1 38 Reviews, and Notices respecting New Books. 



upon the description of these last means of measuring small arcs, he 

 gives the description of the best construction for clamps and tangent 

 screws. We believe that Dollond's clamp, described in Art. 5, page 

 275, is the best for insuring a steady motion to a large instrument 

 like the mural circle. After this the principle of the Vernier is 

 fully described in §. XLVII. Our author next minutely describes 

 the construction of Troughton's Reading Microscope. 



After having described the means of measuring small arcs, our 

 author comes to the description of the principal instruments in 

 an observatory : but preparatory to this, he describes two of the 

 most important and indispensable appendages required in making 

 their adjustment. The first is the Plumb-line, and the second the 

 Spirit-level. 



In §. LIII. we find an account of the most improved construction 

 of the Transit clock, which, as the late Professor Robison has justly 

 described it, is one of the principal pieces of furniture in an obser- 

 vatory. The author divides the different escapements into five 

 classes. They may however be comprised in two, (viz.) those in 

 which the maintaining power of the clock is immediately applied in 

 sustaining the vibrations of the pendulum ; and those in which the 

 same power is only mediately applied to this purpose. In the first 

 class are contained all the first four of our author's classification ; 

 and the second contains the Remontoir and Hardy's Escapement. 

 The performance of those of the first class is open to the practical 

 objections pointed out by our author, viz. the variable nature of 

 the force applied immediately through the train of mechanism, to 

 support the motion of the pendulum, which variation depends on a 

 variety of causes, such as the thickening of the oil, &c. The second 

 class is free from this objection, for the pendulum is only supported 

 by the maintaining power mediately, {i.e.) an independent and in- 

 variable power is put into action by the maintaining power in the 

 first instance, and when put in action becomes independent of the 

 maintaining power, and then acts in keeping up the pendulum. The 

 performance of these clocks is surprisingly exact. To Cumming 

 we believe we are indebted for the remontoir, and we understand 

 that Mr. Browne of Portland-place has one of this construction by 

 Cumming, of which the performance is admirable. But the reader 

 who is interested in this subject, cannot do better than refer to the 

 article alluded to by our author in Rees's Cyclopaedia; in which he 

 has fully treated on the subject with his usual clearness. The es- 

 capement which is described in the work under consideration is 

 Hardy's, which justly deserves to be classed among the most refined 

 improvements of modern horology. 



In §. LX. we find the account of the magnificent Transit instru- 

 ment at Greenwich, constructed by Troughton. 



Our author in §. LXI. adds full instructions for observing, and 

 systematically registering Transits ; and then, in the two following 

 Sections, details the methods of reducing the observed into mean 

 right ascensions. 



