482 



I ought perhaps to add, with reference to Father Secchi's contriv- 

 ance, that he recommends in some cases the cistern of the barometer to 

 be made moveable, instead of the tube. The balance is then disturbed 

 by the efflux of mercury from the tube of the barometer when the 

 pressure diminishes, and by its influx when the pressure increases. 

 Though less elegant, as an application of a principle, it has the ad- 

 vantage of making the suspended mass lighter. It will be seen, by a 

 reference to Maorellan's account of Sisson's instrument, that the 

 weight was such as to require support on friction-rollers, instead 

 of knife edges.* 



Invention of the Cathetometer, — I take this opportunity of adding 

 a historical notice, which has occurred to me whilst making the pre- 

 ceding inquiry. In the twentieth volume of the Philosophical 

 Transactions for 1698, Mr Stephen Gray described a microscope 

 moving on a vertical pillar by means of a micrometer screw, to be 

 used for determining the exact variations of level of a liquid, such as 

 mercury in a barometer or thermometer, and not necessarily con- 

 nected with the apparatus. This instrument accurately corresponds 

 in most respects with that known to French physicists and instru- 

 ment-makers under the name of the Cathetometre, which I have 

 never heard ascribed to any inventor in particular, and which, till 

 very lately, has hardly been recognised in this country. 



2. History of an Anencephalic Child. By Dr Simpson. 



3. On certain Laws observed in the Mutual Action of Sul- 

 phuric Acid and Water. By Balfour Stewart, Esq. Com- 

 municated by Dr G. Wilson. 



The object of this paper was to show that where sulphuric acid 

 combines with water, distinct reference is made to certain definite 

 compounds or hydrates of sulphuric acid. 



* Since this paper was read, I have been enabled to carry back the history 

 of the Balance Barometer, or at least of the experiment described by Desagu- 

 liers, considerably farther. In Cotes's Lectures on Hydrostatics, &c., published 

 by Smith in 1747 (but which were delivered more than forty years previously), 

 the experiment is fully detailed and explained. It is also ascribed to Wallis, 

 as well as an ingenious modification of it well adapted for the lecture table. 



April 1857. J. D. 1\ 



