50 Willson — Mode of Reading Mirror Galvanometers, etc. 



val we find the average of each of the six sets of differences to 

 be respectively l'T8, 1*70, 1*64, 156, 1'54, 1*39, and the aver- 

 age of all the differences to be 1*59. 



Several hypotheses have occurred to me in explanation of 

 this phenomenon, but as I have not as yet been able to put them 

 to the test of experiment I am not prepared to submit any of 

 them, though I intend to test them as opportunity offers. As, 

 however, the objects for which this station is created does not 

 embrace the carrying on of researches for purely theoretical 

 purposes, it may be some time before the desired opportunity 

 for experiments occurs, and hence I desire to place on record 

 this preliminary observation. I ought to add however that the 

 idea has suggested itself to me that we may possibly find in 

 this phenomenon a means for distinguishing between and per- 

 haps measuring the effects of different detonating exj3losives. 



I am deeply indebted to Commander C. F. Goodrich, 

 U. S. N., Inspector in charge of the Torpedo Station, for per- 

 mission to publish this account, and to Mr. Arendt Angstrom, 

 C. E., for the precise drawings used in figs. 1 and 3. 



Torpedo Station. Newport, R. I. 



Art. YI. — Mode of Heading Mirror Galvanometers, etc.; 

 by R. W. Willsox, Ph.D. 



In physical work which requires the observation of small 

 angles of deflection, such as the reading of a reflecting gal- 

 vanometer, it is sometimes found that the use of telescope 

 and scale is inconvenient or trying to the eye, while the method 

 with lamp and scale has other disadvantages beside that of re- 

 quiring a darkened room. 



In many such cases a method of reading may be used, which 

 I do not remember to have seen described, but which I 

 have found so useful that I think it merits description. Though 

 often more convenient than the telescope and scale it does not 

 compare with the latter in accuracy ; but in this respect it is 

 not much inferior to the spot of light, while it is free from 

 some of the most objectionable features of that method. 



Replacing the telescope by a peep hole gives for many pur- 

 poses a very convenient means of reading, where the magnifying 

 power of the telescope can be dispensed with, a vertical line being 

 drawn on the surface of the mirror to fix the sight line. A 

 better plan, however, consists in placing in front of the mova- 

 ble mirror, and as near it as possible, a good-sized piece of thin 

 plate mirror, from half of which the silvering has been re- 

 moved, so that the silvered and unsilvered portions meet in a 



