SCIENTIFIC SUMMARY. 
369 
cess, a film of which only remains in the depression of the matrix, mainly in- 
dicating its size and shape, and so much of the horizontal ramns as includes 
the alveoli of the nine posterior teeth, eight of which are in situ. The 
articular and angular processes, and the fore part of the ramus, have been 
broken away, and there is no indication in the matrix of the entire ramus 
having been imbedded therein ; so it may be inferred, therefore, that the 
mutilation took place prior to imbedding. Enough, however of the parts 
remained to enable Professor Owen to conclude that the fossil was a mammal* 
and to enable him to add another genus and species to the small mammalian 
fauna of the Mesozoic period. 
MECHANICAL SCIENCE. 
Balanced Rudder for Screw Steamers. — Our readers will remember that we 
have already described the invention of Mr. Arthur Rigg for applying fixed 
directing-blades behind the screws of steam-ships, by which the current 
of water driven off obliquely by the screw is turned into a fore-and-aft direc- 
tion, in order that the increased pressure on the concave surfaces of the 
blades, and the diminished pressure on the convex surfaces, may produce 
an additional thrust for driving the vessel, and thus save power otherwise 
wasted. More recently, Professor Rankine and Mr. James R. Napier have 
proposed to modify the form of the balanced rudder abaft the screw so 
as to effect the same purpose, a plan which is applicable in the case of 
vessels which have no aftei-sternpost, in which fixed deflecting blades could 
not be used. In the rudder proposed by Professor Rankine and Mr. Napier, 
the forward edge, when the rudder is standing amidships, is tangential to 
the current from the screw, and the after-edge points directly aft. The 
construction for finding the form of the rudder is given in a paper by Pro- 
fessor Rankine, read before the Institute of Naval Architects. 
On finding the most Economical Rates of Expansion in Steam-engines. — A 
very remarkable paper on this subject was read by Professor Rankine at the 
last meeting of the Institute of Naval Architects, in which a graphic con- 
struction is given for finding the most economical grade of expansion when 
the ratio of back pressure to absolute initial pressure, and the ratio of ex- 
pense which varies with the capacity of the cylinder to the expense which 
varies with the quantity of steam used, are known. 
The following are some of the results obtained : — 
Ratio of Back Pressure 
to Initial Pressure. 
Ratio of Cost of Engine to Cost of 
full Steam. 
0-05 0T0 015 0-20 0'25 
Most economical cut off. 
0-05 
0T6 
0*22 
0-27 
0-30 
0*33 
0T0 
0*20 
0*25 
0’30 
0-33 
0-37 
0*15 
0-24 
0*29 
0-33 
0-37 
0*40 
0*20 
0-28 
0*33 
0-37 
0*40 
0*44 
-r 
