156 



Dr. C. Chree. 



inform me, being in excellent agreement with his travelling instru- 

 ments. Pare Saint-Maur may be regarded as the base station for 

 M. Moureaux's great survey of Prance and Algeria, while Kew 

 Observatory performed a similar function in the surveys of Great 

 Britain and Ireland, by Professor Riicker and Dr. Thorpe. The- 

 existence of the English Channel introduces uncertainty into any 

 conclusions based on the trend of the magnetic lines in France and 

 England, and the instruments employed in the two countries are 

 sufficiently dissimilar to justify scepticism as to their close agreement 

 in the absence of direct experiment. The interest of the comparison 

 is thus far from being limited to the two observatories most imme- 

 diately concerned. 



M. Moureaux's observations at Kew Observatory occupied the 

 afternoon of July 26, and the forenoons of July 27, 28, and 29. On 

 the afternoons of the last three days, observations were made with 

 the Kew standard instruments, by Mr. T. W. Baker, chief assistant 

 at the Observatory. All the observations were made in the " mag- 

 netic house " in the Observatory garden. 



The comparison was really between M. Moureaux's absolute 

 instruments and the Kew absolute instruments, but the observations, 

 being made at different hours of the day, had to be connected through 

 the intermediary of the curves from the self-recording magnetic 

 instruments. The elements recorded photographically are the decli- 

 nation, horizontal force, and vertical force. The value in magnetic 

 units of 1 cm. of the ordinates is known, but the value of the base lines, 

 answering to zero ordinates, of the several curves is to a certain 

 extent variable. The usual practice at Kew Observatory is to 

 treat each month separately, deducing the value of the base line for 

 any element from a comparison of the absolute observations for that 

 month with the curve ordinates at the times of the observations. 



In the case of the declination and horizontal force, the standard- 

 ization of the curves is comparatively simple. In the case of the 

 vertical force, the influence of temperature is unfortunately some- 

 what large, a rise of 1° F. equalling in effect a fall of O0001 C.GLS. 

 unit in the vertical component. There is also the complication 

 that what the curve gives is the vertical force, while what the 

 absolute instrument gives is the inclination. 



Thus to compare inclinometers used at different hours, one has to 

 follow a circuitous route by way of the horizontal and vertical com- 

 ponents, allowing a correction for changes of temperature in the 

 magnetograph room during the observations. 



M. Moureaux observed the inclination early, and Mr. Baker late, 

 in the day, and there happened to be a slight difference in the mean 

 temperature of the magnetograph room at the times of their obser- 

 vations. 



