WITHOUT THE USE OF THE SOUNDING-LINE. 
679 
place near the point of suspension of the instrument, where it can be turned by means 
of a milled-head, without the observer being inconvenienced by the oscillations of the 
instrument relatively to the vessel. Instead of two spiral springs three might be applied, 
dividing the circle equally, probably with some advantage, viz. that of imparting addi- 
tional steadiness to the crosshead in its horizontal position. The letters of reference on 
the drawing, with the references given below, sufficiently describe the mechanical details 
of the instrument. It remains to be shown how an instrument answering to this 
description can be depended upon for giving true indications of the varying depths of 
water below the same, notwithstanding changes of temperature, of atmospheric pressure, 
and of geological formation and condition of the bottom of the sea. 
Influence of Temperature . — In considering the influence of temperature upon the 
instrument, it was necessary to investigate its action upon the component parts sepa- 
rately. The effect of temperature upon the linear dimensions of mild steel, of which 
the instrument is mainly composed, is sufficiently well known. Steel expands, according 
to the experiments of Dulong and Petit, -000012 of its length for every degree Cent, 
rise of temperature between 0° and 100° C. ; and this number agrees closely with expe- 
riments by Regnault, who found the cubic expansion of mercury to be -00018153 per 
degree C., between 0° and 100° C. ; in both these metals the ratio of expansion by heat 
may be considered as strictly arithmetical between ordinary limits of temperature. 
Influence of Temperature on Steel Springs . — Regarding the influence of temperature 
upon the elasticity of springs, we have investigations by M. G. Wertheim*, which show 
a diminution of elasticity with rise of temperature in all metals except iron. This latter 
metal attains its maximum elasticity (according to this author) at 100° C. ; hut annealed 
cast steel agrees with gold and silver and other metals in showing a diminution of 
elasticity with rise of temperature. The results given in the table prepared by 
M. Wertheim show a coefficient of diminution of elasticity for cast steel of -00033768 
per degree Centigrade, the modulus of elasticity at 0° C. being 19561, and at 80° C. 
19014. Before the bathometer was set up, I had experiments made on the variation 
of the elasticity of its spiral steel springs in the range of ordinary temperature, 
which proved this important result, — that the elastic force of well-tempered steel springs 
diminishes with increase of temperature, within the limits of ordinary temperature, in 
an arithmetical ratio. The coefficient which I obtained from these experiments was 
•000258 of diminution of elasticity per degree Centigrade rise of temperature ; and the 
small difference between this and the coefficient deduced from Wertheim’s table will 
be due most likely to a difference of temper in the steel. 
In the bathometer the linear expansion of the springs is compensated by the linear 
expansion of the tube to which they are attached ; and we have therefore only to deal 
with the variation of elastic force which has to be compensated for, in order to make 
the indications of the instrument independent of temperature. 
* Annales de Chimie et de Physique, ser. 3, 1845, xv. 119. “ Sur l’influence des basses temperatures sur 
l’elasticite des metaux.” 
