Thomas Stevenson, Esq., on Harbour Screw-Cramps. 45 
the chains or rods. After a stretch of sufficient length has 
been completed, the bars or chains 6 being slackened, the cross 
rod a is removed, and the bars or chains, being now free, are 
withdrawn. A new cross rod having been previously inserted 
at some convenient joint, the chains are now attached, and 
drawn tight as before. In this way, the open end of the work 
is at each course strongly connected to the stones farther 
back, without any loss of iron, or expenses for boring or 
cutting the stone. 
The cramp, represented in figs. 4, 5, 6,7, and 8, is de- 
signed more particularly for a talus or sea-wall. In this 
case, one end of the chain is made fast to a ring bolt or lewis 
a, in any stone which is at a sufficient distance from the 
open end, while the kneed abutment or anchor-plate c, is 
placed upon the last or outermost stone of each course, 
when the chains 6 are tightened by means of the draw-screw 
e.* This apparatus, in the event of a gale coming on sud- 
denly, could be applied in the course of a few minutes. In- 
stead of the ring-bolt or lewis, a bar or pinch could, in such 
an emergency, be easily driven down between one of the 
joints of the talus wall. This kind of Screw-cramp could, in 
cases of great exposure, as a farther security, be also applied 
transversely or across the courses, so as to extend from low 
to high water mark. 
It is proper, perhaps, in conclusion, to mention, that, in 
order to render the last or outermost stone as steady as 
the others in the same course, no more strain need be ap- 
plied to the chains than is sufficient to make up for the de- 
fect of friction, caused by its not being in contact on all its 
four sides with other stones. The amount of the strain re- 
quired for the chains should therefore, in each case, be pro- 
portioned to the number of the sides of the last or outer- 
most stones which are exposed. It must be borne in mind, 
however, that, although the chains were slack, the last stone 
could hardly be carried away unless the chains were broken. 
EDINBURGH, November 25, 1848. 
* As an additional security, another kneed plate h, may be inserted below 
the stone, and screwed up tight by the draw-screw 1. 
