WILLIAM G. STEVENSON. 197 
sult of modern inquiry, earthquakes are to be attributed 
to many causes acting in such a complex manner that, 
as yet, it is impossible to determine the exact dynamic 
value of any one cause in their production. It seems to 
be well established, however, that ‘‘telluric heats, 
solar heat and variations in gravitating influences ”’ are 
primary, and often the immediate cause ; while, as de- 
pendent or secondary causes, we recognize ‘‘ expan- 
sions and contractions of the earth’s crust, variations in 
temperature, barometrical pressure, rain, wind, the at- 
tractive influences of the sun and moon in producing 
tides in the ocean or the earth’s crust, variations in tbe 
distribution of stress upon the earth’s surface caused by 
processes of degradation and the attractions in the posi- 
tion of isogeothermal surfaces.” 
To one accustomed to the use of microscopes or tele- 
scopes, it is well known, from many unpleasant ex- 
periences, how small a cause, comparatively speaking, 
is required to produce earth tremors which serious- 
ly interfere with the accurate use of these instruments. 
When reflecting a star in a tray of mercury at the 
Greenwich observatory, it has been found that the tread 
of people, from London to Greenwich park, caused 
earth tremors which so disturbed the surface of mercury 
as to render it impossible to complete the observations 
until these earth tremors were overcome by mechanical 
means. 
Prof. Paul, when making a survey at Washington to 
find a suitable site for our naval observatory, found that 
a passing train of cars caused earth tremors which were 
readily noticed at the distance of a mile; while Lieu- 
tenant-Colonel Palmer, when at New Zealand making 
observations on the transit of Venus in 1874, found the 
vibrations of the earth, caused by a railway four hun- 
dred yards distant, so disturbing that he was obliged 
to intrench his instruments in pits three and a half feet 
S81 
