120 REPORT— 1900. 



are given in this report. The instrument, however, promises well, and 1 

 hope next year to give a description of it and numerical results ; now I 

 only propose to explain its general principle. 



A brass casting is permanently fixed to the rock at each of the four 

 stations, and it is the relative vertical movement of these castings which 

 is measured. A stand carrying a microscope can be placed on any of 

 these castings ; it has three feet, each in the form of an inverted V, and 

 these rest on three cylindrical pieces forming part of the brass casting. 

 This is the usual geometrical arrangement, giving six points of contact, 

 and determining absolutely the relative position of the microscope stand 

 and the casting. The microscope is about 4 feet long, and thus the eye 

 is in a convenient position for taking an observation. The microscope is 

 moved vertically in the stand by a micrometer screw, and carries at its 

 lower end a needle pointing vertically downward. The micrometer 

 screw is turned, the microscope is lowered till the needle point touches 

 the surface of some oil contained in a vessel fixed to the rock, and 

 the position of the micrometer screw noted. The microscope and stand 

 are then removed and placed on the other castings, and the observation 

 repeated ; in this way the relative position of the casting at each station 

 to the oil surface is measured. The four oil vessels are connected by a 

 pipe ; the surface of the oil is therefore at the same level. The needle 

 point is illuminated by a mirror fixed in the oil vessel, and the light, 

 leaving it in a nearly horizontal direction, is reflected by a vertical 

 mirror nearly directly backwards, ajud is then again reflected vertically 

 upwards through the object-glass and eyepiece of the microscope. On 

 looking vertically downwards through the microscope, the needle point 

 and its reflection in the surface of the oil are seen as if the eye were 

 placed just above the surface of the oil ; and when the micrometer screw 

 is turned the needle point and its image are seen to approach each other. 

 The moment of contact is perfectly evident ; the needle and its image 

 appear to run into each other in a confused manner, owing to the dis- 

 tortion of the oil surface when the needle point touches it. The delicacy 

 is considerable ; the divisions in the divided head of the micrometer screw 

 correspond to a movement of -j^^ mm., and it is easy to estimate a tenth 

 of these divisions, but I do not think that the readings can be trusted to 

 this amount, and it is proposed only to read to i^^ mm., which is well 

 within the power of the instrument. 



The micrometer readings give the height of each station above the oil 

 surface, and from these readings is deduced the movement at each 

 station relatively to a datum plane. This datum plane is taken at the 

 mean level of the four stations. The necessary calculations also prevent 

 any error arising from change of the oil level due to expansion or 

 evaporation, damage to the needle point, or expansion of the microscope. 



It is hoped that a very small slip at the Fault will be detected and 

 measured, but even if the movement should ever become as much as 

 10 mm. to 20 mm., it can still be measured with great accuracy. It is 

 unlikely that such a movement will damage the lead pipe where it crosses 

 the Fault ; damage to the pipe, however, can be easily remedied without 

 impairing the accuracy of the readings. Some readings have been taken, 

 but it is feared that they are not perfectly trustworthy ; they may, 

 however, be useful in confirming later results. 



