1212 REPORT—1896. 
As the result of many observations, I venture to suggest that the 
causes of the so-called ‘earth tremors’ are twofold :— 
i. Air-currents within the cases. Such currents are produced by a 
cold current of air impinging upon the outside of covers like glass or thin 
metal, but they are not likely to be produced if the covering is made of 
_ thick wood lined with thick felt. They may be produced by an inflow or 
' outflow of air through ill-fitting joints, but what is more likely, as experi- 
ment has shown, by a difference in the rate at which moisture is con- 
densed, absorbed, or given off at different points within a cover. 
2. By movements in the superficial soil outside the building in which 
the instrument is installed. These movements take place in soil whilst it is 
‘freezing or thawing, and after a heavy shower on dry ground. They may 
also be produced at the time when there are rapid but small changes in 
barometric pressure over an area the different portions of which vary in 
their elasticity and resilience. 
Although these suggestions partially destroy the value of many records 
of ‘earth’ tremors, they nevertheless leave us confronted with phenomena 
which it is the interest of all who have to work with instruments having 
‘delicate suspensions to understand more clearly, especially, perhaps, the 
reason that their frequency is so marked at particular hours and seasons. 
Diurnal Wave and Wandering of the Pendulum. 
On May 24, 1896, a drum moving a bromide film at a rate of about 
‘75 mm. in twenty-four hours was placed beneath pendulum T, and records 
‘were taken until June 15. The sensibility of the instrument was such 
that 1 mm. deflection indicated a tilt of 0''-56. 
The records yield the following results :— 
Difference 
Tarthest | Farthest | Range of ° 
Date East / West catia ee Remarks 
| 1 ah al Falla ur | 
May 24 Sieh 18 1:12 5° | Rain last night 
yy 628 5 | Wave slight. Fine, cloudy, N. wind 
» 26 3 <3 = Dull, 8. wind 
See | 24 1:12 is Fine, E. wind 
eos 8 2t 392 6 Fine, N.W. wind 
i 29 3 Glen Fair 
12 21 1:12 
+> 30 8 24 2°80 3 Dull, 8. W. wind 
=) vey 8 22 4:48 | Fine, E. wind 
June 1 ley 22 3°36 8 Fine 
ee D 3 iat 
7 | 19-24 1:68 
(23 6 | 18-24 2:24 Gash Fine 
ee al teG27 dl flies 24 1:68 Cree-| Fine 
ee ei Oe 22 2°24 aay ee Dull, W. wind 
ea 2 6_7 22 5 | Wave slight. Fine rain, W. wind 
det § 24 el en » Rain 
Er 8 4; | ,. practically straight. Fine 
a 19 Aah ES + » Dull, S. wind 
flO Bh = = », Drizzle 
” 1 l 3 ” ” ” ” 
an le 29 ge Fine 
yh ks} 22 i felled Dull 
