386 



Messrs. R. H. Scott and R. H. Curtis. [May 20, 



i 



parallel to the axes of the cylinders. The movement was always in 

 the same direction, namely, towards the disks, whether the ball was 

 moved to the right or left. After the trial of many expedients the 

 defect was finally, in great measnre, overcome by attaching weights to 

 the spindles of the cylinders. It however still exists in the machine 

 to a slight extent, and its effect is to decrease the readings on the 

 cylinders by a very small amount. 



It was decided to employ the analyser, in the first instance, in the 

 determination of temperature constants, and careful comparisons have 

 been made of the results obtained by its means, with those got by 

 actual measurement of the photographs and numerical calculations, 

 as will presently be mentioned, and the accordance is so very close as 

 to prove that the machine may safely be trusted to effect reductions 

 which could only otherwise be accomplished by the far more laborious 

 process of measurement and calculation. 



It will facilitate an apprehension of the method of using the machine 

 to give a somewhat detailed account of the operations involved in the 

 treatment of the curves, with an example of the manner in which the 

 readings of the machine are recorded and dealt with. 



The machine is furnished with three pairs of recording cylinders 

 and disks, numbered consecutively 1 to 6, which give the coefficients 

 for the first three pairs of terms of the expansion, and in addition a 

 seventh cylinder and disk from which the mean is obtained. In the 

 thermograms which supply continuous photographic records of the 

 march of temperature, the trace for twenty-four hours covers a length 

 of 8" 75 inches, while a vertical height of about 0*7 inch* corresponds 

 to a range of ten degrees in temperature ; each thermograph sheet 

 contains the record for forty-eight hours. 



* This value varies slightly for each observatory. 



