end, which is conical, a rope is fastened, so as to enable 
the log to be towed by theship. A modification of this is 
called the “ detached’”’ log, the revolving fan and case 
for the clockwork being two separate pieces. To both 
these logs there are several objections, such as the almost 
unavoidable entrance of salt water to the wheelwork, the 
drag on the ship, which is often as much as 4o lbs. or 
more, the inconvenience of hauling in the log each time 
it has to be read, and lastly, the loss of the whole instru- 
ment, should the towing-line break. 
To obviate these objections, the taffrail log was invented, 
and which goes a step further than the detached loz, by 
taking away the recording portion to the taffrail of the 
ship, and causing the towing rope to transmit the revolu- 
tions of the fan to it. These logs are, in many respects, 
a great improvement on the first ; the registering dial is 
less liable to damage, and is always visible; the tension 
of the towing line is less; it is therefore less liable to 
rupture, and even when this does take place, the fan is 
FIG... 
easily replaced. On the other hand, the unsuitable nature 
of the towing line for transmitting torsional force, is ob- 
vious. Further, the slip of the fan must by seriously 
affected by the constant variation, in length, of the sub- 
merged portion of the line. In Walker’s log these diffi- 
culties are partly met by the use of a governor, consisting 
of a pair of weights fixed to the towing line, and revolving 
with it; but this device can only modify the evil. 
The late Mr. W. Froude, who pointed out other objec- 
tions to existing logs, endeavoured for some time to 
devise an electrical log, in which the revolutions of an 
accurately formed screw should be communicated to the 
deck, of by the revolution of the towing rope, but by an 
electric current in wires carried by it. Eventually he 
succeeded, in conjunction with Mr. Brunel, in construct- 
ing an instrument of this kind, which was applied to Sir 
W. Thomson’s yacht, La//a Rookh, and worked very 
well, till by some mischance, it carried away and was 
lost. Mr, Kelway had, meanwhile, been working inde- 
pendently with the same end in view, and had constructed 
NATURE eae 
585 
an electric log, which he brought before the notice of the 
Admiralty. A trial of this last instrument was under- 
taken by Messrs. Froude and Brunel, on board H.M.S. 
Shah. In this trial, the registering portion was placed 
on the poop, and self-recording apparatus was used, by 
which at every revolution of the fan a pen was lifted 
from a strip of paper moved by clockwork, thus causing 
breaks in the otherwise continuous line. On a line 
parallel to this, time in half seconds was simultaneously 
recorded. The result of this trial was to clearly demon- 
strate the satisfactory action of Mr. Kelway’s log, and it 
was afterwards applied, together with accurately formed 
fans, to H.M.S. /vzs, and the yacht A/erta. Its further 
extension has not been proceeded with, apparently in 
consequence of Mr. Froude’s decease. Quite recently 
the same inventor has put into practice the happy idea of 
placing the electrical log to work altogether under the 
ship’s bottom. The way in which this is done is shown 
(Fig. 1). A water-tight case is securely fixed to the 
bottom plates (AA), inthis case a frame (HH) is moved 
up and down, and in the lower part of the frame the fan 
(R) works, The fan communicates its motion by a vertical 
spindle (M) to a box (N), in which electric contact is made 
and broken eight times in a mile. The wire (00) can 
thus transmit a record of the distance passed over to a 
dial or dials fixed in any part of the ship. 
This invention has the advantage of allowing the 
screw of the instrument to work in water of uniform 
pressure, and to a great extent free from the disturbing 
action of the waves. There is undoubtedly a body of 
water carried along by the surface friction of the ship. 
The depth to which this extends is unknown, but 
there is strong reason to think it is very small, and would 
not therefore affect the fan. The log itself, however, 
offers an excellent opportunity of investigating this ob- 
scure point, since it can easily be raised or lowered to 
different positions. 
The complete instrument is at present being exhibited” 
at the Crystal Palace Electrical Exhibition, and an 
account of its various applications has been recently 
given in a paper by Mr. Kelway. These applications 
are many and important, and the invention, besides 
being very suitable for its original purpose, promises to 
afford valuable information, not to be obtained by the 
use of ordinary logs. Ein Se) Elenoe 
THE TONNAGE QUESTION 
NE of the most interesting papers discussed at the 
recent meeting of the Institution of Naval Architects 
was on the Revision of the Tonnage Laws. The author, 
Mr. W. H. White, is, from his position as Chief Con- 
structor at the Admiralty, as well as from his well-known 
attainments, a singularly impartial and able judge of this 
most difficult question. The occasion which called forth 
the paper was the report on the tonnage question lately 
issued by the Royal Commission, which last year took 
evidence on this subject. The report was, as is well 
known, not signed by all the commissioners. Two of 
them, viz. Mr. B. Waymouth, Secretary to Lloyd's 
Register, and Mr. Rothery, Q.C., the Wreck Commis- 
sioner, wrote independent reports, which differed widely 
from that of the majority, and from each other. 
The state of the Tonnage Laws has for a long time 
past given rise to serious complaints, on various grounds. 
As matters stand at present it is possible for a steamer 
not only to have no tonnage at all, but even, as is the 
case of a vessel well known on the Clyde to have a 
negative tonnage. It is also alleged that the indiscrimi- 
nate measurement of all inclosed spaces on deck for 
tonnage has a direct tendency to produce unsafe vessels, 
by taxing the covering in of the large open spaces above 
the engine, &c., and also by unduly taxing vessels pro- 
vided with hurricane decks, which, from their nature, can 
never be entirely filled with cargo. 
