208 



Professor A. Grav on 





Experiment III. — Wrought-iron Bar. 





Load 



;:. 

 Time in 

 seconds of n~- . 10 5 . 



Load 



Time in 

 seconds of 



n~-.10\ 



in tons. 



20 vibrations. 





in tons. 



20 Tibra:::.i5. 





0-r, 



17-25 



336 





13-2 



574 



05 



16-8 



354 



2 - 7 



13-25 



570 



o-o 



17-1 



342 



o-o 



12-55 



6'>3 





i : 



350* 





13-0 



592 



0-7 



15-25 



430 





13-3 



565 



1-0 



14-05 



507 



312 



12-75 



615t 



00 



14-7 



463 



00 



14-85 



453 



IS 



13-05 



587 



3-4 



12-75 



615 



1-6 



1315 



578 



o-o 



15-2 



433 



00 



IS - 5 



549 



3-8 



12-85 



606 



2v 



1295 



d^ 



4-2 



13-55 



545 



o-o 



12-95 



596 



00 



16-25 



379 



o.o 



13-1 



581 



4-4 



13-55 



545 



0-M 



13-4 



557 



o-o 



1595 



393 



2-4 



12-5 



610 









* After one hour. 



+ After the permanent set. 



XXI. On Sensitive Galvanometers. 



By Prof. A. Geay. M.A. 



To the Editors of the Philosophical Magazine and Journal. 



GKS'TLE^IES', 



I HATE read with interest and also some surprise the re- 

 marks made by Prof. Threlfall in his paper in the Phil. 

 Mag. for December last on the galvanometer invented bv niv 

 brother and myselt'. and described by ns in the " Proceedings of 

 the Eoyal Society." for 1864 (vol. xxxvi/ . The notes of onr ori- 

 ginal observations on the sensitiveness of that instrument are 

 I believe in the possession of my brother. who is now in the 

 United States, and are at present therefore beyond my reach. 

 I have, however, set up the instrument; and am now able to 

 state some results I have obtained with it in its present state. 

 The sensitiveness I have attained is not so great as that stated 

 in onr paper ; but I feel certain, that with the same delicacv of 

 adjustment and suspension as was formerly given to it by mv 

 brother, the former degree of sensibility would be regained. 



But first as to Prof. ThrelfalFs statements and calculations. 

 He says, speaking of the galvanometer of our type which he 

 constructed, and which had a resistance of 15,852 ohms: — 

 " The test for sensitiveness was made l:y running a large 

 Clark cell through 10.000 legal ohms, and a certain small 



