1874.] Meteorological Use of a Planimeter. 437 



ments to which I have just alluded. It will be seen from it that the 

 five-day means so obtained hardly differ from those which are yielded by 

 the direct measurement of the photographic curve by means of the plani- 

 meter. 



The plates in question are obtained by the use of Mr. Francis Gralton's 

 Pantagraph, which transfers the records at a reduced time-scale to zinc 

 plates, which plates are subsequently further reduced and transferred to 

 copper by Wagner's Pantagraph, as explained in the Eeport of the Com- 

 mittee for 1870. 



I therefore hope that the Society will allow me to remark that such a 

 test as this affords a satisfactory proof of the accuracy of the reproduc- 

 tions of our automatic records which are executed in the Meteorological 

 Office. 



The result of these preliminary experiments is that the planirneter 

 means are practically identical with those obtained by treatment of the 

 values of the hourly ordinates. 



It is found that the mean from the photographic record of temperature 

 for one day can be obtained in about the same time as is required for the 

 calculation of the hourly values ; while in the case of pressure the saving 

 of time w r ould be considerable. In both cases the hourly values are 

 supposed to have been previously measured. If, however, the five-day 

 mean from one of the plates of the ' Quarterly Weather Report ' be 

 admissible, the economy of time would be very great indeed. 



It does not appear that the liability to error in following the course of 

 the curve with the tracer of the planimeter is greater than that of mea- 

 suring the ordinates of the curve by a glass scale ; while we escape one 

 serious cause of uncertainty in the latter operation, the difficulty of 

 assigning the exact ordinate to the hour at a period of rapid change of 

 temperature, &c. a case of frequent occurrence ; and we almost entirely 

 dispense with arithmetical calculations. 



It is very unfortunate that the use of the planimeter will not enable 

 us to dispense with the necessity of taking hourly readings, inasmuch as 

 it affords us no means of averaging any but consecutive values, and so 

 renders us no assistance in any determination of the march of meteoro- 

 logical phenomena. 



A further series of planimeter measurements, for pressure and dry- 

 and wet-bulb temperature, for all the observatories for three months is 

 now in progress ; and if, as we hope, the results will prove as satisfactory 

 as those which I have the honour to submit to the Society on the pre- 

 sent occasion, it would appear that there is no further reason why plani- 

 meter means should not be published in future by the Office. 



