315 



that it must be a cause which can be affected by atmospheric influ- 

 ences. 2. They would serve to show whether the shocks that occur 

 in this country coincide in time with those occurring in distant 

 countries, and whether therefore they all originate from a common 

 source, or are otherwise connected. 3. If there were instruments 

 placed in different parts of an earthquaking district, the different ef- 

 fects of the same shock on these instruments would lead to the dis- 

 covery not only of the precise spot on the earth's surface where the 

 shocks were strongest, but likewise of the depth in the earth's crust 

 from which they emanated. 



The difficulty of devising proper instruments for registering the 

 shocks, and especially of measuring their force, was next alluded to. 

 As the instruments can only be made to operate by the movement of 

 the earth's surface, the precise effects on the instruments cannot bo 

 anticipated, as the nature of that movement is as yet very imperfectly 

 known. It seems probable from the observations made last year in 

 Perthshire during the most violent shocks, not only that there is a 

 change of level, but that there is also a slight pi'ogressive motion in 

 the part of the earth's surface affected. In the construction of the 

 instruments therefore, not only their gravitating tendency, but also 

 their inertia, must be taken into account. 



With the view of discovering what instruments could be employed 

 for the purpose of registering the shocks, and of having them set in 

 proper places, under the charge of careful and intelligent observers, 

 a committee was appointed by the British Association at their last 

 meeting in Glasffow. 



A number of instmments had been proposed, several of which were 

 explained ; and three, recently constructed to be sent to Perthshire, 

 were exhibited. Two of those exhibited were constructed on a prin- 

 ciple suggested by Professor Forbes, — being an inverted pendulum, 

 supported by a strong steel spring at the bottom, and loaded with a 

 weight near the top, so that it should vibrate in the direction of the 

 shock, whenever the floor of a house to which it may be attached 

 shall change its horizontal position. The direction and the extent of 

 vibration are indicated by marks left on paper by a soft black-lead 

 pencil rubbing against it, affixed to the upper end of the pendulum. 

 This instrument and most of the others explained appeared likely to 

 be affected only by lateral or horizontal movements of the earth's 

 surface. One or two were also suggested for the purpose of record- 

 ing the vertical movements. 



James Anstruther, Esq. was duly admitted an Ordinary Fel- 

 low of the Society. 



