastic mixture by means of the gun is reduced to a minimum. There would seem to be an almost unlimited field for the practical use of the cement gun. Foundation work and waterproofing below grade should be well adapted to this process. As a means of coating steel to prevent rust and corrosion it should prove superior to the or dinary method of painting, for a cement coating will wear much better than one of paint. The pipe line of the New York aqueduct from the Catskill mountains is now being lined with a two-inch coating of cement and sand in this manner. The inside diameter the pipe is eight feet, eleven inches, so that a man can easily walk through it and perform the spraying process. Smaller pipes can be similarly treated in sections. of Suppose you have old and crumbling walls, fences, sidewalks or leaky roofs that need repairing, or wooden building that you want made fireproof in anticipation of a visit from the fire insurance examiner. There I will be no need of having one's premises littered up for a week or more with a lot of mortar APPLYING CEMENT STUCCO TO AN OLD BUILDING AFTER COVERING IT WITH WIRE MESH. boards and mixers, scaffoldings and similar eye-sores, as in times past. You simply send out a hurry-up call for the cement-gun who comes man, around with his little renovator, and-presto, everything is made spick and span "while you wait." It is something like sending one's clothes to the cleaner, only in this case the cleaner comes to you. It is probable that a great many people have noticed small railroad stations which had been sprinkled with sand, while newly painted, and then given the matter no further thought except perhaps to note that the buildings formed a convenient place upon which to strike matches. This is an idea that has long been employed to make such structures fireproof and to protect them from wear and tear. But WHAT MAY BE DONE IN BUILDING OUT WITH ONE APPLICATION OF THE CEMENT EXAMPLES OF TREE SURGERY. Filling holes in trees with cement and mortar by means of the cement gun. the process is slow and laborious. Imagine turning a garden hose on the same. building, and imagine further that instead of water a stream of liquid cement issues from it, and it will be seen how quickly the same job might be finished. Tree surgery is another thing that seems destined to undergo a revolution if all that is claimed for the cement gun proves practical. There are hundreds of torn, cracked and decaying trees in the private yards, on the streets, and in the public parks of every city that could have their years of usefulness and ornament doubled if they were given proper attention. The trouble is that it doesn't pay to go around and patch them up with hand-made cement or plaster, as it might be termed. Besides, this is an instance wherein it is especially necessary that the plastic product should be applied with the least possible delay after mixing on account of its subsequent constant exposure to the elements. With the cement gun this work can be done so quickly and cheaply that its use in this connection should become very general. In the case of fences and other similar structures, and the interior and exterior walls of entire buildings, a special design of frame work is required. This consists of a wire mesh of the required size, with a wooden backing. The cement is shot upon this and after it has hardened the wooden backing is removed, leaving what is practically a reinforced cement wall. Such walls are claimed to be as fire-proof as they can be made by any known method. In order to demonstrate this, a small building of two-inch by four-inch wood was constructed. Both the inside and outside were covered with building paper and over this was placed a wire mesh reinforcement with one inch of cementsand stucco, leaving a four-inch air space between the walls. A scientifically built fire was then allowed to burn in it for fifteen minutes, after which buckets of water were thrown on the inside hot walls, with the result that only a little of the surface scaled off and not a single crack developed. The outer coating of the rear wall was left off in order to determine whether or not the wooden studding would be affected by the heat, and it was found to be not even charred. A comparative test to determine the respective breaking strengths of handmade and cement-gun made bricks, the latter composed of one part cement to three parts sand, was made after both had been exposed to moist air for one day and immersed in water for twelve days. The hand-made brick broke at 303 pounds, and the cement-gun-made brick at 533 pounds. The cement gun is, of course, not adapted to the use of concrete or other mixtures in which coarse gravel or stone forms one of the ingredients. But a solution of this problem may be found in the construction of a "concrete cannon," and future generations may witness the spectacle of whole towns being literally "bombarded" into existence in the same time that it now takes to erect one building of moderate proportions. For my part, people R. HUGH M. SMITH, biologist and Deputy Fisheries Commissioner, sat at his desk in Washington, holding in his hand a small rectangular piece of glass, upon which was fastened a single scale of one of this year's run of Potomac shad. He was examining it with a magnifying lens. "From this scale," he said, "I have learned more in fifteen minutes about the life history and habits of shad in general than all that I previously knew on the subject put together." It seemed a remarkable statement. But Dr. Smith went on to explain that back of his remark there lay a most important new discovery, in the light of which it will henceforth be possible to ascertain with exactness and certainty the history of any individual fish. For, as has now come to be known, each fish carries on its body the story of its life, told in cipher, as one might say, and this story is repeated word for word, so to speak, on every one of its scales. The language of the fish scale is entirely new to science, but now that biologists have found out how to read it, there is no further difficulty, inasmuch as it is practically the same for all kinds of fishes. Revealing as it does the details. of the life history of various species, the discovery has a most vital bearing upon the interests of the commercial fisheries, to which in the long run it will be worth |