Notices respecting New Books. 301 



The present log-glass runs so as to answer for both the long 

 and short glass; the fanner is used when the ship's rate of going 

 is slow, the latter when she has great velocity. It is intended 

 to be exhausted of air, so that no oxidation of the mercury can 

 take place, nor will it be influenced by the expansion, &c. of the 

 air, which is the case with the sand glasses. The fluid mercury 

 runs equally and more correct than sand, which always forms a 

 hollow cone as it is running. 



Those glasses have been much approved by nautical men ; 

 they are manufactured by Messrs. W. and T. Gilbert, mathema- 

 tical instrument makers to the Honourable East India Company, 

 Leadcnhall-street. 



Should this instrument deserve attention, I beg it may be 

 placed among those valuable instruments and machines in your 

 repository, which are so honourable to this country and to the 

 Society. I am. Sir, &c. 



J. Aikin, Esq. Sec. ^c. H. C. J:ennings. 



Referejice to the Engraving of Mr. H. C. Jennings's Loq- 

 Glass, Plate II. 



Fig. 1, a section, a a the iron collar into which the two glasses 

 /' b are cemented ; c d, two steel pipes fitted in the iron collar, 

 so long as always to be above the surface of the mercury, and 

 the glasses b b are so wide as to hold the mercury clear of the 

 pipes in all positions : e the mercury, which is now running 

 through the smallest aperture c, in 2S seconds (as marked on the 

 glass, fig. 2) ; when inverted, the aperture d allows it to run in 

 14 seconds : yj a steel screwed mouth-piece to introduce the 

 mercury, which may then be cemented up, or a valve may be 

 ap|)lied and the glasses exhausted of air : g a steel cap to screw 

 on to keep all safe. Tl)e glasses are secured by a horizontal cir- 

 cular piece of wood at top and at bottom, which are kept in 

 their places by three screws i i which pass through the tubes or 

 hollow pillars k k, one of which is shown in section : / I the 

 screw nuts. 



Fig. 2 shows the whole together, 11 and 2S being marked on 

 the glasses : they are represented of one-third the real size. 



L. Notices respecting New Books. 

 Lately published, 



1 HP, Grecian, Roman, and (iothic Architecture, considered as 

 applicable to public and private BuiUling-i in this Country. Hy 

 W. Fox. r>i. 



Index 



