156 REPORT — 1867. 



On the Construction of the Lifeboat. By Professor Macdonald. 

 Instead of the common form of the hoat, with a sharp keel, the author suggested 

 the more ample and expanded form of the head of the whale, but rising high out 

 of the water at the bow, having bluff sides, but ending in a long clean run aft, nar- 

 rowing towards the stern, where the moving paddle-wheels or Archimedean screw 

 should be placed. 



On an Lwproved Paddle-ivlieel. By Professor Macdonald. 



On Iron Floating Forts, Iron Harbours, and other Fhatinc) Structures ; and 

 on Daffs Method of Construction of Iron Fabrics. By S. J. Mackie. 



On the Theory of Diagrams of Forces as applied to Boofs and Bnclges. 

 By J. Cleek Max^a-ell, F.R.SS. L. 4- E. 



A roof is made up of a series of vertical frames. A diagram of forces is a figure 

 consisting of straight lines, which represent, both in magnitude and direction, the 

 tensions and pressures in the different pieces between the joints of the frame. The 

 pieces of the frame and the weights acting on it are denoted by capital letters, and 

 the corresponding lines of the diagram by small letters. The diagram is constructed 

 by the following ride, which is sufficient for the purpose : — The frame, including the 

 vertical lines representing the weights, and the diagrams of forces, are reciprocal 

 figures, such that every line in the one is parallel to the corresponding line m the 

 other, and every set of lines which meet in a point in the one figure foi-m a closed 

 figure in the other. It follows from this that the weights, which are all vertical 

 forces, are represented by the parts of one vertical line. The first extension of the 

 principle of the diagram of forces was made by Dr. Eankine in his 'Applied 

 Mechanics.' The theory was generalized by the author in the Philosophical 

 Mao-azine in April 1864. In the present paper it is shown to be connected with 

 the'^theoiy of reciprocal polars in solid geometry, and rules for the construction of 

 diaorams are o-iven. The advantage of the method is that its construction requires 

 only a parallel ruler, and that every force is represented to the eye at once by a 

 separate line, which may be measured with sufficient accm-acj' for all purposes with 

 less trouble than the forces can be found by calcidation. It also affords secuiity 

 ao'ainst error, as, if any mistake is made, the diagram cannot be completed. 



On Covered Life-Boats. By George Maav, F.S.A., F.G.S., Sj-c. 

 In advocating the employment of closely-covered boats for shipwreck service 

 from vessels at sea, the author pointed out the different requirements from those 

 eno-ao-ed in coast service. In rescuing from the land the crews of coast-wrecked 

 vessels, the power of navigation and locomotion was of paramount necessity, 

 whilst in the case of vessels foundering at sea, the means of locomotion was alto- 

 gether of secondary iniportance to such qualities as would provide for the cer- 

 tainty of floatation. To ensure this, the author proposed a light boat-shaped iron 

 caisson, perfectly covered, except a man-hole for access, which woidd be water- 

 tight when closed, and two openings for ventilation. A self-acting valvular 

 an'angement was described, by which water would be perfectly excluded during 

 the occasional breaking of a wave, whilst allowing a free passage of air when 

 not submerged. 



On a new Mode of constructhiy the Surface of Streets and Thoroughfares. 

 By Joseph Mitchell, C.E., F.B.S.E. 



On the Use of Moveable Seats for Slide- Valves. By James 11. jSTapier, F.B.S., 



Murine Engineer, and W.' J. Macquoen Eakkine, C.E., LL.B., F.R.S. 



The great practical convenience of the slide-valve and link-motion as_ means of 



varying the rate of expansion in steam-engines is well known. An objection to 



their use, however, arises from the fact, that the points of admission, cut-off, 



