122 REPORT— 1872. 



arose from small differences between the individual planes, probably differences 

 in the thickness or nature of edge at their ends, diminishing or increasing the 

 body-resistance. 



The initial edge of the planes tried was formed by the sheath or cutwater 

 befoi-e mentioned, which held the plane in its place; and this was tapered in 

 horizontal section, fron? tbe thickness of the plane to about jL inch, the 

 extreme edge being rounded, as shown full size in Plate III. (marked a a). 

 At the tail edge there was no sheath, and the board was simply cut off square, 

 as shown in the same figure. Clearly, if there was any great body-resistance 

 due to these blunt ends, as the line a a on Plate VII. seemed to imply, then 

 the slight differences in thickness which existed between the different planes 

 might be sufficient to account for the discrepancies between their results. 

 Accordingly experiments were tried with certain of the planes, of various 

 lengths, already tried, but substituting a cutwater having a very thin edge, 

 tapering to the thickness of the plane in 6 inches, as shown in horizontal 

 section on Plate III. (marked b). This alteration produced a slight diminu- 

 tion in the amount of the resistance of all the planes, but rather a greater 

 reduction in the short planes than in the long ones. The difference, though 

 almost too small to show, is indicated by the line a! a', Plate VII. Experiments 

 were then tried with the tail edges of the planes, tapered in the same manner 

 as the initial edge (c, Plate III). This was tried with the result (indicated by 

 the plain line a, Plate VII.) of a ver}- much larger reduction in the resistance ; 

 and this reduction was likewise relatively greater in the shorter lengths. 



In order still more correctly to obtain a value by which the results of all 

 the planes already tried might be corrected for the varying thicknesses of 

 their after edges, an experiment was tried with one of the thickest planes 

 (2 feet 6 inches long) by reducing the thickness of its square-edged tail to 

 that of the thinnest of the planes (a reduction of perhaps -^^ inch). The 

 results of this are shown on Plate V., where the resistance of the plane with 

 the thick after edge is shown by the line marked a'", and the resistance of 

 the same plane with the tail thinned by the line marked a", and that with 

 the tail tapered to a perfectly sharp edge by the line a. 



It is worth notice on this point that the difference between the reduction 

 of resistance found throughout these experiments on sharpening the tail edge, 

 and that foiuid on sharpening the initial edge, seems to be entirely owing to 

 the difference between the original foi-ms of the blunt cutwater and the blunt 

 tail (the former being partly tapered and rounded, while the latter was cut 

 quite square). This was proved by trying the above-mentioned 2-feet 6-inch 

 plane, with its after edge sharpened like the original blunt cutwater. These 

 results are shown on Plate V. by the dotted lines a' a', a a, from which it 

 may be seen that the difference between the rounded edge and the perfectly 

 sharp edge is comparatively small. 



The application of the corrections obtained as I have here described to the 

 results of the experiments previously made gave diagrams of resistance in 

 terms of length similar to the line marked a" a", Plate VII., the discrepancies 

 between the jolanes disappearing when thus corrected. But these experi- 

 ments did not include any made with shorter planes than 1 foot 6 inches, 

 that being the shortest length that could be constructed with the existing 

 cutwater ; and in order to make it complete, it was most desirable to extend 

 the lines as far as possible towards the zero-point of length and of resistance, 

 by trying very short and thin planes, so as to test the nature of the curve 

 close to the origin, and discover whether any body-resistance remained owing 

 to the thickness of the plane hitherto tried. 



