The vertical arc of this alidade is graduated to 30 minutes and read by vernier and magnifier to 5 minutes. The two long edges of the base are beveled; one is equipped with a parallel rule. Since the pedestal and telescope can fold down flat against the base, the instrument is easily portable. Casella offered a similar instrument, describing it as "Reeve's Pattern Folding
Alidade." E. R. Watts & Son was established in London in 1857.
Ref: C. F. Casella & Co., Ltd., Surveying and Drawing Instruments and Appliances (London,
about 1940), p. 30.