Literary Notices. 155 



serted in the earth, should be at different levels, in order that a 

 current should be developed in it, since the positive electricity of 

 the air increases as the poles are approached; and hence, in a 

 similar way, the negative electricity of the earth produced by in- 

 ductions. There is, therefore, usually a difference of intensity be- 

 tween the earth at the poles and at the equator ; and as the electric 

 state is not static, a wire in a direction from the equator to either 

 pole should indicate a current, which experiment shows actually to 

 take place. 



Photographic Mariners' Compass. — A simple means of register- 

 ing a ship's course by photography has recently been invented. 

 The instrument employed is the compass by which the vessel is 

 steered, modified for the purpose. A small aperture is made in the 

 line representing the north on the card, and in it is placed a lens. 

 Under the card is sensitized paper, which is made, by clockwork, 

 to pass along with a regulated speed. Were the vessel constantly 

 to move in the plane of the magnetic meridian, the paper, after de- 

 velopment, would show a straight line, of a length dependent on the 

 time of the voyage ; but every deviation to the east or west will be 

 marked, by the light which illuminates the compass, on the paper, 

 which moves under the aperture in the card, the latter being im- 

 movable. If the paper is divided by transverse lines, the length of 

 time during which the vessel was steered in any particular direc- 

 tion will be indicated by the corresponding deviations from a line 

 corresponding with the intersection of the planes of the magnetic 

 meridian and the horizon. 



LITERARY NOTICES. 



Outline Lunar Map. Zones II. and IV. Areas IV. a and IV. £, 

 with Catalogue of two hundred and three objects, the whole deduced 

 from Photographs, Observations, and Measurement, by W. R. Birt, 

 F.R.A.S. Under the direction of the Lunar Committee of the British 

 Association. (From the Report of the British Association for the 

 Advancement of Science, for 1866.) (Printed for private circulation.) 

 — Mr. Birt, the Secretary of the British Association Moon Com- 

 mittee has evidently made his arduous and laborious task a labour 

 of love, and the positions of the great map comprised in Zones II. 

 and IV., areas IV. a and IV. £, now issued together with the 

 elaborate, accompanying, and explanatory, pamphlet, bear ample 

 testimony to the value of his work. The pamphlet states that 

 " the scale of two hundred inches in the moon's diameter appears 

 to be the smallest that can be xised with advantage in the present 

 state of selenography." This dictum embodies the deliberate 

 opinion of the distinguished astronomers who form the committee, 

 and must therefore be received with great respect. No doubt 

 much may be said for it ; but as few observers have any oppor- 

 tunity of seeing portions of the moon on anything like so large a 



