4-30 Notices respecting New Books. 



the lines of collimation of two telescopes previously made 

 parallel to each other. 



VI. Compare two sets of observations, one of which was 

 made before, and the other afler the object-glass and eye- 

 piece had been taken out of the instrument and replaced 

 reversed in situation. 



VII. Fix one spirit-level to the upper surface, and another 

 to the under surface of an inflexible bar attached to the tele- 

 scope. Make observations with the latter direct and inverted, 

 and repeat them with the bar reversed in direction. 



VIII. The error may be found by the double level affixed 

 to the tube, without reference to the telescope. 



IX. Were the bar of the double level moveable about a 

 short horizontal axis projecting from either side of the tube, it 

 might be inverted by making it describe exactly half a revolu- 

 tion. The two levels could then be made parallel to each 

 other, and the reversing of the bar would be unnecessary. 



X. Place the instrument between two telescopes pointing 

 at each other. Then make the line of collimation of the 

 instrument parallel to that of either telescope, and reverse it 

 within its Ys, which will bring its object-glass opposite to that 

 of the other telescope. Having made the line of collimation 

 of the latter parallel to that of the instrument, observe the dif- 

 ference of inclination, or deviation of parallelism of the lines 

 of collimation of the two telescopes, the instrument being re- 

 moved from between them. 



XL Take out the eye-tube and substitute an object-glass 

 placed at a distance from the wires equal to its sideral focus. 

 Point another telescope at, for instance, the original object- 

 glass, and make their lines of collimation parallel. Reverse 

 the instrument within its Ys, and note the deviation of paral- 

 lelism of the lines of collimation of the additional object-glass 

 and that of the (proof) telescope. 



[To be continued.] 



LXV. Notices respecting New Books. 



Illustrations of the Geology of Yorkshire ; or a Description of the Strata 

 and Organic Remains of the Yorkshire Coast : accompanied by a 

 Geological Map, Sections, and Plates of the Fossil Plants and Ani- 

 mals. By JOHN PHILLIPS, F.G.S., Keeper of the Museum of the 

 Yorkshire Philosophical Society, &c. York, 1829. 4to. pp. 192. 

 Twenty-four Lithographs. 



[Concluded, from page 354.] 



THE third chapter, containing descriptions of the Coast of York- 

 shire from Spurn Point to Redcar, is illustrated by a coloured 



geological 



