G.—ENGINEERING 121 
tion that on horizontal joints there should be a vertical compressive stress 
at the water face equal to not less than the water pressure at the joint. 
Such compression in the masonry would tend to prevent access of water 
to any joint or crack. 
The late Sir Benjamin Baker, in 1904-5, at the time when I was his 
chief assistant, was faced with the problem of raising the Aswan dam. 
(At present the dam is being raised a second time.) ‘The investigations 
after the disaster in France had shaken confidence in the accepted method 
of gauging the stability of a dam, and in 1904 a memoir was published 
entitled Some Disregarded Points in the Stability of Masonry Dams, by 
Prof. Karl Pearson and Mr. Atcherley. By mathematical investigation 
the authors concluded that although a dam might satisfy the usual condi- 
tions regarding the stresses on horizontal planes, it might still be subjected 
to dangerous tensile stresses on vertical planes in the vicinity of the 
downstream toe. That conclusion seemed most unlikely to engineers 
interested in the subject, but however incredible it might seem, it de- 
manded attention as coming from so eminent a mathematician. In 
arriving at their results, the authors of the memoir based their calcula- 
tions on an assumed law governing the distribution of shearing stress 
across the base. The unsatisfactory state of affairs could only be cleared 
up by determining the distribution of shear and other stresses. 
Jointly with my friend the late William Gore, I made an attempt to do 
this, and we embarked on a series of elaborate experiments with india- 
rubber models. 
It is just thirty years ago, and although I was busily engaged assisting 
Baker, and Gore was assisting the late G. F. Deacon, we did the whole 
work in our own time independently of the office work. Rubber had been 
used already by several workers as an elastic medium for strain investiga- 
tions, but I think the methods we devised for getting at the stresses were 
an advance on earlier work. Our investigations were described and 
discussed at the time at the Institution of Civil Engineers * and in 
Engineering,’ in which journal there was a long correspondence on the 
subject. 
The models were made of slabs of rubber 1 in. thick with a smooth 
white surface, and shaped to represent the transverse section of a dam. 
The model was strained by weights carefully adjusted to represent the 
water pressure against the face and the weight of the masonry, on the 
assumption that the masonry had a specific gravity of 2:25. ‘The model was 
divided into sections, and the ‘ masonry weights ’ were hung on transverse 
pins put through the rubber. Plates pulled by cords against the water 
face represented the water pressure. To ensure the exact relative posi- 
tions of the loads, the model was so shaped that when fully strained it 
had the correct profile. A network of lines was ruled on the rubber, 
and large-sized photographs on plate-glass were taken under the strained 
and unstrained conditions. Corresponding lengths on the two negatives 
could be measured accurately, and from them the strains and stresses were 
calculated. The intensity of shear at various points was measured by 
3 Minutes of Proceedings Inst. C.E., vol. 172, 1907-8. 
4 Engineering, 1905, 1907. 
