678 
DE. C. W. SIEMENS ON DETEEMININ Gr THE DEPTH OE THE SEA 
easier and safer method of relying for its final adjustment upon the result of actual 
working. The limits to the sensitiveness of action of the instrument are chiefly imposed 
by the diaphragm itself, which must be maintained near its neutral position, because its 
elastic range is limited and discordant with the range of the spiral springs. It is 
desirable on this account to make the diaphragm of as thin and flexible metal as 
possible, and to make the annular indentations as deep as they can be made. This 
consideration led me to try a diaphragm of silk impregnated with solution of india- 
rubber, which diaphragm has the advantage of being more flexible than one made of 
metal, but is liable, on the other hand, to stretching under the constant pressure of the 
mercury. A diaphragm of thin steel plate has been found to be sufficiently flexible for 
the purposes of the instrument. 
It was desirable to avoid levers, pulleys, and other such working parts in the instru- 
ment, which parts are liable to derangement from stretching, bending, and abnormal 
expansion, which would make the instrument liable to change its zero position. I have 
therefore had recourse to a micrometer-screw with electrical contact, which, with great 
solidity and simplicity of parts, affords the advantage of a long and accurately divided 
scale. 
Beading of Bathometer . — The micrometer-screw passes vertically through a boss 
below the centre of the diaphragm, which is attached to the tube by means of two 
insulating supports of ebonite. A galvanic battery is connected through one pole to 
the body of the tube, and by the other to the boss through which passes the micro- 
meter-screw. An alarum or galvanometer is comprised in the electrical circuit, which 
is closed whenever the end of the micrometer-screw touches the extreme point of the 
crosshead supporting the centre of the diaphragm, and therefore the weight of the 
mercury column. The galvanometer and alarum are so constructed that one element 
is sufficient to produce the signal, as, if a number of elements were employed, discharges 
of currents would ensue and affect the surfaces of electrical contact. It is important 
to clean these surfaces from time to time, by passing a sheet of stout paper or of fine 
emery-paper between them. A graduated circle is provided to indicate the precise 
angle through which the micrometer-screw is moved from its zero position when its 
point touches the end of the crosshead, an event marked by the sounding of the alarum 
or motion of the galvanometer-needle. The points of contact on the crosshead and on 
the micrometer-screw are made of platinum in the usual way ; but the contact-piece 
carried by the screw is attached to the same through the medium of a strong and 
short horseshoe spring, the object of which is to soften the contact between the two 
points, and thus allow of the natural oscillations of the weighty column as influenced 
by the motion of the vessel. The pitch of the micrometer-screw being 5 millims. 
nearly, and the graduated circle being divided into 1000 equal parts, it follows that 
each division of the scale through which the screw is turned raises the contact-point 
•005 millim., a quantity which is intended to represent the depth of a fathom. The 
micrometer-screw is turned by a wheel geared into a pinion, which is brought up to a 
