xvu 



the beach at Table Cape, which have interested me by their curious condition 

 of perforation, which I have attributed to the action of dropping water. 



"These pieces are broken off the dividing walls of an assemblage of shallow, 

 more or less circular basins, which occurs below high water-mark, and covers 

 some hundreds of square feet. The perforations in their original position 

 are vertical. The basins are quite smooth within unless where there is a 

 mound of the same character in the centre, and they remind me of what 

 we always find on the floor of caves where there is much dropping of water 

 charged with carbonate of lime. Pools form on the floor. The water is sent 

 ofi" in circles from the centres of drip. The lime is deposited where it meets 

 a check, and there ridges are built up. In the case of a floor of fine sand, 

 the sand itself would be driven off from centre of drip and deposited in the 

 same way — to be itself, as it consolidated, acted upon by other drip. 



"If my conjecture is correct, the land at Table Cape once extended con- 

 siderably to the eastward, and contained at this spot a large cave, the floor 

 of which remains. I am aware that opinions I much respect attribute the 

 perforations to the action of boring animals ; all I can say is, that looking 

 at the isolation of the effect, and its general character, that explanation does 

 not satisfy me. 



" The absence of a stalagmitic character about the specimens would be 

 accounted for by the absence of lime in the formation." 



His Excellency the President read a paper " On the Magnetic Variation at 

 Hobart, and its change in amount since last determined." 



Mr. C. H. Grant observed that this Society, and the public generally, 

 were under a deep obligation to His Excellency for bringing under their 

 notice the very practical matter now occupying their attention. Owing to 

 the system of survey of properties now adopted in this colony, it is certain 

 that the variation in the magnetic deviation must lead to errors and con- 

 fusion, resulting in much annoyance and litigation. In settled districts 

 where the boundaries were kept well defined on the ground, their correct 

 compass bearings at any time was a matter of comparatively small im- 

 portance ; but in some cases such lines had been originally marked in a very 

 temporary manner, and a surveyor now running them afresh must take 

 different directions, and therefore quite alter the character of the property. 

 As a matter of fact, great difficulty is now constantly felt in reconciling new 

 surveys of separate properties with those made formerly, or with the dis- 

 trict maps ; and, in despair, the attempt to do so is often abandoned, causing 

 serious loss both of time and money. Now that property had become more 

 valuable and divided, it was time, he thought, that a special survey depart- 

 ment should be organised under a Surveyor- General, who would iusist upon 

 all surveying operations being conducted on a strictly scientific basis, both in 

 the field and office, and that all the work done should be thoroughly checked 

 and corrected with known points. One of the first matters to eogage atten- 

 tion should be the careful determination and official record of the magnetic 

 deviations from year to year in different localities. 



Captain Stanley remarked, that in order to determine with accuracy the 

 variation of the compass, a good azimuth compass was a matter of great 

 importance ; it was also very necessary to take magnetic bearings on 

 numerous points of the compass ; he had made observations while conduct- 

 ing the Marine Survey of Victoria with what was considered a splendid 

 instrument, and yet on certain portions of the arc the beariogs were as 

 much as 55min. in erroi-. The only way to obtain the variation with accuracy 

 was by a multiplicity of observations, or by what was known as a "Mean of 

 Errors." In this way it was that Captain Flinders succeeded in fixing his 

 Astronomical positions ; he would observe perhaps one hundred lunars, and 

 though various of his results were much in error, the mean was surprisingly 

 exact. In taking observations for the variation of the compass, it was a matter 

 of importance that a situation should be chosen from which all-round 

 observations could be obtained : he had observed the variation at Swan 



