i873»] Notices of Books. 125 



by the usual care for accuracy where tables are concerned. The 

 engineering student should at once add it to his library. 



Notes on River Basins. By Robert A. Williams. London : 



Longmans and Co. 1872. 

 This is a collection of short notes on river basins, drawn up 

 from the works of Petermann and Milner, Mackay, Long and 

 Porter, McLeod, and others. The source, course, drainage, 

 mouth, and tributaries of each river are given ; and the, area and 

 other details of the lakes of England, Scotland, and Ireland are 

 clearly laid down. The work appears well adapted to the use of 

 pupil-teachers and schoolmasters. 



Reports on Observations of Encke's Comet during its Return in 

 1871. By Asaph Hall and Wm. Harkness, Professors of 

 Mathematics, U.S. Navy. Washington. 1872. 

 The astronomer and those interested in the science of Astronomy 

 will welcome this able pamphlet. Many difficulties have oc- 

 curred in the observation, especially in the use of the spectro- 

 scope. The spectrum of the comet was very faint ; hence it 

 was necessary to remove the photographed micrometer scale of 

 the spectroscope. In its place was inserted a brass plate, pierced 

 with a hole 0*00796 of an inch in diameter, moved by means of 

 a micrometer-screw. The light passing through the hole is 

 reflected from the surface of the prism, and appears, in the field 

 of view of the spectroscope telescope, as a bright disc, with an 

 apparent diameter of 36' 55", which can be made to traverse the 

 whole length of spectrum by turning the micrometer-screw. 

 " The illumination of the disc can be adjusted to the brightness 

 of the spectrum under observation with the greatest nicety. If 

 it is required to be very brilliant, the direct light of a lantern 

 may be thrown into the hole : a less degree of brightness may 

 be secured by passing the light through a piece of ground-glass ; 

 and finally, the luminosity may be varied down to absolute invi- 

 sibility by reflecting the light into the hole from the back of the 

 observer's hand held at a suitable angle. This last plan was 

 employed in the case of the comet. The micrometer head is 

 half an inch in diameter, and divided to one-tenth of a revolu- 

 tion, while each complete revolution of the screw moves the 

 brass plate 0*0181 of an inch, which corresponds to an angular 

 distance of 14' 40-5"." 



In using this micrometer, the readings on the line whose place 

 was to be determined were habitually made alternately with 

 readings on a sodium-line, produced by the flame of an alcohol- 

 lamp with a salted wick held before the object-glass of the large 



