Astronomical Methods of Observation. 135 



degree counterbalanced by the defects of its construction. 

 May we, therefore, infer that the smaller class of portable 

 circles of the constructions of Troughton and Kater which 

 admit of repeating the observations near the meridian a 

 sufficient number of times to insure accuracy, are, from the 

 compactness of their structure, their stability and accuracy 

 of motion, superior to the repeating circle. Of all these 

 circles Kater's is the cheapest and susceptible of great 

 accuracy when not too small. The size I would venture to 

 recommend would be about 6 or 8 inches in diameter with 

 telescopes magnifying 20 or 30 times and the three verniers 

 each reading 10". To those who are willing and able to 

 afford the expense, one of Troughton's altitude and azimuth 

 circles of 10 or 12 inches in diameter would prove an ex- 

 cellent instrument, though for travellers it would be rather 

 too heavy. In that case Kater's would be a good substitute 

 and its efficiency will be shown in the following obser- 

 vations. 



(To be continued.) 



Article VIII. 



On the Curved Figures 'produced by rapidly rotating Disks. 

 By Charles Tomlinson, Esq. 



( Continued from Vol. iii., page 44. J 



Since the publication of the last paper, I have taken adr 

 vantage of a visit from my friend Mr. Dodd, further to in- 

 vestigate several additional facts on this subject. 



A part of the experiments before detailed, as well as one 

 by Professor Wheatstone, was performed simply by viewing 

 the rotating disk for one instant of time during various 

 sudden flashes of light, the result being the resolution of 

 the various figures on the disk (whatever they may be) into 

 precisely their original stationary arrangement ; but the ex- 

 periments with a slitted disk are of a more extended na- 

 ture, a new element entering into the resulting phenomena, 

 namely, the generation of curved lines where none pre- 

 viously existed, and the consequent production of new 

 figures of considerable beauty. 



By referring to vol. iii. page 42, the reader will find tliat 



