J. C. Jacobsen writes about mash efficiency

In this, Jacobsen senior asks his son to make some calculations as to the efficiency of mashing at William Younger.

14 April 1869.
Dear Carl!
It interests me greatly to see that you are busy with the investigation of the extract quantity of the malt in England and that you can have the malt for a brew weighed (by the way, I am very surprised that this does not always take place at Younger. Are they in Burton equally accurate?) From the quantity of the wort I assume that you can have the exact measure stated in the fermentation vessel, for the measure in the kettle is not sufficiently accurate.- It is quite a long time since I have made accurate observations on the ratio between malt and extract, but at that time I found the result to be in the closest agreement with Balling's statement, according to which in practice one can calculate about 56% of the weight of the malt; i.e. When you measure the wort in the fermentation tank after the cooling, calculate the specific gravity of the wort according to the percentage strength (of which you will find a table at Balling) and multiply the weight of the wort thus calculated by the percentage strength. Since the specific gravity of the water (62 Danish #(pound signs) per cubic foot) is given by the highest degree of density of the water at 4 R [5º C], a reduction should actually be made for the difference between the specific gravity of the water at 4 R [5º C] and at 14 R [17.5º C], which is the temperature at which both Balling's and Kaiser's saccharometers are adjusted, but I have not previously taken this reduction into account. When I now, one of the first days, repeat the examination with all possible accuracy, I will also take the aforementioned reduction into account. It will then be seen, when we compare the yield in Edinburgh and at Carlsberg, whether the English method of mashing delivers an equally large yield as the Bavarian, which I do not quite believe. However, it must be taken into account that the English malt of good Scottish barley is certainly more floury than the Danish and should therefore give a greater yield even of equal weight. Nous verrons!

The bit about weighing the malt is interesting. It sounds like Younger were using volume quarters. Which wouldn't necessarily weigh 336 lbs. Meaning that the brewing record wouldn't necessarily record the exact weight of malt used. Just the volume.

56% of the weight of the malt seems like an awfully high figure. 90 lbs of extract per quarter of malt is about the best you could expect. Which is only around 26% of the weight of the malt. Am I missing something?'

By Bavarian method, I assume that he means a triple decoction. I, too, would love to know how the efficiency of the two methods of mashing compares. Though the superior quality of UK malt might have muddied the comparison. AS Jacobsen himself remarks.

With me, as you know, a part of the last sparge goes into the next mashing as mash water, but since this is repeated in an equal amount throughout the entire cycle of all mashings, it has no influence on the result. I assume the same is the case with Younger. 

That sounds like a return wort to me. Not sure if Younger did that or not, as it isn't mentioned in the brewing records. But it was fairly standard practice. The big London brewers certainly did it.

I would not advise you to concern yourself with microscopic examinations at the present time. They require, first of all, long preparation and practice just to be able to see correctly, and would also require a longer study of the later investigations of Pasteur and others, which are partly found scattered in various Annals, etc. - If you do not have a microscope at your disposal like mine, which magnifies with perfect clarity 1500 times, linearly, you would not be able to see anything with certainty. I have been busy with other things for some time, but I now have various samples ready, which I am going to analyze microscopically. 

1869 was quite an early date for having a microscope in a brewery. I seem to recall that Whitbread only invested in one in the 1870s. And they were ahead of the game in the UK. 1500 times magnification sounds pretty impressive. And must have been an expensive instrument.