328 CARNEGIE INSTITUTION OF WASHINGTON. 



of the magnetic latitude. Similarly, magnetometers not infrequently, for 

 one reason or another, give values of the magnetic declination, or of the hori- 

 zontal intensity, erroneous by amounts which must be taken into account for 

 satisfactory work, whether in magnetic surveys or at magnetic observatories. 



The sources of these instrumental errors are found to be partly in some imper- 

 fection in construction of instrument and partlj^ in imperfect determination of 

 instrumental constants, as for example, of the moment of inertia of the chief 

 magnet, of the distribution coefficients, etc. Numerous examples might readily 

 be cited to show the great care required in the purchase of magnetic instruments, 

 and in the determination and control of the constants, if the aim is to determine 

 the terrestrial-magnetic elements within the requisite and possible accuracy. 



The Department of Terrestrial Magnetism has had occasion to design and to 

 construct in its own workshop various types of magnetic instruments to meet 

 the demands of accurate field work on land and at sea. In its latest type of 

 theodolite-magnetometer, there has been added an earth-inductor attachment 

 for the measurement of dip. This one instrument thus suffices for both 

 the astronomical and the magnetic work of land magnetic expeditious. It 

 has been used with success in a difficult land expedition, across the deserts in 

 the northwestern part of Australia, this being the first time that an earth 

 inductor has been utilized for extensive field work. This combined magnet- 

 ometer and earth inductor, though weighing but 15 pounds (7.2 kg.), has 

 met fully the varied requirements of magnetic work. An accuracy has been 

 reached with this field instrument comparing very favorably with and even 

 at times exceeding that obtained with the standard large types of magneto- 

 meters in use at many observatories. Thus the uncertainty in the declin- 

 ation and inclination need not be over .'2, and in the horizontal intensity not 

 over 1/5000 part or 0.02 per cent (about 4 units in the fifth decimal c. g. s. 

 for Washington) . 



It has been found possible by aid of the accumulated data to decide now on 

 "international magnetic standards," designated I. M. S., which apparently 

 yield values of the magnetic elements with an absolute accuracy of about O.'l 

 or .'2 in declination and inclination, and about 0.01 or 0.02 per cent in the value 

 of the horizontal intensity. 



On the basis of these international standards, the corrections of 29 observa- 

 tory standards, excluding those manifestly defective, range from +1.'8 to 

 — 1'5 in declination (east declination being counted positive), +l.'l to — l.'l 

 in inclination (inclination of north end of needle below horizon being regarded 

 as positive), and +0.14 to —0.21 per cent of the value of the horizontal 

 intensity. The average corrections, regardless of sign, are: 0.'5 (declination), 

 0.'5 (inclination), and 0.06 per cent (horizontal intensity). 



The experience gained during the past decade has proved that, when ade- 

 quate precautions are taken, it is possible to preserve the constancy of magnetic 

 standards, 5 to 10 years, with an accuracy of 0.'2 or less in declination and 

 inclination, and 0.015 per cent or less in horizontal intensity, i. e., within about 

 3 units of the fifth decimal c. g. s. unit for a magnetic observatory in medium 

 latitudes. Serious discontinuities have been introduced at times in observatory 

 series by lack of requisite precautions, inadequate control of instrumental con- 

 stants, or changes in constants based on limited experimental data. 



General results of the work in atmospheric electricity aboard the 'Carnegie, 1909-1914. L. 

 A. Bauer. Proc. Am. Phil. Soc, vol. 54, No. 216, 14-17 (January-April 1915). 

 Philadelphia. 



A paper presented before the annual meeting of the American Philosophical 

 Society at Philadelphia on April 24, 1915. It contains a general account of the 

 progress made by the Department of Terrestrial Magnetism in its work in 



