THE MAGNETIC OBSERVATORY AT MADISON. 457 



tion, except with a variation in the intensity of the vertical component 

 of terrestrial magnetism. 



In making the observations, care must he taken that the tempera- 

 ture be kept as nearly constant as possible, a magnet losing about one 

 ten-thousandth part of its power for each degree Fahrenheit of in- 

 crease in temperature ; also, that no magnetic bodies are present to 

 influence the magnets ; that the instruments be secured from all me- 

 chanical interference ; and that all the adjustments be true. 



Let us see how these requirements are met. The observatory has 

 been built underground, and has double walls and roof, there being a 

 space of two feet between the outside and inside walls. A differential 

 thermometer placed within shows a daily variation in temperature of 

 but 1 or 2 Fahr. In case of artificial heat being required, a brick 

 stove has been built, with which it would be impossible to cause a 

 sudden change in temperature. 



The observatory has been placed without the influence of iron 

 water or gas pipes, and in its construction and furnishing no iron has 

 been employed, all metallic supports, mountings, etc., being of brass, 

 copper, or zinc. The reservoirs of the lamps used have to be taken 

 outside to be filled, since to bring a so-called tin oil-can within the ob- 

 servatory would seriously disturb the instrument. As before men- 

 tioned, visitors to the observatory leave outside whatever of iron 

 they may have about them. An abnormal variation in the movements 

 of the magnets at the Key West Observatory is thought to have been 

 caused by the landing of some heavy guns in the vicinity, and their 

 subsequent transportation past the observatory. 



The mounting of the instruments upon heavy blocks of stone and 

 their close incasement reduce the chance of mechanical interference 

 to a minimum. 



To still further guard against errors of observation, there are 

 special adjustments in the several instruments. 



The magnet of the declinometer is suspended by a skein of one 

 hundred fibres of silk, the utmost pains being taken to reduce the 

 torsion to a minimum. The length of skein is at least three feet, so 

 that any residual torsion has the less effect. It seems impossible, 

 however, to entirely get rid of this disturbing element. The records 

 of the Key West observations show that the torsion of the suspension 

 skein changed rapidly during the first five months after the suspension 

 of the magnet, and did not become constant even after six years. 



As variations in temperature do not affect the direction of the line 

 of action of the magnetic force, no temperature adjustment is required 

 for the declinometer. 



The magnet of the bi-filar magnetometer is rigidly cennected with 

 a small glass rod of the same length as the magnet. Over the ends 

 of this rod slip two zinc tubes, of such length as to reach within about 

 five millimetres of its centre. At the inner end of each tube is at- 



