At the turn of the 19th and 20th centuries Edwin Harrington, Son & Co. were based at Seventeenth and Callowhill Streets, Philadelphia, USA, in an area famous for its many machine-tool builders. Although the Company made conventional lathes, they were perhaps better known for their range of "Extension" or sliding-bed lathes. This was not a machine type exclusive to Harrington - many other makers produced similar machines - but Harrington and Son - besides manufacturing Radial and Multiple Spindle Drills and Chain Hoists, appeared to specialised in them and this page illustrate ones produced by the company in the early 1900s. In addition, at the end of the page, can be seen a standard gap-bed Harrington in a derelict condition, as recovered during 2016 from a workshop by the restoration group of the Amador Sawmill and Mining Association in Jackson, California.
All Harrington sliding-bed lathes had a two-part construction, with the upper arranged to slide on the lower so that the distance between centres, and the width of the gap, could be increased, or decreased, as a particular job demanded. The idea was to combine several machines in one: with the gap closed the machine became a stout, backgeared, screwcutting engine lathe  (or 'centre' lathe to British readers) with the capacity for regular work. When the bed was extended, long, thin or irregularly shaped work, perhaps with large diameters at inconveniently placed intervals, could be also be accommodated. This flexibility in sizing and set up allowed smaller engineering shops to take in work which should, nominally, have been beyond their capacity - or in the case of a larger enterprise, would have ensured that the biggest lathes were left free to be used for the heaviest jobs. The Harrington lathes were built in four sizes, named according to the swing above the upper and lower beds, and all were backgeared and screwcutting - the leadscrew being carried by the upper bed and connected to the changewheel drive by  a pair of gears, the lower of which was connected to a splined 'power shaft'.
With the makers specifying a countershaft speed of between 45 and 85 r.p.m, and in combination with a "triple-reduction" backgear - an enormous 54 : 1 ratio on the largest lathe - the result was a bottom speed slow enough to allow the successful machining on the outer edges of work up to 5 feet in diameter. In addition to backgear, and the four or five-step headstock cone pulleys, the two smaller lathes each had a two-speed gearchange on the headstock and the larger ones three - giving, in the best cases, a selection of 45 different spindle speeds. The drive could be by either a traditional wall or ceiling-mounted flat-belt countershaft system (or overhead line shafting) or from a built-on variable-speed motor driving direct though exposed spur gears at the rear of the headstock. Quite how the latter system sounded when well worn, and being worked hard, is best left to the imagination.
Of deep section and braced internally by cross ribs, the lower bed carried an upper section sliding in deep guides and constructed with stiffening cross ties placed at frequent intervals. When the required length of top bed had been wound out, the upper and lower sections were locked together by T bolts and the outer, overhanging part of the upper bed supported on two loose screw jacks. On the smallest machine the top section was extended by a screw, with its operating handle positioned at the end of the bed; on all larger versions this drive was augmented by a reduction bevel box to ease the load of moving what was a considerable and very heavy lump of cast iron.
When the top section was extended, and the gap opened, an extra carriage support, fitted as standard, came in to play; positioned under the front edge of the apron, it reached down to floor level where a machined 'way' was formed for it along the lower edge of the bottom bed.
Power sliding and surfacing feeds from a separate power shaft were fitted to all models, the leadscrew only being used when generating threads; power to the feeds was taken through a clutch and gear drive with all the controls grouped exclusively on the apron. As an option, power feed could also be fitted to the top (tool) slide - where it could be made to operate at any angle..

Facing a large casting in the smallest lathe (the 28"/48") with its centres raised by 9". Note the long extension arm carrying the tool post and the rather narrow flat belt drive- the cut could only have been a light one, despite the bracing arm which can be seen reaching down to a slideway machined along the lower edge of the bed at floor level.
On the two largest lathes the extended toolholder was not required, their compound slide rests being large enough to reach right across the gap when turning faceplate jobs.

No spotless overalls, safety goggles or corporate logos for this unemancipated turner - though it is hoped that he would normally have watched the job from the tool-side of the operation and not where the photographer had asked him to stand.

Even tricky jobs could be accommodated by the addition of some simple equipment. In this set up a large drum is being turned by a toolpost mounted on a rest secured to the carriage and allowed to slide through a support fastened to the headstock end of the bed.  Note the size of the faceplate in the background at the tailstock end of the bed …

Drive could be by either a traditional flat-belt countershaft system (or from overhead line shafting) or from a built-on variable-speed motor though exposed spur gears at the rear of the headstock. 
In addition to the "Triple backgear" the lathe (a 28"/48") had a two -speed gear engaged by a sliding key - the selector handle can be see immediately behind the faceplate.
Notice the very largest backgear, just visible behind the faceplate and outboard of the front spindle bearing.

The direct motor drive on the 36"/60" lathe incorporated 3-speed change gears - in addition to the triple backgear and 4 or 5-step cone pulley drive.

36" and 60" Extension Lathe. The compound slide rest was mounted at the very edge of the saddle - with no support at all to its left; the top slide was adjustable for position on the very narrow cross slide in two transverse slots - just like a model engineering lathe.
This picture gives a much clearer view of the huge backgear carried behind the faceplate - and outboard of the headstock-spindle bearing.

Largest of the Harrington Extension lathes were the 48" and 72"-swing models. The ratio of the triple backgearing was 53.3 to 1 and the combined "leverage" of the backgear and cone-pulley drive 221 to 1 - this was a lathe meant to run slowly, rather than fast. All regular threads, including the 111/2" pipe thread, from 1/2 to 14 t.p.i could be cut by the changewheels supplied.
The front spindle bearing was 6" in diameter and 9" long, the far less heavily loaded rear bearing a mere 3.5" by 4.5". Flat belt 4.5" wide drove the 5-step cone pulleys, the largest of which was 13.5" in diameter.
The bed was 15 feet long and, when fully extended, could accept 15 feet between centres. The standard faceplate was 5 feet in diameter but when the "extension-turning" accessory was in use (see the bottom of Page 1) a special 48" faceplate was used instead.
For its time this was a relatively heavy lathe, turning the scales at 19,000 lbs - or 8.5 tons

An early Harrington lathe of conventional construction with a gap bed in complete, as-found condition. The original flat-belt drive to the spindle has altered to a roller chain arrangement, the drive coming from an automobile gearbox. The lathes age is indicated by the use of a rear-mounted leadscrew, with a powershaft running along the front face of the bed and a lever, to operate the clasp nut, mounted at the front within easy reach of the operator.
Possibly of a unique design, the tailstock used either a cam or nut that encircles the tailstock spindle to lock it in position, this being operated a tommy bar inserted into a series of holes just visible forward to the two large nuts protruding from the top face.
From catalog illustrations this model appears to have been the smallest of the three gap-bed lathes listed.