226 Royal Society : — Mr. F. Galton on verifying 



so as to staud upon its parallel face, and in this position y can be 

 brought into perfect contact with z. It is plain that when this is 

 done, X is at right angles to the parallel faces of a, y to those of h, 

 and z to those of c. There will now be little difficulty in working 

 the fourth face of each bar. These bars can now be used for 

 bringing into exact shape a rectangular Y groove cut in a block of 

 metal ; and when this has been done the measuring-plane at the 

 end of the bar can be dealt with. Thus, the bar is placed with one 

 edge (k) in the groove, and examined by a true plane ; it is then 

 turned through 180° so that the edge opposite to Tc is now in the 

 angle of the groove ; both the face of the block and the end of the 

 bar are now formed until the contact of both with a " true plane " 

 is perfect in both positions. A similar process is then performed 

 when the bar has its remaining edges successively in the groove. 

 When perfect contact in all these positions has been attained, the 

 end of the bar is at right angles to its axis. 



It is obvious that these tests depend on simple geometrical facts, 

 which would yield perfect results if they could be perfectly applied; 

 they are in fact applied with a degree of accuracy that comes indefi- 

 nitely near to perfection. 



We have dwelt so long on the machines and the principles in- 

 volved in the construction of their parts, that we have little room 

 to speak of the rest of the volume. This is the less to be regretted, 

 as a good deal of it is controversial : e. g. the object of Chapter 2 

 is apparently to prove that the Committee on standards carried out 

 a method for the construction of primary standards inferior to Sir 

 J. AVhitworth's. We must not, however, conclude our notice 

 without just mentioning that several other topics are introduced 

 besides those adverted to above — such as the adaptation of the 

 measuring-instrument to the bores of guns, as well as others of a 

 more general character, as the value of uniformity of system 

 in mechanical work. The book, which will be seen to be one of 

 great interest, is written very clearly and accurately, as might be 

 anticipated from the names of its authors. The illustrations seem 

 to have been executed with great care, and are very good ; and, in 

 particular, the four engravings of the measuring-machine are ex- 

 ceedingly beautiful specimens of mechanical draughtsmanship. 



XXX. Proceedings of Learned Societies, 



ROYAL SOCIETY. 

 [Continued from p. 151.] 

 March 15, 1877.— Dr. J. Dalton Hooker,C.B., President, in the Chair. 

 T^HE following paper was read : — 



-*-• " Description of the Process of verifying Thermometers at 

 the Kew Observatory." By Prancis Gralton, P.B.S. 



It may be of interest to describe the method recently adopted 

 at the Kew Observatory of verifying thermometers by comparison 

 at different temperatures with a standard instrument, since a large 

 proportion of the various thermometrical determinations made by 



