270 CARNEGIE INSTITUTION OF WASHINGTON. 



Besides the so-called "magnetic-deviation errors" of the compass, arising 

 from the magnetic materials in the vicinity of the compass, there are other 

 errors which make themselves seriously felt, only, however, while the aero- 

 plane is turning. The latter are called "dynamic-deviation errors"; their 

 magnitude depends upon the tilt of the aeroplane, the magnetic dip, and the 

 heading of course of the airship. 



While the dynamic deviations may be large during turns of the aeroplane, 

 yet they disappear, practically, when straight flight is resumed. Accordingly, 

 there is doubt as to the desirability of adopting movable compensating devices 

 such as suggested by Starling, which, while effective during aeroplane turns, 

 might introduce magnetic deviations of a more permanent character during 

 the more usual straight flights. If such devices are used they will require 

 careful control. 



In connection with the use of the compass in aerial navigation, an interesting 

 scientific question comes up as to the change of the Earth's magnetic field, or 

 of the magnetic elements with altitude above the surface. Magnetic experi- 

 ments of this nature were made in balloons by Gay Lussac and Biot in 1804, 

 which were repeated, with more success, a half century later by Glaisher. The 

 available observations to date do not possess, however, the requisite refine- 

 ment, and it is hoped that some day a non-magnetic airship and the necessary 

 instrumental appliances will be available for conducting a magnetic survey of 

 the aerial regions in the same manner as that employed in the ocean-magnetic 

 survey of the non-magnetic ship, the Carnegie. 



Experiments concerning "magnet-photography." L. A. Bauer and W. F. G. Swann. 

 Physical Rev., ser. 2, vol. 9, 563-564, June 1917. (Abstract of paper presented 

 at the joint meeting of the American Physical Society and the American Asso- 

 ciation for the Advancement of Science, New York, December 26, 1916.) 



In the Scientific American for November 4, 1916, are recorded a number of 

 experiments by F. F. Mace, on the influence of a magnetic field in stimulating 

 photographic action of a set of miscellaneous articles. In Mace's experiments, 

 both electromagnets and permanent magnets were used. The photographic 

 plate was placed, film side up, on the top of the magnet, and the articles were 

 placed on the film. The whole was covered with a bell-jar, which was bound 

 around with black cloth and evacuated. After allowing the apparatus to 

 stand in a hght-tight room for lengths of time which varied in different experi- 

 ments, impressions of the articles were obtained. 



In the present paper is described a repetition of Mace's experiments mth 

 an electro-magnet, the plate used being a Seeds No. 30. The articles used 

 were: a disk of hard rubber, a disk of white pine, a lead ring, a hexagonal iron 

 nut, an iron key, a copper washer, a piece of amber, and a lead disk. The bell- 

 jar was exhausted with a Gaede box-pump for about half an hour, and then 

 sealed off. After an exposure of 13 days, distinct impressions of the articles 

 were obtained. The metallic articles and the hard rubber showed up lighter 

 and the other articles darker than the ground. The grain of the wood was 

 distinctly visible, but it is well known that wood will produce an action of this 

 kind on a photographic plate, independently of the action of a magnetic field, 

 the effect being primarily due to the resin in the wood. The appearance 

 presented by the images of the metals was such as would be produced if they 

 had acted as shields to a general radiation or other source of influence extending 

 over the plate as a whole, and the main interest of the exjieriment here cited 

 lay in the fact that the general darkening of the ground of the plate appeared 

 to be much greater than was the case in the absence of the magnetic field, as 

 subsequent experiments showed. (For continuation of experiments, see 

 page 273.) 



