16 Alex. Gerard. Esq., on Pendulum Observation*. 



to conceive that the sudden shower should have produced so 

 great an effect in a few minutes, if acting merely by con- 

 tracting the side of the tower exposed to it, on account of 

 granite being a very slow conductor of heat ; but, upon the 

 hypothesis of the effect being due to a modifying influence 

 over the whole adjoining region to windward, that difficulty 

 is removed. And with regard to my own apparatus, so 

 strongly sheltered from the direct action of the sun, and placed 

 not twenty feet from the ground, the same difficulty occurs. 



It is of course impossible to find a building entirely free 

 from unequal expansion, or from tremor, occasioned by wind 

 or other causes ; but the experiment might be brought to a 

 decisive test by hanging a pendulum down the shaft of a very 

 deep mine which was not being wrought at the time, or by 

 floating a powerful telescope upon mercury, after the man- 

 ner of the horizontal collimator, and directing it in succession 

 to four equidistant marks placed in the four cardinal points. 

 The apparent position of the marks with respect to the hori- 

 zontal wires of the telescope might be altered by unequal 

 refraction at different hours of the day, but being equally 

 distant from the observer, they would by this cause be all 

 affected equally. If, therefore, the horizontal wires should 

 continue to cut the marks at the same points, or at corre- 

 sponding points, at all hours of the day, it would be obvious 

 that no change had taken place in the level of the mercury ; 

 whereas, if the intersections did not correspond, a change of 

 level, and consequently of the plumb-line, proportioned to the 

 discordance, would be equally manifest. 



The satisfactory trial of the experiment, in either of these 

 methods, would imply a command of time, ground, and as- 

 sistants, beyond the reach of most private individuals, but, if 

 undertaken by Her Majesty's Government, might be con- 

 ducted at comparatively little expense by the machinery 

 already in operation in the Trigonometrical Survey. 



Should this communication appear to you of sufficient im- 

 portance, the insertion of it in your Journal may be instru- 

 mental in securing a settlement of the question in the above 

 indicated or some other decisive manner. I have the honour 

 to be, &c. Alexr. Gerard. 



