220 PROCEEDINGS OF SECTION A. 
nearly east and west through the city, this line for a length of 
50 or 60 miles is the locus of the points at which the deflection 
of the direction of gravity northwards is a maximum, amounting 
to nearly 6” in the average, while along a parallel line 18 miles to 
the south are found the points of maximum deflection southwards. 
Midway between these lines are found the points of no deflection. 
Thus there is plainly indicated the existence beneath the surface, 
if not of a cavity, yet of a vast extent of matter of very small 
density.” 
The magnitude of some of the local deflections and the close 
accord between those arrived at by observation and those given 
by the triangulation of the Caucasus may be seen from the 
following list, which has been taken from a valuable report on 
gravity disturbances by Dr. Helmert.* It may be mentioned 
that in the same report the accuracy of Clarke’s elements is 
amply illustrated :— 
Deviation from the Vertical in the Caucasus. 
Latitude. D ea 
Name Longi- i Differ- 
F ; : tude. (Cone ence. 
Astronomical.} Geodetic. Observed. puted. 
° , u” ie} / “ ° , Y u u 
Pestchanovkopsky | 46 14 45°81 46 14 48°70 | 41 6} — 2°89 0; — 2°89 
TR UISSESV Ae merrcte etererecrs 45 8 1°97 45) 7 52731 41 56 + 9°66 + 10°83 —117 
Jekaterinodar ....| 45 0 51:27 45 0 46°91 88 58] + 436] + 236 + 2°00 
Georgiewsk ....:. 44 9 29°27 44 9 19°30 43 30 + 9°97 + 12:09 — 2°12 
Ws NO? saan nb505007 43751) 22413 43 51 21:07 | 46 38 + 1°06 + 2°65 — 1°59 
Jekaterinograd ....| 43 49 7°03 43 48 54°41 44 41 + 12°62 + 12°62 | Assumed 
zero. 
Alexandrowskaja..| 43 29 813 | 43 28 49°99 | 44 8) +1814] +1945 | —131 
Wladikawkas...... 43 1 40°24 43 1 448 44 43 | + 35°76 | + 38°76 | — 3:00 
PetrOwiske seen tee 42 59 36°7 42 59 18714 47 33 + 18°56 + 16°41 + 2°15 
In our survey no attempt has been made to estimate the 
deflections due to irregularities in the distribution of the adjacent 
surface matter, owing probably to the labour involved in making 
the necessary topographical surveys, and also to the doubt which 
exists as to the radius within which such irregularities may be 
considered as having effect. 
Allusion has been made, when dealing with the altitudes of the 
stations, tothe advantage which would be derivedfrom theexistence 
of a photo-topographical survey in default of amore accurate survey, 
but in connection with the determination of the distribution of 
hill masses in the neighbourhood of the stations at which astro- 
nomical observations have been made such a survey would also 
prove of the highest value. In the absence of any means of 
correcting for the local deflections of the zenith, it is certainly 
* Comptes-Rendus des Séances de la Commission Permanente de l’Association Géodésique 
Internationale, 4 ’Observatoire de Nice, 1887. 
