340 Prof. G. H. Darwin. On an Apparatus for [Dec. 15, 



The process by which the special hourly heights have hitherto 

 been obtained is the entry of the heights observed at the mean solar 

 hours in a schedule so arranged that each entry falls into a column 

 appropriate to the nearest special hour. Schedules of this kind were 

 prepared by Mr. Roberts for the Indian Government.* The suc- 

 cessive rearrangements for each sort of special time were made by 

 recopying the whole of the observations time after time into a series 

 of appropriate schedules. The mere clerical labour of this work is 

 enormous, and great care is required to avoid mistakes. 



All this copying might be avoided if the observed heights were 

 written on movable pieces. But a year of observation gives 8,760 

 hourly heights, and the orderly sorting and re-sorting of nearly 9,000 

 pieces of paper or tablets might prove more laborious and more 

 treacherous than recopying the figures. 



It occurred to me, however, that the marshalling of movable 

 pieces might be reduced to manageable limits if all the 24 observa- 

 tions pertaining to a single mean solar day were moved together, for 

 the movable pieces would be at once reduced to 365, and each piece 

 might be of a size convenient to handle. 



The realisation of this plan affords the subject of this paper, and it 

 will appear that not only is all desirable accuracy attainable, but that 

 the other requisite of sudh a scheme is satisfied, namely, that the 

 whole computing apparatus shall serve any number of times and for 

 any numrer of places. 



The first idea which naturally occurred was to have narrow sliding 

 tablets which should be thrown into their places by a number of 

 templates. It is unnecessary to recount all my trials and failures, 

 but it will suffice to say that the slides and templates require the 

 precision of a mathematical instrument if they are to work satis- 

 factorily, and that the manufacture would be so expensive as to make 

 the price of the instrument prohibitive. 



The idea of making the tablets or strips to slide into their places 

 was then abandoned, and the strips are now made with short pins on 

 their under sides, so that they can be stuck on to a drawing board in 

 any desired position. The, templates, which were also troublesome to 

 make, are replaced by large sheets of paper with numbered marks on 



* An edition of these computation forms was reprinted by aid of a grant from 

 the Royal Society, and is -sold by the Cambridge Scientific Instrument Company, 

 but only .about a dozen copies now remain. In the course of the preparation of 

 the " guide sheets " of the method proposed in this paper, I found that there are 

 many small mistakes in these Indian forms , but they are fortunately not of such a 

 kind as to produce a sensible vitiation of results. I learn that the mistakes arose 

 from a misunderstanding on the part of a computer employed to draw up the forms. 



The accuracy of my guide sheets was controlled by aid of Mr. Eoberts's forms, 

 and it was the occasional discrepancy between my results and the forms which led 

 to the detection of the errors referred to. 



