liv REPORT — 1869. 



(C) Errors in tabulating from the traces. 



It will, in the first place, be necessary to discuss some arrangement for en- 

 suring the entry under the projyer date into the tabulation forms of the mea- 

 surements from each curve ; for even supposing that by the method now de- 

 scribed we can ensure the proper dating of the curve, yet the tabulations from 

 this curve may be entered under the wrong date in the tabulation form. 



The appropriate cheek would seem to be the independent entry from the 

 journal of the Standard readings reduced. For if either of these two inde- 

 pendent entries be wrongly made, this will be seen by a non-coincidence 

 of the reduced readings when compared with the simultaneous Standard 

 readings. Oiu' security becomes, therefore, the security which we have that 

 these two independent readings cannot both be erroneously entered, and this 

 may be converted into a certainty if the assistant at the Central Observatory 

 sees that the journal readings are entered under their proper dates into the 

 Barograph tabulation forms. 



Having thus ascertained the entry into the tabulation forms imder their 

 proper dates of the tabulations and of the reduced standard readings, we come 

 next to inquire what check there is for accuracy of tabulations ; and here we 

 may consider separately the cases of large and small errors. 



But before proceeding to this part of the subject it may be desirable to say 

 a few words regarding the system of Barograph tabulation . 



The progress made in tabulating the Barograms up to the date of publica- 

 tion of the last Report of the Committee has been described in that Eeport. 

 The first operation is to measui-e by the aid of a simple tabulating instru- 

 ment, carrying a scale with a vernier attached to it, and capable of being 

 read to the thousandth of an inch, the whole depth of the Barogram for every 

 hoiu'. 



This system is nevertheless laborioiis, implying two measurements and one 

 subtraction for each hour, besides the application of tables of conversion, 

 and the consequence is the liability to make an occasional mistake. But 

 although at first it is absolutely necessary to have in the ease of the 

 Barograph a tabulating instrument measuring inches, in order by its means 

 to determine the constants of each instrument, yet when once these instru- 

 mental constants have been accurately determined, it has been foimd ser- 

 \dceable to replace the tabulating instrument by another which gives the true 

 pressure in one measurement, instead of requiring two measurements, one 

 subtraction and one conversion. Instruments of this nature have been ob- 

 tained by this Committee for their various observatories, by which the labour 

 of tabulation has been greatly reduced and acciu'aoy of result much in- 

 creased. 



Nevertheless there is still the liability to make an occasional blunder, and 

 as this may take the shape of a large error, it is necessary to devise some 

 means for detecting and obviating all such mistakes. 



The best remedy appears to be the use of a simple kind of subsidiary tabu- 

 lating instrument, consisting of an ivory scale having a breadth equal to one 

 hour of the time-scale, by means of Avhich the hoarly depth of the Barogram 

 may be read to the hundredth of an inch. If these readings be compared 

 with the readings taken independently by the tabulating instrument, any 

 error in the latter will be at once discovered ; for the errors to which the 

 tabulated measurements are liable are such as five hunclrcclths of an inch, or 

 one-tenth of an inch, — errors of a large size, which may easily be detected by 

 the system of sudsidiary measurement. 



