REPORT ON GEOPHYSICAL LABORATORY 191 



sand and sheathed in planks. The buildings are built on systems 

 of piles inclosing those of the piers. Now these piers are so stable 

 that in the Cryogenic Laboratory observations with the most delicate 

 galvanometers are entirely practicable when five condensing pumps 

 are at work in a room only a few yards distant, while in the observa- 

 tory there is not the least trouble in using the quicksilver mirror. 

 On the other hand, at the Bureau des Poids et Mesures wagons 

 on a high road some hundreds of yards away shake the piers, and 

 the quicksilver mirror is seriously disturbed even at the bottom of 

 the catacombs near the Paris Observatory. It would seem to me 

 that the mud underlying Leiden damps the vibrations of the piers 

 much as cataracts would do, and that this is the only probable ex- 

 planation of their success, which I understand to have been attained 

 without special reference to the efficacy of mud for this purpose. 



Research Called for. 



The subject is one needing and deserving research. In the well- 

 known Julius suspension, means are adopted both to secure damping 

 and to place the instrument in a node of vibration. The extra- 

 ordinary efficacy of this suspension is well known. It appears to me 

 perfectly possible to devise piers, after proper investigation, in which 

 vibrations will likewise be damped and in which the upper surface 

 will lie in a node. 



While the results in Leiden show that mud underlain by firmer 

 material is not a bad foundation for a laboratory, no one, I take it, 

 would deliberately select such a situation if solid rock or a firm 

 saprolite (decomposed rock in place) were available. 



Construction of Laboratory Building. 



All the most delicate experiments of a laboratory would be car- 

 ried on on piers and in the basement or the first floor of the building, 

 but great stability sufficient for a large class of experiments can be 

 obtained in a second floor by the use of masonry arches. For this 

 reason I cannot recommend for a laboratory the steel beam construc- 

 tion now used in ordinary buildings. In such buildings the oscilla- 

 tions due to wind would be very sensible, and any jars due to moving 

 apparatus or similar causes would be communicated to other portions 

 of the building much more readily than in an arched construction. 

 'Professor Wiechert has measured quantitatively the deflection of 

 his main laboratory building by the wind. 



