326 REPORT — 1851. 



The magnetic variations are recorded by Mr. Ronalds's instruments either 

 on silvered plates or on prepared paper. The silvered plates are supposed 

 to have the advantages of greater sensibility to the impressions of light, and 

 pf requiring in consequence a less time of exposure to the light, so that 

 movements of a more rapid character or of more transient duration may be 

 recorded by them ; — and also of producing more sharply defined traces, and 

 of being free from the defects occasioned by the inequalities of surface to which 

 paper is subject, and by those caused by the stretching and shrinking of 

 paper when moistened and when dry. The paper, on the other hand, is sup- 

 posed to have an advantage in the circumstance that the actual traces them- 

 selves can be preserved, whereas the preservation of the original traces made 

 on the plates would be out of the question on account of the expense, ex- 

 cept on very particular occasions. Towards a just appreciation of the prefer- 

 ence due to the plates or to the paper in this application of Photometry, it 

 is necessary to take into the account the practical purposes for which the 

 record is obtained and has to be employed. The mere inspection of the 

 trace, as shown either on the plate or on paper, no doubt, possesses an in- 

 terest both to the uninstructed and to those more familiar with the magnetic 

 phaenomena ; and it may be, that amidst the great variety of those phaeno- 

 mena there may be some which may have light thrown upon them by pecu- 

 liarities only discoverable by a very close examination of the trace itself. 

 But as in astronomy the advancement of the science has more particularly 

 followed from the combination of measuring apparatus with optical power, 

 so in terrestrial magnetism the analysis of the various periodical and other 

 causes, which in their joint action produce the variations which the photo- 

 graphical traces record, requires that the traces should undergo tabulation as 

 a preliminary step to their practical application ; arrangements for rapid and 

 exact tabulation are therefore as necessary as the photographic means of 

 making a trace*. 



Mr. Ronalds has adopted for the trial which is now in progress the plan, 

 which as far as can be judged at present appears the most advantageous, of 

 taking the trace? on plates, from which they are tabulated with great accu- 

 racy and with tolerable rapidity by the aid of ingeniously contrived appa- 

 ratus, and when the tabulation is completed the traces themselves are copied 

 by the hand with a graver's tool on transparent gelatine paper, an operation 

 which requires about a quarter of an iiour for each trace of twenty-four 

 hours, and can be done after a little practice with very considerable accuracy. 

 The copies thus made are arranged in a journal and preserved. The gela- 

 tine paper, which is of French manufacture, but is easily obtained in London, 

 is found to answer extremely well for the purpose, is durable, and bears 

 handling well ; so well indeed that impressions are freely taken on this paper 

 by means of printer's ink and a small press, from the copies of the trace 

 on gelatine paper employed as if it were a metal plate. These impressions 

 are useful to send by post on days of unusual disturbance to other obser- 

 vatories. 



* In the application to Government made in 1845 by the President of the British Asso- 

 ciation (Sir John Herschel) to " encourage by specific pecuniary rewards the improvement 

 of self-recordini^ magnetic and meteorological instruments," the following were stated as the 

 reasons which had induced the British Association to direct the application to be made : — 



1. " The great ultimate saving of time and expense which would take place in observa- 

 tories established for the purpose of magnetical and meteorological determinations, if such 

 apparatus were improved to a degree admitting its indications to be confidently trusted in 

 the absence of an observer, and recorded in a manner suitable for scientific use. 



2. "The advantages which would accrue to those sciences from continuous registry of 

 their phaenomena, instead of observations taken, as at present, at intervals of time." 



y 



