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PROCEEDINGS. 509 
distortion much larger than these experiments indicated, in his experi¬ 
ence in the field. 
A simple diagram with eight points marked upon it in the form of a par¬ 
allelogram, with an included figure showing the relations of the points 
after the sheet had become distorted, was then referred to in illustration, 
and the following rules announced : 
1st. A station made with three points that are on the lines of contrac¬ 
tion, the resecting lines forming nearly right-angles at their intersection, 
will give the true position in relation to all the points on the sheet. 
2d. A similar condition of right-angle intersection at the station, but 
the lines of resection forming diagonals to the lines of contraction, will 
give the worst possible position for the station. 
3d. A station made with three points on one of the “ lines of contrac¬ 
tion ” will give the correct orientation of the table. 
4th. In eliminating errors of the points due to distortion, those situated 
on the lines of contraction require no allowance, however distant. 
The errors liable to arise in conducting an extensive plane-table trian¬ 
gulation were then referred to, and a method of correcting distances 
measured on a distorted sheet was briefly explained, and the advisability 
of constructing squares on all sheets before taking them into the field was 
recommended. 
Mr. William Hallock read a paper on The Flow of Solids. 
[Abstract.] 
The question whether solids possess any of the properties of liquids, or 
what conditions will impart such properties to them, is one of ever-increas¬ 
ing interest and importance, alike to the student of molecular physics in 
general or of the earth’s crust in particular. 
The temperature rises as we penetrate the earth; hence, if no other in¬ 
fluences affect the substances, the earth has a liquid center with this solid 
crust. 
Astronomical and mechanical facts seem to demand a considerable 
rigidity. 
Thompson has even demanded a rigidity equal to that of glass or steel. 
Geological phenomena require a considerable liquid-like motion. With 
rising temperature as we penetrate the earth’s crust, we also have rising 
pressure, which probably increases the rigidity of the materials. Can 
we not satisfy the demands of both geology and astronomy or mechanics ? 
In the glaciers we have the grandest examples of the flow of solids. 
Henri Tresca proved that lead and some other substances would flow and 
follow the laws of flowing liquids. W. Spring has extended the list. 
Mousson actually liquefied ice. These observations have led many to 
advocate the idea of a liquefaction by pressure. Others, having in view 
the results of Bunsen, Hopkins, Amagat, and others, maintain that the 
melting-point is raised by pressure, the rigidity increased. Solids can be 
made to flow; hence that property cannot be used to characterize them. 
