1871.] Magnetic Survey of the East of France in 18G9. 39 



circle, on the north brow of the hill, the zenithal direction is disturbed to- 

 wards the north and the astronomical latitude is too great. 



There is only one point in this investigation upon which a doubt can be 

 suggested as possible, namely the evaluation of the micrometer-scale. It 

 was thus conducted : — The micrometer-plate contains 26 wires, and the 

 fixed part of the instrument contains 25 crosses, each interval being nearly 

 256". With this arrangement every wire-interval is measured with great 

 ease, and the whole series of 25 intervals is accurately obtained in terms of 

 the micrometer. By placing the instrument in a proper position, the same 

 intervals are obtained in time of the star's transit, which is easily converted 

 into arc. The comparison of these gives the value of micrometer-divisions 

 which has been employed. 



The following verification, of somewhat inferior accuracy, has been made 

 by measures of the instrument. It appears that the ray of light passes 

 through 0'9 inch of glass, 35*3 inches of water, and 0'8inch of air, nearly 

 (the measure of the last being slightly uncertain). Remarking that the 

 dividing surfaces are horizontal and plane, it is easily seen that the micro- 

 meter-scale ought to be such as is due to an air-telescope whose length in 

 inches = °~ + -||^ + 0*8 = 27*8 inches. And from this, with observation 

 of transit of the star, it was found that the measure of 25 intervals of wires 

 ought to be 0*8693 inch: as measured with a pair of compasses, it was 

 found sometimes 0"8/l, sometimes 0*8/5 . The agreement is fully as close 

 as can be expected from the rudeness of the operation, and shows distinctly 

 that there can be no error of principle in the method of evaluing the 

 micrometer-scale. 



V. "Magnetic Survey of the East of France in 1869/' By the 

 Rev. S. J. Perry and the Rev. Y\ T . Siogreayes. Communi- 

 cated by the President. Received July 13, 1871. 



- - (Abstract.) > 



This paper contains the results of a series of magnetic observations 

 taken in the east of France during the months of August and September 

 1869, and is a continuation of the paper on the survey of the west of 

 France, published in the Philosophical Transactions for 1870, p. 33. 



No change was made in the observers, nor in the methods of observa- 

 tion, during the two surveys ; and the only alteration in the instruments 

 was the substitution in 1869 of a Jones theodolite iu lieu of the small 

 altazimuth by Cook used in 1868. 



Observations were made at twenty-one stations in the following order : — 

 Paris, Rheims, Metz, Strasbourg, Issenheim, Dole, Mont Rolland, Dijon, 

 Lyons, Avignon, Marseilles, Monaco, Montpellier, Grenoble, N, D. de 



