1831.] 
Proceedings of Societies : 
93 
vertically at this altitude, appeared to be formed of alternate strata of “shell rock, 
black slate, and horizontal sandstone.” 
The terebratulse, detached and in mass, were particulurly remarked in the Laitche 
Lang Chain, the third great ridge of the Himmalaya, at an elevation of seventeen 
thousand feet, altogether distinct from the other formation. 
Dr. Gerard is preparing a geological section of the Speetee valley, which will 
materially assist in developing the facts of his most interesting researches. 
6 . — A stuffed animal, brought from Van Diemen’s Land, by Dr. Henderson, was 
presented, by his permission, for the inspection of the Society, by Dr. Grant 1 . 
7 . — A fragment of Calcareous Rock, received from Dr. Govan, of Soobatoo, was 
presented by the Hon’ble Sir Charles Grey; supposed to contain the fossil impres- 
sion of a lizard's tail. 
8 . A paper was read “ on Artificial Hydraulic Cements, by Lieutenant W. Saund- 
ers, Madras Engineers,” communicated by the Secretary 2 . 
The thanks of the Society having been voted for these various contributions, the 
Class proceeded to the inspection of the new Compensation Bars provided by the 
Hon’ble Company, for the measurement of base lines in the Grand Trigonometrical 
Survey of India. They were placed along the apartment exactly as prepared for 
use, with the various apparatus mounted, to shew the construction, the adjustments, 
&c. Captain Everest, Surveyor General and Superintendent of the Trigonometrical 
Survey, entered into a complete and highly interesting explanation of the subject. 
He first adverted to the great difficulty of performing, within certain limits of 
accuracy, that apparently simple operation of measuring a straight line ; — the num- 
ber of linear units in use among different nations, all at variance, though nominally 
derived from the same primitive sources, the human foot, and the barley corn : the 
very recent date of their actual lengths being compared by Captain Kater’s experi- 
ments under the orders of Parliament. He traced the origin of the modern mea- 
surements of the earth to M. Riclier’s observation on the diminished length of the 
seconds pendulum at Cayenne, in 1672, and the consequent investigation of the 
figure of the earth by Huygens and Newton, which stimulated the exertions of the 
philosophers of France, Italy, Sweden, and last of all, England, to the practical 
solution of this important problem, by the measurement of meridianal arcs in dif- 
ferent parts of the world. As the chief accuracy of the results of these depended 
on the base or portion of the line actually measured, every means were sought of 
attaining perfection in the instruments used for this purpose. Wooden rods were 
subject to warp and lengthen, from humidity;— chains to wear at the joints, and 
vary with alternations of heat and cold ; — glass rods were troublesome and fragile. 
The French had at last the merit of introducing a compound bar of platina and 
brass, which, by the unequal expansion of the two metals, always pointed out its 
own correction for temperature ; but Colonel Colby, of the Irish Trigonometrical 
Survey, the inventor of the present apparatus, has surpassed them, by doino- away 
with all correction whatever. The principal of his invention, as far as it is possible 
to explain it in so brief a notice, is as follows. Brass expands nearly half as much 
again with heat as steel ; two bars of these metals, ten feet in length, are in prac- 
tice, therefore placed side by side, say one inch apart, and firmly united together in 
the centre ; upon heating these, the brass bar will expand one part in three more 
than the other : if therefore their ends be united together by a moveable lever, 
projecting two inches beyond the steel bar, (making the whole length of the lever 
three inches,) then a point at the end of this lever will alwavs remain fixed, because 
we have two similar triangles superposed, the sides of which are three and two 
inches respectively, and their variable bases (i. e. their expansion) also in the same 
ratio. There are similar levers at both ends of the double bars, so that the distance 
between the fine dots adjusted upon these, remains invariable. There are three 
pairs of bars, which are placed in extension along the line, and adjusted by means of 
a joning telescope : the ends of the bars are kept six inches apart, to prevent any 
s ia e or disturbance, and this distance is set off by means of double microscopes, so 
fitte as to bisect the dots of two adjacent bars : these microscopes are also com- 
pensa ec , that their distance apart may be invariable like that of tbe bars. It will 
be su cient to give some idea of the perfection of this mode of measuring, by stat- 
ing, iat in an experimental trial of the rods in Lord’s Cricket Ground, a length of 
Ine hundred and sixty-seven feet was twice measured with a difference in result of 
only 55 of an inch, which is equal to of an inch per mile. Former operations 
of the best geologists, have frequently varied upwards often inches in the mile. 
^The thanks of the Class were voted to Captain Everest, and the Meeting adjourned 
i ml 16 notlce w h lc h accompanied this specimen will be found in our next number. 
I Ins paper was published in our last number, p. 54 , 
