1871.] 



Presents. 



273 



surface, as all other illustrative experiments hitherto known to me have 

 done, but by showing an effect of diminished fluid pressure throughout 

 more rapidly moving portions of the finite mass of the fluid itself. This 

 effect consists of a slight degree of expansion, depending on the air not 

 being perfectly incompressible. The volume occupied by the more rapidly 

 moving portions becoming slightly augmented, the remainder of the fluid 

 would be condensed were the whole contained within an altogether fixed 

 boundary. A moveable portion of this boundary (that is, the surface of 

 the liquid in your tube) yields and shows to the eye the effect of the di- 

 minished pressure through the rapidly moving portions. 



No branch of abstract dynamics has had a greater charm for the ma- 

 thematical worker than hydro-kinetics, but it has not hitherto been made 

 generally attractive by experimental illustrations. Such refined and beau- 

 tiful experiments as those you describe, and especially your seventh, tend 

 notably to give to this branch of dynamics quite a different place in popular 

 estimation from that which it has held ; but what is perhaps of even 

 greater importance, they help greatly to clear the ideas of those who have 

 made it a subject of mathematical study. 



Yours truly, 



William Thomson. 



Presents received December 8, 1870. 



Transactions. 



Batavia : — Koninklijke Natuurkundige Yereeniging in Nederlandsch 

 Indie. Natuurkundig Tijdschrift. Deel 31. Afl. 1-3. 8vo. Batavia 

 1869. The Society. 



Bern: — Naturforschende Gesellschaft. Mittheilungen, aus dem Jahre 



1869. Nr. 684-711. 8vo. Bern 1870. The Society. 

 Bremen: — Naturwissenschaftlicher Yerein. Abhandlungen. II. Bd, 



II. Heft. 8vo. Bremen 1870. The Society. 



Cambridge, Mass. : — American Association for the Advancement of 

 Science. Proceedings. Seventeenth Meeting held at Chicago, Illi- 

 nois, August 1868. 8vo. Cambridge 1869. The Association. 



Graz : — Naturwissenschaftlicher Yerein fur Steiermark. Mittheilungen. 

 II. Band. II. Heft. 8vo. Graz 1870. * The Society. 



Leipzig : — Konigl. Sachsische Gesellschaft der Wissenschaften. Abhand- 

 lungen. Math.-phys. Classe, Band IX. No. 4, 5. Phil.-hist. Classe, 

 Band Y. No. 6, 7. 8vo. Leipzig 1870. Berichte iiber die Yerhand- 

 lungen. Math.-phys. Classe, 1869, 2, 3, 4 ; 1870,1,2; Phil.-hist. 

 Classe, 1868, 2, 3; 1869, 1, 2, 3. 8vo. Leipzig 1869-70. 



The Society. 



London : — Pathological Society. Transactions. Yol. XXI. 8vo. London 



1870. The Society. 



VOL. XIX. Y 



