Iviii EEPORT — 1869. 



TmSEMOGEAPH. 



The accuracy of the Thermograph results is liable to be deranged by three 

 causes : — 



(1) By a cause depending on the situation and exposure of the instru- 



ment. 



(2) By instrumental deficiencies, and especially the arrangements con- 



nected with the wet bulb. 



(3) By a deficient system of tabulation. 



Sihuition of Instruments. 



The situation of their various Thermographs was a point carefully con- 

 sidered by the Meteorological Committee, and there is no reason to think 

 that the effect of local peculiarity is considerable in the case of any of their 

 instruments. 



In the Report for 1867 this subject was alluded to, and the restdt of 

 simultaneous comparisons made at Kew between the readings of two sets of 

 dry and wet bulbs was given for the month of Februaiy, one of these sets 

 being placed in a frame detached from the main building of the observatory, 

 and the thermometers having very small bulbs, the other set being the wet- 

 and dry-bulb Standard Thermometers of the Thermograph frame. 



The result seemed to indicate that the local peculiarity of either frame 

 was comparatively small ; indeed, taking the average of the month, there 

 was no residual difference between the dry bulbs, while, on the whole, the 

 Thermograph wet bulb stood 0°*12 higher than the other. 



A similar comparison made for the month of July gave no residual differ- 

 ence either for the dry or wet bulbs. 



Dr. Robinson, of Armagh, has hkewise made a similar comparison between 

 his Thermograph dry bulb and another Thermometer placed at a higher 

 elevation, and has obtained as the result of 150 observations made during 

 the months of April and May, a mean difference indicating that the Thermo- 

 graph Thermometer read on the whole 0°-27 less than the other, "While 

 this difference is not large. Dr. Robinson is of opinion that the upper ther- 

 mometer is more liable to be affected by the sun, and that the Thermograph 

 Thermometer is in consequence the most correct. No other observations 

 have been made on the subject, 



Instrumen tal JDe/iciencies. 



The u'ct-hulb arrangements are peculiarly liable to go wrong, and the fol- 

 lowing course of action is suggested in order to reduce this source of error to 

 a minimum. 



The Standard Thermometers should be read at least five times a day at 

 those moments when the light is cvit off by the clock arrangement. The 

 light remains cut off by this arrangement for four minutes, and it is neces- 

 sarj^ to read the Standard Thermometers at the le(jinnimi of this interval; 

 the exact points in the curves corresponding to certain known readings of 

 the Standards may thvis be determined. "When the Standards are read, the 

 observer ought to notice if both wet bulbs are acting properly. If both are 

 right, the sign y' should be made after the recorded temperature of the wet 

 Standard. If the Thermograph wet bulb is wrong, the sign t should be 

 made, and if the Standard wet bulb is wrong, the sign s. Either wet bulb, 

 if found wrong, ought to be put right at once. Should it happen that the 

 wet bulbs are frozen at the moment of obsciTatiou, the present temperature 



