Jot the Advancement of Science, 273 



quarter of an hour to copy the twenty-four hours work of the instru- 

 ment. Both Sides of the trace being marked, this can be done after a 

 little practice with very considerable accuracy. The copies thus made 

 are arranged in a journal and preserved. The original plate, except 

 in rare cases, is then cleared off for new work. The gelatine paper 

 IS transparent, durable, and bears 'handling so well that impressions can 

 be freely taken on paper with printers' ink and a small press; which 

 on days of unusual disturbances can be sent by post to other observa- 

 tories. The measuring and taking the means and tabulating the trac- 

 ings are a far more serious operation as far as time is concerned ; but 

 under the able management of Mr. Welch, to whom Mr. Ronalds has 

 intrusted this department of the experimental trial, this can be all ac- 

 complished for the twenty-four hours' work in about three-quarters of 

 an hour, — and thus the great advanlage of producing a continuous rec- 

 ora IS obtamed. The plates as used at Ivew are twelve inches long, 

 and GYevy twelve hours are reversed, thus giving a second tracing on 

 fich plate. The breadth there used is three inches, which is found to 

 be^ ample. But in parts of the globe where great disturbances pre- 

 vail and where at the same time great precision is required in the 

 variations of small amount, a larger field than three inches may be 

 V desirable and may be given. It has been found by the concurrent 



experience of several that the tracings on the plates can be read off 

 with full confidence to the 500th of an inch. This gives in the three- 

 inch plate, 1,500 distinctly recognizable points. Taking the declino- 

 meter as an easily understood example, a single division of the scale 



IS made equal (o six seconds : the range of the scale will therefore 

 extend to 6X1,500—9,000 seconds, or 2° 30'. 



Great stability and freedom from shaking of Mr. Ronalds's appara- 

 tus is obtained by binding together all parts which have reference to 

 each other by marble or metal, and only using wood for outside casing. 

 Another great advantage is, the constancy of the zero line, which ia 

 •nsured by mechanical contrivance. The zero line has thus a perma- 

 nent magnetic value which remains unchanged as well from day to day 

 ^s at all times of the same day. The time of the exposure to the 

 light by which each part of the tracing is at present taken is one and 

 a half minutes. It is, however, probable that by improvements in 

 photographic preparations, the trace may hereafter be taken by ex- 

 posure of much less duration ; also that the length of the magnets, 

 now twelve and fifteen inches, may be greatly reduced. Both these 

 improvements appear to be required if we desire that the trace should 

 iDdividualize perturbations succeeding each other with great rapidity. 



The Astronomer Royal said that, while he admitted that at Kew the 

 photographic method of registering these or similar observations was 

 nrst introduced successfully, and that the methods there used were es- 

 sentially the sa«ne as those of Mr. Brook, which had been adopted at 

 the Royal Observatory, Greenwich, he could not help thinking that 

 there was one important distinction. In Mr. Brook's method, a sheet 

 of photographic paper wrapped on a cylinder received and recorded 

 the working of the instrument; and these original records, carefully 

 Gated, were kept. Where silver plates were used it was of course 

 impossible to preserve the original records ; after the tracings were 



Second SEam, VoL XII, No. 35.— Sept., 1851, 86 



