KEPORT OF THE KEW COMMITTEE. liil 



ought to be evenly put on Trithont any bagging or bulging ; as, if it bulged, 

 besides giving a bad result, it might come into contact with the end of the 

 temperature adjustment bar. 



Care ought to be taken that there is no want of llglit, especially in the case 

 of a low barometer ; and finally, great precaution should be taken to avoid 

 Jinger-marJcs and every species oi had photography . 



(B) Errors of Time and Date. 



Suppose that the sheet has been placed in an unexceptionable manner 

 upon the Barograph cylinder, the next point is for the operator to set the 

 instrumental clock before starting to correct Greenwich mean time, as given by 

 his chronometer. Now the instrumental clock has an arrangement for cutting 

 oif the light for four minutes every two hours, beginning to do so two minutes 

 before an even hour and ending two minutes after it, and the practice is for 

 the observer to read the Standard Barometer about five times every day at 

 periods two minutes after even hours, as ascertained by his chronometer, or 

 when the light should be about to be restored after having been cut off by the 

 clock-stop. If therefore the instrumental clock keeps good time and its stop 

 acts, and if the observer reads the Standard Barometer correctly and at the 

 proper moment as ascertained by his chronometer, and if he finally reduces 

 his curves properly, the near coincidence between the corresponding curve 

 and Standard readings will be a good practical test, not only that all these 

 operations have been properly performed, but also that throughout the curve 

 the instrumental clock keeps good time with the chronometer. A further 

 check with regard to time is afforded by the comparison made between the 

 chronometer and the instrumental clock at the moment when the curve is 

 taken off the cylinder, the results of which are recorded on the curve. 



The clock may sometimes possibly stop, or the clock-stop may go wrong. 

 Without discussing minutely these possibilities, it may be sufficient to state 

 that when any such misadventure occurs the curve ought to be inspected by 

 the Director of the Central Observatory. 



There still remains the question of date. The security that a curve is 

 rightly dated depends ultimately on the strong improbability that an obser- 

 ver at any of the observatories should make a mistake with regard to the 

 first day of the week. When therefore he returns the Barograph journal 

 filled up, we may be quite certain that the observations entered on the line 

 with Sunday were really made on that day, although he may possibly put 

 the wrong day of the month on the form beside it. 



Again, the photographic operator when he takes off a curve, should mark 

 on the back in pencil the day of the week and month when the curve was 

 taken off, and should also, after drj^ing, write upon its face the hour and day 

 of piittiug on and taking off as recorded by the joiirnal. If, therefore, the 

 accuracy of the observer in assigning the proper day of the month to Sunday 

 be checked at Kew as each week's journals are transmitted to that establish- 

 ment, and if it also be seen that the date written in pencil on the back of the 

 curve corresponds to that written on its face, and if the times of starting and 

 ending of the curve as described in front are found to agree with the curve 

 itself as measured by a simple time-scale, there can hardly be any doubt that 

 the curve has been properly dated ; if there still remain any doubt it wiU be 

 dispelled when the tabulations from that curve are examined and it is found 

 that the tabulated readings agree well with the simultaneous readings of the 

 Standard Barometer. 



