181 



sultant and one of its compounds. These figures the author denomi- 

 nates imperfect resultants. 



In each series of transitions there are certain points which are in- 

 variable during all the changes : these are quiescent points, formed 

 by the nodal lines of one figure intersecting those of the other, and 

 the centres of vibration, where the maxima of positive or negative 

 vibration agree in each component mode of vibration. The points of 

 compensation are changeable. Transitional figures appear when the 

 sides of the plate are nearly, but not exactly, equal. 



The author next considers the figures obtained on square plates of 

 wood and other substances, having different degrees of elasticity in 

 different directions. He concludes this part of his paper by an ac- 

 count of some optical means of representing the figures noticed by 

 Chladni. 



March 21, 1833. 



WILLIAM GEORGE MATON, M.D., Vice-President, in the Chair. 



A paper was read, entitled, "An Account of two cases of inflam- 

 matory Tumour produced by a deposit of the Larva of a large Fly 

 (CEstrus humanus) beneath the Cutis in the Human Subject; accom- 

 panied with Drawings of the Larva." By John Howship, Esq. Com- 

 municated by Charles Hatchett, Esq. F.R.S. 



The first of these two cases is that of a soldier stationed on the banks 

 of the Marawina river in Surinam, who had a large boil on the back, 

 from which a maggot was pressed out. The second case, which oc- 

 curred at Santa Anna, in the district of Maraquita, in Columbia, is 

 that of a carpenter, who had for some months a large boil on the 

 scrotum, from which a living larva was extracted. A description of 

 this larva, drawn up by Mr. Curtis, is given by the author, together 

 with a drawing of the specimen. The author proposes giving to it 

 the name of the CEstrus humanus. 



The reading of a paper, entitled, " Experimental Researches in 

 Electro-magnetism," by the Rev. William Ritchie, LL.D. F.R.S. was 

 commenced. 



March 28, 1833. 



The Rev. JAMES CUMMING, M.A., Vice-President, in the Chair. 

 The reading of Dr. Ritchie's paper was resumed and concluded. 



This communication consists of three parts. In the first part the 

 author shows that the common deflecting galvanometer, in which the 

 deflecting forces are assumed to be as the tangents of deflection, is 

 founded on false principles, and consequently leads to erroneous re- 

 sults. The wire forming the coil is of considerable thickness, and 

 therefore there is no fixed zero from which the deflections can be 

 reckoned. The length of the coil, also, being generally short, occa- 



